In this article, we shall study Thomson’s model of an atom and its deficiencies and Rutherford’s atomic model and its deficiencies.

Thomson’s Model of an Atom:

J. J. Thomson, in 1898, proposed that an atom possesses a spherical shape (radius approximately 10^–10 m) in which the positive charge is uniformly distributed. The electrons are embedded into it in such a manner as to give the most stable electrostatic arrangement. Many different names are given to this model, for example, plum pudding, raisin pudding, or watermelon. It can be visualized as a positive charge with plums or seeds (electrons) embedded into it (watermelon). An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over the atom.

Although this model was able to explain the overall neutrality of the atom but was not consistent with the results of later experiments.

Basis of Rutherford’s Atomic Model:

Alpha particles from a radioactive substance were made incident on the thin foil of gold of thickness 10^-7 m. After passing through the foil, the alpha particles were detected at various places on the ZnS screen or photographic plate. The following four types of particles were detected.

• Most of the alpha particles just passed through without any deviation as if there is empty space.
• A few alpha particles were deflected through smaller angles.
• A few alpha particles deviated through larger angles.
• Very few rebounded back This larger deflection is possible only if α-particles collide with heavy and positively charged particles inside the atom because like charges only repel each other.  This massive +ve charge is at centre of an atom and called nucleus.
• Very few alpha particles were rebounded i.e. they deviated through 180°. This concludes that the nucleus is very small as compared to the volume of the atom.

Conclusions :

• Based on the experiment Rutherford concluded that most of the alpha particles having positive charge went through the foil undeflected. Hence there must be empty space in the atom.
• Some of the particles were deflected.  It is due to the positive charge present in a very small space within the atom. He called this centrally cored and positively charged region of an atom as a nucleus. The rest of the portion of the atom should be empty.

Rutherford’s Atomic Model:

From the observations of his experiment, Rutherford put forward the concept of his atomic model.

The atom consists of the centrally located positively charged nucleus. The whole mass of an atom is concentrated in the nucleus. Around the nucleus, there is empty space in which the negatively charged electrons revolve in different orbits. Rutherford’s model of an atom is also called as a planetary model of an atom.

Limitations Of Rutherford’s Atomic Model:

• The negatively charged electrons revolve around the nucleus in a circular orbit, hence they possess centripetal acceleration. According to the classical theory of electromagnetism, accelerated charge radiates energy continuously. Therefore, the
  electron should radiate energy while going round the nucleus losing its energy continuously. therefore, it should approach nearer the nucleus while going round emitting radiations of increasing frequency and finally falling in the nucleus. Thus it should move in a spiral path and should emit continuous spectrum and thus structure atom is not stable. Actually, the spectrum observed is line spectrum of definite frequency, and hence a modification of Rutherford’s atom model was necessary.

• This model of an atom fails to explain the distribution of electrons in different orbit around the nucleus.
• According to Rutherford’s model of an atom, the atomic spectrum should be continuous. But atomic spectrum is found to be discontinuous. Rutherford model fails to explain the discontinuity of the atomic spectrum.
• This model also fails to explain the line spectra of atoms, which show discrete lines, each line corresponds to a fixed frequency.

Science > Chemistry > Atomic Structure > Rutherford’s Atomic Model

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In last article, we have discussed, the discovery of electron and characteristics of electron. In this article, we shall discuss the discovery of proton and neutron and their characteristics.

Discovery of Proton:

Proton was discovered by E. Goldstein in 1886. He performed the same experiment as performed by J.J. Thomson but used perforated cathode. He found that on passing an electric discharge through a gas under reduced pressure, rays containing positive particles move towards the cathode.  As they appear to arise from the anode they are called anode rays or canal rays. They are found to contain positively charged particles called protons.

Origin of Positive Rays:

When an electric discharge is passed through gas at very low pressure in the discharge tube cathode rays are produced. The cathode rays consist of a stream of high-speed electrons. When these fast-moving electrons strike the atoms or molecules of the gas present in the discharge tube, they remove one or more electrons from the neutral atoms or molecules. Thus positive ions of gas are formed. These ions which move towards the perforated cathode kept midway in the tube and constitute the positive rays coming through the perforated cathode.

Characteristics of Canal Rays:

• They travel in a straight line in the opposite direction to that of cathode rays.
• Canal rays produce fluorescence when incident on zinc sulphide screen.
• Unlike cathode rays, the positively charged particles depend upon the nature of gas present in the cathode ray tube.
• These are simply the positively charged gaseous ions.
• The charge to mass ratio of the particles is found to depend on the gas from which these originate.
• Some of the positively charged particles carry a multiple of the fundamental unit of electrical charge.
• The behaviour of these particles in the magnetic or electrical field is opposite to that observed for electron or cathode rays.

Characteristics of Protons:

• Protons are positively charged.
• Protons are located in the nucleus.
• Protons have mass 0f 1.0078 a.m.u. (1.672 ×  10^-27  Kg.). This mass of a proton is considered as unit mass ( 1 a.m.u. ).
• Mass of one proton is almost equal to the mass of one hydrogen atom.
• The proton carries a positive charge of  1.6 ×  10^-19  C. This charge carried by the roton is considered to be the unit positive charge.
• Proton is denoted by   1 H 1  or 1 P 1. I.e. it has unit positive charge and unit mass.
• All atoms contain proton.

Discovery of Neutron:

In 1920 Rutherford proposed existence of the third neutral particle in an atom. Neutron was discovered by Sir James Chadwick in 1932. He observed that when Beryllium (Be) is bombarded by α -particles; particles having no charge and mass equal to that at proton were produced (ejected).  He called them as neutrons.

Characteristics of Neutrons:

• Neutrons have no charge i.e. they are electrically neutral.
• They are located in the nucleus of an atom.
• Mass of neutron is of 1.008665 a.m.u  (1.675 ×  10^-27  Kg ). For practical purpose, this mass is assumed as unit mass.
• Mass of neutron is nearly as that of the proton.
• Neutron is denoted as 0 n 1.

Other Subatomic Particles:

Mesons, positrons, neutrinos, antiprotons are other subatomic particles. Other subatomic particles discovered recently are quarks, antiquarks, pions, gluons, boson and god particle.

