Nuclear Reactions:

A reaction in which there is a change in the composition of the nucleus is called a nuclear reaction.

Characteristics of Nuclear Reactions:

• There is a change in the composition of the nucleus.
• In nuclear reaction rearrangement of nucleons ( constituents of the nucleus) takes place and thus the composition of nucleus changes
• In nuclear reaction sum of atomic mass numbers and the sum of atomic numbers of reactant nuclei, should be equal to that of product nuclei.
• The rate of nuclear reaction is independent of temperature.
• In a nuclear reaction, the energy values are given in MeV per nucleus transformed.

Chemical Reactions:

Reaction in which there is change in chemical composition of reacting substances is called chemical reaction.

Characteristics of Chemical Reactions:

• There is a chemical change in reacting substances.
• In chemical reaction redistribution of electrons and rearrangement of atoms takes place
• The total number of atoms of each element must be the same on both sides of the chemical equation.
• The rate of a chemical reaction depends on temperature.
• In chemical reaction heat of reaction is given in kJ per mole.

Distinguishing Between Nuclear Reaction and Chemical Reaction:

Types of Nuclear Reactions:

Artificial Transmutation:

The process of conversion of a nucleus of atom one element into that of another element by using high-velocity projectiles is called as artificial transmutation.The new element form may or may not be radioactive.

• Target: The nucleus of an atom which is bombarded by high-speed particles is known as the target.
• Projectile: A high-speed (accelerated) particle that strikes the nucleus of an atom (target) to carry out artificial transmutation is called a projectile. Projectiles used are  ₁H¹ (proton), ₂He⁴ (α -particles), _-1 e⁰( β – particles),  _on¹(neutron), ₁D² (deuterium) etc.
• Emissions: Particles ejected in artificial transmutation along with recoil nucleus is called emissions.
• Recoil Atom or Recoil Nucleus: Atom produced in an artificial transmutation having nearly the same atomic number and mass number as that of the target atom is called a recoil nucleus.

More examples with different projectiles are given below.

Using  α – particles as projectile:

Using Protons as Projectile:

Using Deuterons as Projectile:

Using Neutrons as Projectile:

Neutrons are the Best Projectiles:

• The process of conversion of the nucleus of an atom one element into that of another element by using high-velocity projec­tiles is called as artificial transmutation. There is a positive charge on the nucleus while the negative charge is carried by extranuclear electrons.
• There is a positive charge on the particles like protons and deuterons. Due to the positive charge on them, they experience a greater force of repulsion while penetrating the positively charged nucleus of the target atom.
• Neutrons do not carry any charge. Due to chargeless nature, they are not deflected by extranuclear electrons similarly they are not repelled by the positive charge of the target nucleus. Hence neutrons are easily absorbed by the nucleus and the transmutation is carried out effectively. Hence neutrons are the best projectile in the transmutation of elements.

Artificial or Induced Radioactivity:

The process of conversion of a stable non-radioactive atom of one element into an unstable radioactive atom of another element by bombardment with high-speed projec­tile is called induced or artificial radioactivity.

The recoil atom decays at its own rate. The radioactivity exhibited by recoil nucleus itself is called artificial radioactivity.

In certain artificial transmutation reactions, product undergoes spontaneous disintegration even on stoppage of bombardment of projectile.  During such nuclear reactions positrons ( ₊₁ e⁰),  neutrons  (₀n¹), electrons (_-1 e⁰) or  γ rays are emitted.

Some of the induced radioactivity reactions using different projectiles are,

Using  α – particle (₂He⁴):

Using protons (₁H¹):

Using Deuterons (₁D²):

Using Neutrons:

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. It is a spontaneous process shown by nuclei of heavy elements with an atomic number greater than 83.

• α, β, γ rays are emitted.
• No naturally occurring radioisotope emits positrons.
• I daughter element is radioactive hence it further undergoes disintegration and a series of radioelements is obtained.
• This reaction cannot be controlled as it is independent of external factors.
• A very large amount of energy is released.

Artificial Radioactivity:

The process of conversion of a stable non-radioactive atom of one element into an unstable radioactive atom of another element by bombardment with high-speed projec­tile is called induced or artificial radioactivity. It is a non-spontaneous process.

• Positrons ( ₊₁ e⁰),  neutrons  (₀n¹), electrons (_-1 e⁰) or  γ rays are emitted.
• No artificially synthesized radioisotope emits α particles.
• The daughter element is non-radioactive hence a series of radioelements is not obtained.
• By controlling the flow of projectiles the number of nuclei undergoing artificial transmutation can be controlled.
• A small amount of energy is released.
• Artificial radioactivity can be introduced into lighter nuclei.

Difference Between Natural Radioactivity and Artificial Radioactivity:

Nuclear Fission:

Nuclear fission is a process in which heavy nuclei break into lighter fragments of elements giving a very large amount of energy.

This reaction is used in atom bomb in an uncontrolled manner while it is used in a nuclear reactor in controlled manner.

Nuclear Fusion:

Nuclear fusion is a process in which nuclei of lighter elements combine to form a nucleus of the heavier element.

This process is used in the production of a hydrogen bomb

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Estimation of Age of Rock and Bone Samples:

Estimation of Earth’s Age (Uranium Dating Technique):

The radioactive uranium (²³⁸U )ore is found with nonradioactive lead (²⁰⁶Pb). The end product of radioactive decay of uranium is lead. A sample of radioactive ore of uranium is obtained and is analyzed for the radioactive uranium and nonradioactive lead.

