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.

Science > Physics > Nuclear Physics > Discovery of Proton and Neutron

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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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