Discovery of X-rays:

X-Rays were discovered by Wilhelm Roentgen in 1895. He discovered that when cathode rays strike metal with a high atomic number, radiation of very short wavelength was emitted. He was unable to explain the nature of the emitted rays hence he called these rays as X-rays.

Characteristics of X-Rays:

• These rays are capable of penetrating through wood, paper, and flesh but are stopped by bones and metallic substance.
• X-Rays are electromagnetic waves.
• They are chargeless.
• X-rays produce fluorescence in many substances. The fluorescence in different substances has different characteristics.
• X-ray kill some form of animal tissues.
• X-ray affect photographic plates.
• X-rays travel by velocity of light (3 x 10⁸ m/s) in air or vacuum.
• They are not deflected by electric or magnetic fields.
• They ionize air or gas through which they pass.

Moseley’s Contribution:

Moseley in 1913 found that each element when bombarded by high-velocity electrons, emit X-rays of different frequency.  Thus the atomic number of many elements was determined accurately by the following relationship.

Atomic number α square root of the frequency of X-rays. He gave relation Where a and k are constant. Z is an atomic number and υ is the frequency of X-ray. Thus he stated that frequency of radiation from elements depends upon the number and arrangement of unit particles in their atoms. Hence atomic number (z) is a fundamental property of all elements.

Important Terms and Concepts:

Atomic number (Z) :

The number of protons (positive charge) present in the nucleus of an atom of a particular element is called the atomic number of that element. It is denoted by letter ‘Z’.

Neutron number (N): 

The number of neutrons present in the nucleus of an atom is known as neutron number. It is denoted by ‘N’

Mass number (A): 

The total number of protons and neutrons present in the nucleus of an atom of the element is called mass number. The mass number is denoted as ‘A’.

A  =   Z + N

Representation of Atom in Symbolic Form: 

Generally, every atom X is represented as

Concept of Isotopes, Isobars, and Isotones:

Isotopes: 

Different atoms of the same element having the same atomic number but having different mass numbers are known as isotopes.

Examples :

Characteristics of Isotopes:

• Different Atoms of the same element having the same atomic number but having different mass numbers are known as isotopes.
• Isotopes are the atoms of the same element.
• They have the same atomic number but different mass numbers.
• They have the same number of protons but the different number of neutrons.
• Since they have the same atomic number they show the same chemical properties.
• They occupy the same positions in the periodic table.

Isobars: 

Atoms of the different elements having a different atomic number but having same mass numbers are known as isobars.

Examples :

Characteristics of Isobars:

• Atoms of the different elements having the different atomic numbers but having the same mass numbers are known as isobars.
• Isobars are the atoms of different elements.
• They have the same mass number but different atomic numbers.
• They have a different number of protons and neutrons.
• Since they have different atomic number they show different chemical properties.
• They occupy different positions in the periodic table.

Isotones: 

Atoms of the different elements having the different atomic number, different mass number but having same neutron number are known as isotones.

Examples:

Characteristics of Isotones:

• Atoms of the different elements having the different atomic number, different mass number but having same neutron number are known as isotones.
• Isotones are the atoms of different elements.
• They have the different mass numbers and different atomic numbers.
• They have the same number of neutrons.
• Since they have a different atomic number they show different chemical properties.
• They occupy different positions in the periodic table.

Science > Chemistry > Atomic Structure > Discovery of Proton and Neutron

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In this article, we shall study earlier concepts of an atom, discovery of electron, and its characteristics.

600 B.C. Indian saint and philosopher Maharshi Kanad proposed that matter is made up of the smallest individual particles. He called these particles ‘paramanu’. Around 400 B.C. The Greek Philosopher Democritus suggested that all the matter is composed of tiny, discrete, indivisible particles. He called these particles ‘atoms’. Both the ideas are of conceptual nature and didn’t have any experimental evidence.

Assumptions of Dalton’s atomic theory:

The first concept was given by John Dalton. The postulates of his theory are:

• Every element is made up of extremely small particles called an atom.
• The atoms are indivisible and they can neither be created nor be destroyed. Atoms of the same element resemble each other in all respects but differ from the atoms of other elements.
• When chemical compounds are formed they do so by the combination of atoms of different elements in a simple proportion of whole numbers.
• Atoms of different elements may combine in more than one proportion to form different compounds.

Evidence of Electrical Nature of Matter:

When a glass is rubbed with silk or ebonite is rubbed with fur, electricity is generated. Electricity gets transferred from one point to another point through certain substances. This phenomenon indicated that the matter has electrical nature.

Michael Faraday in 1832 passed electricity through the solution and he called the phenomenon as electrolysis. He observed that charged particles migrate towards oppositely charged electrodes. During this process, they accumulate on the electrode or escape out as a gas at the electrode. On the basis of his experiments, he proposed the laws of electrolysis. These laws of electrolysis given by Michael Faraday provide a relation between matter and electricity. These laws assumed the discrete nature of electricity. These discrete particles are called electrons by Loney.

Fundamental particles of an atom:

Protons, neutrons, and electrons that make up an atom are known as the fundamental subatomic particles. Protons and neutrons are present in the nucleus of an atom, they are called intranuclear particles or nucleons. Electrons revolve around the nucleus in a circular orbit. They are called extra-nuclear particles.

Discovery of Electrons:

Electrons were the first of sub-atomic particles to be discovered, by J.J. Thomson in 1859. J.J. Thomson made a detailed study of the discharge of electricity through gases under very low pressure. This experiment was performed using a cathode ray tube (Crooke’s tube).  It consists of a glass tube connected to two metal electrodes at two ends.  There is a side tube connected to a vacuum pump to reduce pressure.

When a gas is subjected to a high potential (5000 to 10000 V) at low pressure, the glass wall of tube glows with fluorescent light.  This glow is because of the bombardment of glass by rays emitted from the cathode.  As the rays arise from the cathode, they are called cathode rays. These rays were found to consist of negatively charged particles with a negligible but definite mass.

Cathode rays produce heat energy when they collide with the matter. It is due to the kinetic energy possessed by the cathode rays. Cathode rays also rotate the paddle wheel in their path. They move from the negative electrode to the positive electrode. Which clearly indicate that the cathode ray consists of a negatively charged particle. Thomson called these particles as negatrons. Stoney changed this name to electrons.

Thomson also showed that the electrons are present in atoms of all elements. Thus electrons are the fundamental particles of an atom.

Charge to Mass Ratio of Electron:

J. J. Thomson measured the ratio of electrical charge (e) to mass (m) of cathode ray particles using specially designed cathode ray tube.

Construction of Apparatus:

The apparatus consists of a discharge tube containing gas at a very low pressure about 0.01 mm of mercury. The discharge tube has cathode C at one end and fluorescent screen S at the other. Anode A consists of a cylinder with a fine bore.

The electric field can be applied between the plates P1 and P2. Plate P1 is positive and plate P2 is negative

The magnetic field can be applied perpendicular to the electric field and perpendicular to the plane of the diagram and into the plane.

Working:

Cathode emits electrons and they are collimated by cylindrical fine bore anode. The velocity of electrons depends on the potential difference between the cathode and the anode. When no field is applied. The electrons move in a straight line and forms spot at O at the centre of the screen.

When cathode rays are passed through an electric field created by applying a potential across the plates P₁ and P₂ only. It is found that the cathode rays particles get deflected towards the positive plate.

When cathode rays are passed through the magnetic field created by applying a strong magnetic field only, it is found that the cathode rays particles get deflected in a circular path.

Thomson applied both the fields simultaneously and adjusted its value such that the spot remains at the centre of the screen at O. Knowing values of V, B and d and using following formula the e/m ratio can be determined.

Where E = V/x. The value of e/m is 1.758820 x 10¹¹ C kg^-16.

Thomson repeated the experiment for different materials of the cathode and found that the e/m ratio is always the same. From this, he concluded that the particles present in cathode rays (electrons) are fundamental particles of any atom of all matter.

Equation of Path of Parabolic path of Electron:

The path of an electron in the electric field is parabolic whose equation is given by

Where y = deflection of the path of the electron in the y-direction

x = Distance between the parallel plates

e = Charge on electron

E = Intensity of the electric field

m = Mass of electron

v = velocity of electron

Radius of Circular Orbit of Electron:

The radius of the circular orbit of an electron in the magnetic field is given by

Where r = radius of the circular path of an electron in magnetic fields

e = Charge on electron

B = Magnetic induction

m = Mass of electron

v = velocity of electron

Alternate Equation for e/m Ratio:

Millikan’s Oil Drop Experiment:

In the first step, the oil drops are allowed to fall between the plates in the absence of an electric field. Due to gravity, they accelerate first, but gradually velocity decreases up to certain minimum value due to air resistance. Then the drops start moving downward with a constant velocity called terminal velocity (v₁). The terminal velocity of the drop is measured. As the velocity is constant, the air resistance and weight of the drop are equal in magnitude. Thus the net force acting on the drop is zero.

In the second step, an electric field is produced in the chamber. A likely looking drop is selected and kept in the middle of the field of view by adjusting the voltage. So that the drop is moving down with constant velocity called terminal velocity (v₂). Thus net force acting on it is zero. Thus the electric force is balancing the downward forces.

The charge on an electron is given by

Where η = Coefficient of the viscosity of gaseous medium (air)

v₁ = Terminal velocity of the oil drop when an electric field is not applied

v₂ = Terminal velocity of the oil drop when an electric field is applied

E = Intensity of the electric field between plates

f = Density of oil

σ = density of the gas (air)

g = acceleration due to gravity

Millikan repeated the experiment by varying the strength of x-rays used for ionization of air no. of times. Due to which the no. of electrons attaching to the oil drop varied. Then he obtained various values for the charge and is found to be an integral multiple of 1.6 x 10^-19 C.

In 1923, Millikan won the Nobel Prize in Physics in part because of this experiment.

The charge on an Electron:

Scientist R. A. Millikan in his oil-drop experiment determined the charge on the electron and he found that the charge on an electron is 1.6022 x 10^-19 C.

Mass of an Electron:

Using e/m ratio and charge on the electron, the mass of an electron is found to be 9.1094 x 10^-31 kg.

Characteristics of Electrons:

• Electrons are negatively charged.
• They are revolving in circular orbits around the nucleus.
• They have mass 0f 0.00055 a.m.u.  (9.11 × 10 ^-31 Kg.). This mass of an electron is negligible compared to other particles. Hence it is taken as zero.
• Its mass is 1/1837 times that of the proton.
• Electrons carry a negative charge of 1.6 × 10 ^-19 C. This charge carried by the electron is considered to be a unit negative charge.

Science > Chemistry > Atomic Structure > Discovery of Electron

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Radioactivity was discovered by French physicist Antoine Becquerel in 1896. The phenomenon of spontaneous and continuous and uncontrollable disintegration of an unstable nucleus accompanied by the emission of active radiations is called natural radioactivity. The substance which exhibits radioactivity is called a radioactive substance. e.g. Uranium, thorium, radium, etc. The radiations emitted by the radioactive substance are alpha particles, beta particles, and gamma radiations.

Characteristics of Natural Radioactivity:

• These characteristics are also called as Rutherford-Soddy’s radioactive disintegration theory
• Radioactivity is a purely nuclear phenomenon.  The nucleus of a radioactive substance is unstable and such unstable nucleus undergoes spontaneous breakdown (disintegration). The process continues till a stable nucleus is obtained.
• As radioactivity is the nuclear phenomenon it is unaffected by chemical combination. i.e. the element will exhibit radioactivity in free as well as a combined state.
• Radioactivity is a spontaneous process. It is independent of external factors like temperature, pressure, and the state of existence of substance or catalytic action. Hence the process of radioactive disintegration is uncontrollable using these factors.
• The nucleus of radioactive element emit alpha  (α), beta(β) and gamma (γ)  radiations and gets converted into the nucleus of another element.
• The elements undergoing disintegration is called parent element and a new element formed is called a daughter element. Daughter element has different chemical and physical properties as compared with that of its parent element.
• During disintegration, besides emission of alpha  (α), beta(β) and gamma (γ) radiation, a large amount of energy is liberated in form of γ rays. When γ rays are given out no new element is formed.
• The time taken, by a radioactive substance to disintegrate half of its initial quantity is called as half-life period.  Half-life period is a characteristic property of every radio element.
• When radioactive substance emits one α -particle mass number of daughter element reduces by 4 units and atomic number reduces by 2 units. When a radioactive substance emits one β – particle, the atomic number of daughter element increases by one unit but the mass number remain unchanged.
• The rate of disintegration at any instant is directly proportional to the radioactive nuclei present at that instant.
• Thus the rate of disintegration depends on nature and the original amount of the radioactive substance. It is an exponential process and practically never gets completed.

Characteristics of α – particles:

• These are positively charged particles.  So α-rays are called α – particles rather than α -rays.
• Actually, α particles are helium nuclei having 4 unit mass and 2 unit of positive charge.
• They are deflected towards a negative plate of the electric field.
• They have greater ionizing power.
• They have least penetration power.
• They can affect a photographic plate.
• They travel in straight line.
• They have a velocity which is about    1/10 th that of light.
• When radioactive substance emits one α -particle, the mass number of daughter element formed is 4 units less and the atomic number is 2 units less.

Characteristics of β-particles:

• β- Rays are negatively charged particles. So they are called β – particles rather than β – rays.
• β- particles are nothing but high-velocity electrons having unit negative charge and negligible mass.
• These rays are deflected towards + ve plate of the electric field.
• They have less ionizing power as compared with that of α- rays5.   They have greater penetration power than that of α – rays
• They affect a photographic plate to the much higher extent than the α- particles.
• They do not travel in straight line.
• They have a greater velocity than that of the α- rays very close to that of light.
• When a radioactive substance emits one β -particle, the atomic number of daughter element formed is one unit higher but the mass number remains unchanged.

Characteristics of γ – rays:

• γ-rays are nonmaterial
• They are electromagnetic radiations.
• They are chargeless, hence remain undeflected due to the electric or magnetic field.
• They have very low ionizing power.
• They have high penetration power.
• They have a very little effect on a photographic plate.
• They travel in straight line.
• They have a velocity equal to that of the light.
• When radioactive substance emits γ rays there is no change in atomic number and mass number.

Distinguishing between α particles and β particles:

Distinguishing between α particles and γ rays:

Distinguishing between β particles and γ rays :

Science > Chemistry > Atomic Structure > Radioactivity

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Before the discovery of the atomic nucleus, there were ideas that all atoms are composed of hydrogen atoms (called by William Prout “protyles”). This hypothesis is known as Prout’s hypothesis. According to this hypothesis, the hydrogen atom was the only truly fundamental particles, and that the other atoms were actually groupings of various numbers of hydrogen atoms (protyles). In this article, we shall study the discovery of proton and neutron, their characteristics and importance.

Discovery of Proton:

Proton was discovered by E. Goldstein in 1886. He performed the same experiment as performed by J.J. Thomson but used perforated cathode. He found that on passing an electric discharge through a gas under reduced pressure, rays containing positive particles move towards the cathode.  As they appear to arise from the anode they are called anode rays or canal rays. They are found to contain positively charged particles called protons.

Origin of Positive Rays:

When an electric discharge is passed through gas at very low pressure in the discharge tube cathode rays are produced. The cathode rays consist of a stream of high-speed electrons. When these fast-moving electrons strike the atoms or molecules of the gas present in the discharge tube, they remove one or more electrons from the neutral atoms or molecules. Thus positive ions of gas are formed. These ions which move towards the perforated cathode kept midway in the tube and constitute the positive rays coming through the perforated cathode.  In 1907 a study of deflection of these rays in a magnetic field revealed that the particles making up the ray were not all the same mass. The lightest ones, formed when there was some hydrogen gas in the tube, were calculated to be about 1840 times as massive as an electron.

Characteristics of Canal Rays:

• They travel in a straight line in the opposite direction to that of cathode rays.
• Canal rays produce fluorescence when incident on zinc sulphide screen.
• Unlike cathode rays, the positively charged particles depend upon the nature of gas present in the cathode ray tube.
• These are simply the positively charged gaseous ions.
• The charge to mass ratio of the particles is found to depend on the gas from which these originate.
• Some of the positively charged particles carry a multiple of the fundamental unit of electrical charge.
• The behaviour of these particles in the magnetic or electrical field is opposite to that observed for electron or cathode rays.

Characteristics of Protons:

• Protons are positively charged.
• Protons are located in the nucleus.
• Protons have mass 0f 1.0078 a.m.u. (1.672 × 10^-27 Kg.). This mass of a proton is considered as unit mass (1 a.m.u.).
• Mass of one proton is almost equal to the mass of one hydrogen atom.
• The proton carries a positive charge of 1.6 × 10^-19 C. This charge carried by the proton is considered to be the unit positive charge.
• Proton is denoted by   1H1 or 1P1. I.e. it has a unit positive charge and unit mass.
• All atoms contain protons.
• Baryons are massive particles that are made up of three quarks in the standard model. The proton is a baryon and is considered to be composed of two up quarks and one down quark.

Discovery of Neutron:

Since the time of Rutherford, it had been known that the atomic mass number A of nuclei is a bit more than twice the atomic number Z for most atoms and that essentially all the mass of the atom is concentrated in the relatively tiny nucleus.  In 1920 Rutherford proposed the existence of the third neutral particle in an atom. But up to 1930 proton-electron hypothesis was accepted.

An experimental breakthrough came in 1930 with the observation by the German nuclear physicist Herbert Becker and Walther Bothe that bombardment of beryllium with alpha particles from a radioactive source produced neutral radiation which was penetrating but non-ionizing. They observed that the penetrating radiation was unaffected by electric fields and hence, they assumed it to be gamma radiation. In the year 1932, Frederic Joliot-Curie and Irene Joliot-Curie demonstrated that these rays have the potential to eject protons when it strikes paraffin or any H-containing compounds. The experiment proved that the assumption that the rays to be gamma rays was wrong. Because a photon that does not have mass cannot be capable to release a particle 1836 times heavier than an electron (protons). Therefore, it was concluded that the ejected rays cannot be photons.

Chadwick’s Experiment:

Neutron was discovered by Sir James Chadwick in 1932. He performed the same experiment performed by Frederic Joliot-Curie and Irene Joliot-Curie and used different bombardment targets other than paraffin. 

He fired alpha radiation at the beryllium sheet from a polonium source. This led to the production of an uncharged, penetrating radiation. These radiations were made incident on paraffin wax, having relatively high hydrogen content. The range of the liberated protons was measured and the interaction between the uncharged radiation and the atoms of several gases was studied by Chadwick. The particle ejected was found to have a mass equal to that at proton and no charge.   He called these particles as neutrons.

₄Be⁹ +   ₂α⁴   ⟶  [₆C¹³]  ⟶  ₆C¹² + ₀n¹

Characteristics of Neutrons:

• Neutrons have no charge i.e. they are electrically neutral.
• They are located in the nucleus of an atom.
• Mass of neutron is of 1.008665 a.m.u  (1.675 ×  10^-27  Kg ). For practical purposes, this mass is assumed as unit mass.
• Mass of neutron is nearly as that of the proton.
• Neutron is denoted as 0 n 1.
• Baryons are massive particles which are made up of three quarks in the standard model. The neutron is a baryon and is considered to be composed of two down quarks and one up quark.

Importance of Discovery of Neutron:

• After the discovery of the neutron, every chemical element present in the periodic table was modified and written accordingly.
• With Chadwick’s announcement, Heisenberg then proposed the proton-neutron model for the nucleus.
• It carries no electric charge, it is used as a projectile in nuclear reaction allowing it to split the nuclei of even the heaviest elements.
• Neutrons found a wide variety of uses, from examining the structures of different materials to determining water content in soil and treating tumours.
• It is used in fission nuclear reaction in a nuclear reactor for the production of nuclear energy
• It is extensively used in nuclear engineering and research.

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In this article, we shall discuss the composition of nucleus and concept of nuclear radius, nuclear vlume, and nuclear density.

Geiger Marsden Experiment:

A narrow beam of alpha particles from the radioactive source was incident on a thin gold foil. The scattering of alpha particles takes place. The scattered alpha particles were detected by a detector fixed on a stand. The deviation of alpha particles from their original path is called the scattering angle. They observed that most of the alpha particles just passed through without any deviation as if there is empty space. A few alpha particles were deflected through smaller angles. A few alpha particles deviated through larger angles. This larger deflection is possible only if alpha-particles collide with heavy and positively charged particles inside the atom because like charges only repel each other.  This massive +ve charge is at the centre of the atom and called the nucleus. Very few alpha particles were rebounded i.e. they deviated through 180°. This concludes that the nucleus is very small as compared to the volume of the atom.

Rutherford’s Model of an Aom:

From the observations of the above experiment, Rutherford put forward the concept of his atomic model. The atom consists of a centrally located positively charged nucleus. The whole mass of an atom is concentrated in the nucleus. Around the nucleus, there is empty space in which the negatively charged electrons revolve in different orbits. The total positive charge of the nucleus is equal to the total negative charge on orbiting electrons. Hence atom is electrically neutral. Rutherford’s model of an atom is also called as a planetary model of an atom.

From the x-ray technique, it is observed that the charge on the nucleus is Ze and it is positive. Where e is the magnitude of the charge on one electron. Thus it was considered that the nucleus consists of z number of positively charged particles. Each particle carries a charge of + e. These particles are termed as protons. In 1932 Chadwick proved the existence of neutral particles in the nucleus. These neutral particles are called neutrons. The mass of neutron and proton is almost the same.

Nucleus of Hydrogen is an exception because it contains only one proton and no neutron. The lightest positive charge found had the mass of the hydrogen atom and carried a positive charge equal to the electronic charge. This led to the idea that all atoms were made up from hydrogen hence the name proton (which comes from the Greek word protos which means first) was given to indicate its importance.

In an atom total number of protons in neutron is equal to the number of electrons in the atom. Hence total amount of positive charge on nucleus is balanced by total charge on electrons. Hence as a whole atom is electrically neutral.

Nucleus of different elements are different and has unique composition. Size of nucleus is of an order of 10^-14 m.

Constituents of Nucleus:

The particles protons and neutrons present in the nucleus are collectively called as the nucleons. The number of protons present in the nucleus of an atom is called the atomic number. It is denoted by the letter ‘Z’. The number of neutrons present in the nucleus of an atom is known as the neutron number. It is denoted by ‘N’. The total number of protons and neutrons present in the nucleus of an atom of the element is called the mass number.  The mass number is denoted as ‘A’.

A  =   Z + N or   N  =   A –  Z

The mass of an atom is measured in a unit called atomic mass unit (a.m.u.)

Proton-Electron Hypothesis:

Before the discovery of Neutron by Chadwick in 1932, it was considered that proton and electron were constituent particles of nucleus. This consideration was based on following hypothesis:

• During radioactive decay, beta particles are emitted and they were proved to be fast moving electrons.
• As atom is electrically neutral If A is atomic mass of an element, then it should contain ‘A’ number of protons and (A – Z) electrons in the nucleus. Where Z is number of positive charge in nucleus (number of protons).
• Also Z no. Electron outside the nucleus i.e equal no. of protons and electrons in an atom, some electrons inside and some electrons outside the nucleus. Thus in helium nucleus 2He4, there are 4 protons and 2 electrons in the nucleus. Two compensate the extra positive charge this hypothesis assume there are 2 more electrons which revolve around the nucleus.
• This theory was suitable to explain emission of alpha and beta particles by radioactive element.

Shortcoming of Proton-Electron hypothesis:

According to Heisenberg’s uncertainty principle, if an electron is to exist inside the nucleus, it should have energy of at least 200 MeV. But beta particles (fast moving electrons emitted by radioactive element) do not have kinetic energy more than 4 MeV, Thus on the basis of wave mechanics, the existence of electron in the nucleus cannot be justified.

The experimental values of angular momentum of nuclei differ widely from the calculated values on the basis of proton-electron hypothesis.

If electron exist in nucleus the nuclear magnetic moment cannot be less than the magnetic moment of electron. But the magnetic moment of nucleus is only about one thousandth times that of electron. Hence electron cannot be a constituent of nucleus.

Proton-Neutron Hypothesis:

This hypothesis was proposed by Chadwick after the discovery of neutron in 1932.

A nucleus of mass number ‘A’ and atomic number ‘Z’, contains Z protons and ‘A – Z’ no. of neutrons inside the nucleus.  ‘Z’ no. of electrons are revolving around the nucleus. Thus the atom is electrically neutral.

Merits of Proton-Neutron Hypothesis:

• The existence of neutron in nucleus satisfies the Heisenberg’s uncertainty principle.
• It explains the emission of beta particles from radioactive nuclei, as a result of inter conversion of proton and neutron
• It solves the problem of nuclear spin. Nuclei having odd mass no. will have half integral spin and those with even mass no. will have integral spin.

Size of Nucleus:

The nuclear radius is the distance from the centre of the nucleus at which the density of nuclear material decreases to one-half of its maximum value at the centre. Nucleus has no definite boundary, but its radius is given by following relation:

R = R_o A^1/3

Where R_o is constant for all nuclei and its value is 1.2 x 10^-15 m

A = Mass number of nucleus

Thus radius of nucleus depends on the mass number of nucleus.

Example 01:

Find size of nucleus of oxygen ₈O¹⁶.

For oxygen nucleus A = 16

R = R_o A^1/3

R = 1.2 x 10^-15 x (16) ^1/3

R = 3.02 x 10^-15 m

Ans: The radius of nucleus of oxygen ₈O¹⁶is 3.02 x 10^-15 m.

Example 02:

The nuclear radius of ₈O¹⁶ is 3x 10^-15 m. What is nuclear radius of ₈₂Pb²⁰⁵?

Given: R_O = 3x 10^-15 m

To Find: R_Pb =?

Solution:

We have R = R_o A^1/3

Thus R α R_o A^1/3

Ans: The radius of lead nucleus is 7.02 x 10^-15 m or 7.02 fermi

Example 03:

The nuclear radius of ₁₃Al²⁷ is 3.9 x 10^-15 m. What is nuclear radius of ₈₄Po²¹⁶?

Given: R_Al = 3.9 x 10^-15 m

To Find: R_Po =?

Solution:

We have R = R_o A^1/3

Thus R α R_o A^1/3

Ans: The radius of polonium nucleus is 7.8 x 10^-15 m or 7.8 fermi

Volume of Nucleus:

Volume of nucleus is given by

Thus the volume of the nucleus is directly proportional to the mass number of the nucleus.

Example 04:

Find Volume of nucleus of chlorine ₁₇Cl³⁵.

For chlorine nucleus A = 35

Volume of Nucleus = 7.24 x 10^-45 A    m³

Volume of Nucleus = 7.24 x 10^-45 x 35   m³

Volume of Nucleus = 2.53 x 10^-43 m³

Density of Nucleus:

Where m = mass of each nucleon

Notes:

• Nuclear density is independent of mass number of nucleus.
• It is nearly same for all nuclei
• It has very high value. Such high densities can be found in white dwarf stars.
• Its value is not same inside the nucleus. It is maximum at the centre and gradually decreases as we move away from the centre of the nucleus.
• The nuclear radius is the distance from the centre of the nucleus at which the density of nuclear material decreases to one-half of its maximum value at the centre.

Example 05:

Find the density of nuclear mass in ₉₂U²³⁸. If R_o = 1.5 fermi and mass of each nucleon = 1.67 x 10^-27 kg.

Example 06:

Find the density of nuclear mass hydrogen nucleus. If R_o = 1.2 fermi and mass of each nucleon = 1.67 x 10^-27 kg.

Example 07:

Obtain an approximate ratio of radii of the gold isotope ₇₉Au¹⁹⁷ and the silver isotope ₄₇Ag¹⁰⁷. What is the density of their nuclear densities?

We have R = R_o A^1/3

Thus R α R_o A^1/3

The nuclear density is independent of mass number of nucleus. Hence ratio of their nuclear densities is 1.

Ans: The approximate ratio of radii of the gold isotope ₇₉Au¹⁹⁷ and the silver isotope ₄₇Ag¹⁰⁷ is 1.225 and the ratio of their nuclear densities is 1.

Science > Physics > Nuclear Physics > Nuclear Structure

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Transformation of radioactive element into another element (radioactive or non-radioactive) is known as radioactive decay or disintegration. In radioactive decay, the nucleus of a radioactive element called parent undergoes a spontaneous change accompanied by the emission of radiation and the formation of the nucleus of a new element called the daughter. The physical and chemical properties of the daughter may be different from its parent.

There are three types of Radioactive Decay

Alpha Decay:

Alpha decay is a type of radioactive decay in which a particle with two neutrons and two protons (Helium nuclei) is ejected spontaneously from the nucleus of a radioactive atom. During alpha decay, an atom’s nucleus sheds two protons and two neutrons. Alpha particles are Helium nuclei.

Alpha decay occurs in very heavy elements (having nucleons 210 or more) like uranium, thorium, and radium. Nuclei of these elements have a large proton to neutron ratio, which makes these elements neutron-rich. This richness makes alpha decay possible. These nuclei are so large that the short-range nuclear forces holding the nucleons together are unable to counterbalance the electrostatic repulsion among the large number of protons in them. Therefore, in an attempt to achieve greater stability by reducing their size, they emit an alpha particle.

When radioactive substance emits one α -particle, the mass number of daughter element formed is 4 units less and the atomic number is 2 units less.

The daughter element (Th) occupies two positions to the left of the parent element (U).

Energy Change and Distribution:

The α-decay process is “fueled” by the rest mass energy difference of the initial state and final state.  i.e. the alpha-decay of Uranium can occur spontaneously (without an external source of energy) because the total mass of the decay products and an alpha particle is less than the mass of the original substance uranium. Thus, the total mass-energy of the decay products is less than the mass-energy of the original nuclide. The difference between the initial mass-energy and the final mass-energy of the decay products is called the Q value of the process, or the disintegration energy. Thus, the Q value of an alpha decay can be expressed as

Q = (m_X – m_Y – m_He) c²

Since alpha particles has a high binding energy, its formation within the nucleus causes release of sufficient energy which becomes available for escape. The energy Q is shared by the daughter nucleus Y and the alpha particle. Mostly the energy is taken by the alpha particle.

As per Hans Geiger and John Mitchell Nuttall (Gamow Theory), short-lived isotopes emit more energetic alpha particles than long-lived ones. Or the value of Q increases the half-life period decreases.

Beta decay:

A nucleus that decays spontaneously by emitting an electron or a positron is said to undergo beta decay. When a radioactive substance emits one β -particle, the atomic number of daughter element formed is one unit higher but the mass number remains unchanged.

In 1930 Pauli proposed that during beta decay, the proton in the nucleus is transformed into a neutron and vice versa. Thus there are three types of beta decay

Positron Emission:

If a proton is converted to neutron, by β+ decay. In this conversion neutron to proton ratio increases.

The change is accompanied by emission of antineutrino

Electron Capture

If a proton is converted to neutron, by electron capture. In this conversion neutron to proton ratio increases.

The change is accompanied by emission of neutrino

Positron Emission:

if a neutron is converted to a proton, it is known as β- decay. In this conversion neutron to proton ratio decreases.

The change is accompanied by emission of neutrino

Due to the change in the nucleus, a beta particle is emitted. The beta particle is a high-speed electron when it is a β- decay and a positron when it is a β+ decay.

The symbols nu bar and nu represent antineutrino and neutrino, respectively; both are neutral particles, with very little or no mass. These particles are emitted from the nucleus along with the electron or positron during the decay process. Neutrinos interact only very weakly with matter; they can even penetrate the earth without being absorbed. It is for this reason that their detection is extremely difficult and their presence went unnoticed for long.

The above explanation shows why the mass number A of a nuclide undergoing beta decay does not change; one of its constituent nucleons simply changes its character (proton into neutron or neutron into a proton).

Types of Beta Decay

There are three types of Beta Decay:

Electron Emission

The process of ejection or emission of electron from the nucleus is known as electron emission. After the emission, the charge of the nucleus increases by one.

Electron Capture

Electron capture is the phenomena where the nuclei decay by capturing one of the electrons that surround the nucleus. This leads to a decrease of one in charge of the nucleus.

Positron Emission

It is the third form of beta decay. A positron is an antimatter equivalent of an electron & has the same mass as of an electron, but bares the opposite charge of an electron. Positron decay produces a daughter nuclide with one less positive charge on the nucleus than the parent.

Energy Change and Distribution:

The difference between the initial mass-energy and the final mass-energy of the decay products is called the Q value of the process, or the disintegration energy. Thus, the Q value of an alpha decay can be expressed as

Q = (m_X – m_Y – m_e) c²

The energy Q is shared by the beta particles, and the antineutrino /neutrino in all proportions with each other. Daughter element being heavy carries negligible energy. When the antineutrino grabs whole of the energy, the beta particle is emitted with zero energy and vice-versa. Thus beta particles come out with a continuous range of energy which remains conserved.

Change in Momentum:

The antineutrino conserves the momentum also. Before the emission of the beta particle, the momentum of the parent nucleus is zero. The antineutrino is emitted along with beta particle with a momentum which is exactly equal to the sum of the momenta of the beta particle and daughter nucleus.

Gamma Decay:

There are energy levels in a nucleus, just like there are energy levels in atoms. Gamma decay is the nucleus’s way of dropping from a higher energy level to a lower energy level through the emission of high energy photons. Most of the time, gamma decay occurs after the radioactive nuclei have undergone an alpha or a beta decay. When a nucleus is in an excited state, it can make a transition to a lower energy state by the emission of electromagnetic radiation. As the energy differences between levels in a nucleus are of the order of MeV, the photons emitted by the nuclei have MeV energies and are called gamma rays. Unlike, alpha decay and beta decay, the parent nucleus does not undergo any physical change in the process, daughter and parent nuclei are the same.

Gamma rays are emitted by the nucleus, particle decay or annihilation reactions. It is to be noted that X-rays are emitted by electrons (either in the orbits or in outside applications like particle accelerators, synchrotrons radiation etc)

Most radionuclides after an alpha decay or a beta decay leave the daughter nucleus in an excited state. The daughter nucleus reaches the ground state by a single transition or sometimes by successive transitions by emitting one or more gamma rays. A well-known example of such a process is that of ₂₇Co⁶⁰. By beta emission, the ₂₇Co⁶⁰ nucleus transforms into ₂₈Ni⁶⁰ nucleus in its excited state. The excited ₂₈Ni⁶⁰ nucleus so formed then de-excites to its ground state by successive emission of 1.17 MeV and 1.33 MeV gamma rays. This process is depicted in the following energy level diagram.

Science > Physics > Nuclear Physics > Concept of Radioactive Decay

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Radioactivity was discovered by French physicist Antoine Becquerel in 1896. He found that certain compounds of uranium emitted invisible radiations which affected photographic plates. It is also found that Thorium and its compounds also show these properties. Madame Curie and Piere Curie discovered two elements namely ‘Radium’ and ‘Polonium’ and found that they also exhibit these properties. Radioactivity can be studied under two headings natural radioactivity and artificial radioactivity. In this article, we shall study the basics of natural radioactivity.

The phenomenon of spontaneous and continuous and uncontrollable disintegration of an unstable nucleus accompanied by the emission of active radiations is called natural radioactivity. The substance which exhibits radioactivity is called a radioactive substance. e.g. Uranium, thorium, radium, etc.

Characteristics of Natural Radioactivity:

• These characteristics are also called as Rutherford-Soddy’s radioactive disintegration theory.
• Natural radioactivity is a purely nuclear phenomenon.  The nucleus of a radioactive substance is unstable and such an unstable nucleus undergoes spontaneous breakdown (disintegration). The process continues until a stable nucleus is obtained.
• As Natural radioactivity is the nuclear phenomenon it is unaffected by chemical combination. I.e. the element will exhibit radioactivity in free as well as the combined state.
• Natural radioactivity is a spontaneous process. It is independent of external factors like temperature, pressure, and state of the existence of substance or catalytic action. Hence the process of radioactive disintegration is uncontrollable using these factors.
• The nucleus of the radioactive element emits alpha, beta particles, and gamma radiations and gets converted into the nucleus of another element.
• The element undergoing disintegration is called a parent element and a new element formed is called a daughter element. The daughter element has different chemical and physical properties as compared with that of its parent element.
• During disintegration, besides emission of alpha and beta particles and gamma radiation, a large amount of energy is liberated in the form of gamma rays. When gamma rays are given out no new element is formed.
• The time taken by a radioactive substance to disintegrate half of its initial quantity is called a half-life period.  A half-life period is a characteristic property of every radio element.
• When radioactive substance emits one alpha particle mass number of daughter element reduces by 4 units and the atomic number reduces by 2 units. When a radioactive substance emits one beta particle, the atomic number of daughter element increases by one unit but the mass number remains unchanged.
• The rate of disintegration at any instant is directly proportional to the radioactive nuclei present at that instant.
• Thus the rate of disintegration depends on nature and the original amount of the radioactive substance.
• No radioactive substance emits both alpha and beta particles simultaneously. Gamma rays are emitted along with both alpha and beta particles.

Rutherford’s Experiment to Study Natural Radioactivity:

Rutherford analyzed the radiations of radioactive substances. He reported three types of radiations depending upon the effect of the magnetic or electric field upon them.  These are alpha and beta particles and gamma rays.

The apparatus consists of an evacuated metal chamber with a photographic plate at the top. A small quantity of the radioactive substance is placed in a hole of the lead block. A strong electric field is applied between the plates.

When there is no electrical field the radioactive emissions move in a straight line but when the electrical field is applied the emission gets split into three distinct points on the photographic plate.

Observations and Conclusion:

• The rays which get deviated towards the negative plate are positively charged and are called as alpha rays. The deflection of alpha particles is slightly less.
• The rays which get deviated towards the positive plate are negatively charged and are called as beta rays. The deflection of beta particles is more.
• The rays which do not get deviated and move straight are not charged and are called gamma rays.
• By applying a strong uniform magnetic field at right angles to the diagram the same effect can be observed.

Effect of Magnetic Field:

Characteristics of α – rays:

• These are positively charged particles.  So α-rays are called α – particles rather than α -rays.
• Actually, these particles are helium nuclei (₂He⁴) having 4 unit mass and 2 units of a positive charge.
• They are deflected towards the negative plate of the electric field and the magnetic field.
• The small deflection in the electric or the magnetic field indicates that they are comparatively heavier particles.
• They have greater ionizing power (100 times that of beta particles and 10000 times that of gamma rays). Their tracks in cloud chamber are continuous.
• They have the least penetration power ( 1/100 that of beta particles and 1/10000 that of gamma rays). They can be stopped by 0.1 mm thick aluminium.
• They are scattered when passing through the foils of gold and mica. This property was used by Rutherford to propose the planetary model of an atom.
• They produce fluorescence in substances like zinc sulphide and barium platinocyanide. Using scintillations on the fluorescent screen the number of alpha particles can be counted.
• They can affect a photographic plate.
• They travel in a straight line.
• They produce heating when stopped.
• They have a velocity which is about 1/10th that of light. The velocities of alpha particles emitted by different radioactive materials are different but for the same element, the velocity is the same.
• Their range varies from substance to substance. In the air, it is 2.7 cm for uranium and 8.7 cm from thorium. It also depends on the pressure of the medium.
• When radioactive substance emits one µ -particle, the mass number of the daughter element reduces by 4 units and atomic number by 2 units.
• They produce incurable burns on the human body.

Uses of Alpha Particles:

• Alpha particles are most commonly used in smoke alarms. These alarms contain a tiny amount of decaying Americium between two sheets of metal. The decaying Americium emits alpha radiation. A small electric current is then passed through one of the sheets and into the second one. When the field of alpha radiation is blocked by smoke, the alarm goes off. This alpha radiation is not harmful because it is much localized and any radiation that might escape would be stopped quickly in the air and would be extremely difficult to get into the human body.
• Due to the high velocity of emission alpha particles are used for bombarding the nuclei in the transmutation of one element into other.

Characteristics of  β-rays :

• β – Rays are negatively charged particles. So they are called β – particles rather than β – rays.
• β – particles are nothing but high-velocity electrons (_-1e⁰) having a unit negative charge (1.6 x 10^-19 C) and negligible mass. These are not orbital electrons. They are electrons emitted by an atom.
• These rays are deflected towards the +ve plate of the electric field or the magnetic field. Their deflection is larger than that of alpha particles.
• The range of beta particles for a particular radioactive substance is not definite.
• They have less ionizing power as compared with that of α- rays. It is 1/100 that of alpha particles and 100 times as that of gamma rays. As beta particles cannot produce ionization continuously, their tracks in cloud not appear to be continuous.
• They have greater penetration power than that of α – rays. It is 100 times that of alpha particles and 1/100 times that of gamma rays. It can pass through a 1 mm thick sheet of aluminium.
• They produce fluorescence in substances like calcium tungstate, zinc sulphide and barium platinocyanide.
• They affect a photographic plate to a much higher extent than the α – particles.
• They do not travel in a straight line.
• They have a greater velocity than that of the α- rays very close to that of light. There is enough variation in the velocities of beta particles emitted by the same radioactive material. Hence variation in the extent of deflection in the electric field and the magnetic field is observed.
• Since the velocity of beta particles is comparable with that of light, their mass increases with the increase in their velocity. The new mass is given by the expression.

Where m_O = rest mass of electron

v = velocity of beta particle

c = velocity of light

• When a radioactive substance emits one β -particle, the atomic number of daughter element increases by one unit but the mass number remains unchanged.

Characteristics of γ – rays.

• gamma rays are non-material waves.
• They are electromagnetic radiations.
• They are chargeless, hence remain undeflected due to the electric or the magnetic field.
• They have very low ionizing power.
• They have high penetration power. They can pass through a 30 cm thick iron block.
• They have more effect on a photographic plate than beta particles..
• They travel in a straight line.
• They are diffracted by crystals in the same way as X-rays.
• They have a velocity equal to that of the light.
• When radioactive substance emits gamma rays there is no change in the atomic number and the mass number.
• They are absorbed by substances and give rise to the phenomenon of pair production. They strike the nucleus of some atom, where they lose their existence and an electron and a positron are formed.

It is to be noted that gamma rays are similar to X-rays but their sources of origin are different. X-rays are produced by the transition of electrons in one energy level to another energy level. Thus it is atomic property. While gamma rays are produced due to nuclear activity. Thus gamma rays are nuclear property.

Soddy’s Group Displacement Laws:

• Whenever the parent element emits an α- particle, the daughter element produced has the atomic number less by 2 units and the mass number less by 4 units, so the daughter element occupies 2 positions to the left with respect to its parent element in the periodic table.
• Whenever the parent element emits one β – particle, the daughter element produced has the atomic number greater by 1 unit but the mass number remains the same. So the daughter element occupies one position to the right with respect to its parent element in the periodic table.
• Whenever the parent element emits one γ – ray, the daughter element produced has the same atomic number and the same atomic mass. So the daughter element occupies the same as its parent element in the periodic table.

Example:

Next Topic: The Concept of radioactive Decay

Science > Physics > Nuclear Physics > Natural Radioactivity

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