Let the quantities, N =  ²³⁸U moles, N_o = ²³⁸U moles+ ²⁰⁶Pb moles

Knowing the mole ratio (²⁰⁶Pb/²³⁸U) in the sample of the ore. The age of the earth can be estimated. It is about 4.5 billion years.

Example 01:

The mass ratio (²⁰⁶Pb/²³⁸U) in the sample of the ore is 0.008:1. Estimate the age of the mineral. Take the half-life period of uranium as 4.51 billion years.

Solution:

Given: The mass ratio  ²⁰⁶Pb : ²³⁸U = 0.008:1, Half life period = t_1/2 = 4.51 billion years = 4.51 × 10⁹ years

To Find: Age of mineral = t = ?

The mass ratio is  ²⁰⁶Pb : ²³⁸U = 0.008:1

Their mole ratio is (0.008 / 206) : (1/238) = 0.0092 : 1

Example 02:

On analysis of a rock sample, the relative numbers of atoms of ²⁰⁶Pb and ²³⁸U is 0.25. Estimate the age of the rock. Take the half-life period of uranium as 4.51 billion years.

Solution:

Given: The atom ratio  ²⁰⁶Pb : ²³⁸U = 0.25, As they are monoatomic elements, their mole ratio is same as their atomic ratio. Half-life period = t_1/2 = 4.51 billion years = 4.51 × 10⁹ years

To Find: Age of rock = t = ?

Example 03:

On analysis of a rock sample, the molar proportion of uranium to lead is 1:3. Estimate the age of the rock when it was lead-free. Take the half-life period of uranium as 4.5 billion years.

Solution:

Given: The mole ratio  ²⁰⁶Pb : ²³⁸U = 3:1, Half-life period = t_1/2 = 4.51 billion years = 4.51 × 10⁹ years

To Find: Age of rock = t = ?

The age of the rock is 1.50 x 10¹⁰ years

Carbon Dating:

Radioisotope ¹⁴C is used to estimate the age of archaeological and biological specimens. This technique was first time developed by W. F. Libby.  The process in which age of fossils of plant or animal origin is determined by measuring the quantity of ¹⁴C present in the sample is called carbon dating.

The living matter always contains a definite amount of radioactive carbon ₆C¹⁴. Due to the interaction between neutrons produced by cosmic rays and atmospheric ¹⁴N, radioactive ¹⁴C is formed.

Half-life period of ¹⁴C is redefined by IUPAC at 5730 ± 40 years. This radioactive carbon reacts with atmospheric oxygen and radioactive carbon dioxide is produced. This radioactive CO₂ enters in living plants with normal CO₂ which is used by plants to prepare carbohydrates by photosynthesis. These carbohydrates are then consumed by living animals. During the life cycle of the living organism, ¹⁴C decays into ¹²Cbut it is again made up. A state of equilibrium is ultimately attained and a living plant or an animal on an average gives 15.3 disintegrations per minute per gram. Once the animals or plants are dead, ¹⁴C isotope will start disintegrating in them without the intake. Hence ¹⁴C count of dead plants or animals will be the measure of time lapsed since the animals or plants were alive. These counts are compared with that in the environment.

In the carbon dating method sample of dead tissue is burnt to obtain carbon dioxide and the ratio of ¹⁴C to ¹²C is obtained and probable age can be calculated using the formula.

The rapid disintegration of ¹⁴C  generally limits the dating period to approximately 50000 years. Thus carbon dating method cannot be used to find the age of samples older than 50,000 years.

Example 04:

In the carbon dating method, the amount of ¹⁴C isotope in a piece of wood is found to be one-fifth of that present in a fresh piece of wood. If the half-life period of ¹⁴C is 5577 years, calculate the age of the wood piece.

Solution:

Given: N = 1/5 N₀, Half-life period = t_1/2 = 5577 years

To Find: Age of wood piece = t = ?

Example 05:

A wooden sample gave an activity of 16 β particles per second while freshly cut wood samples gave an activity of 64 β particles per second. If the half-life period of 14 C is 5770 years, find the age of the wooden sample.

Solution:

Given: N = 16 β particles per second, N₀ = 64 β particles per second, Half-life period = t_1/2 = 5770 years

To Find: Age of wood piece = t = ?

Short Cut: to reduce activity from 64 β particles per second to 32 β particles per second it will take 5770 years and to reduce activity from 32 β particles per second to 16 β particles per second it will take another 5770 years. Thus totally it will take 11540 years.

Potassium-Argon Dating:

This method is used by geologists for dating very old archaeological materials and rocks. They have used this method to date rocks as much as 4 billion years old. The principle of the method is that some of the radioactive isotopes of Potassium, Potassium-40 (⁴⁰K), decays to the gas Argon as Argon-40 (⁴⁰Ar). Potassium (K) is one of the most abundant elements in the Earth’s crust (2.4% by mass) obtained in mica, feldspar and hornblendes. One out of every 10,000 Potassium atoms is radioactive Potassium-40 (⁴⁰K). They each have 19 protons and 21 neutrons in their nucleus. If one of these protons is hit by a beta particle, it can be converted into a neutron. With 18 protons and 22 neutrons, the atom has become Argon-40 (Ar-40), an inert gas. For every 100 K-40 atoms that decay, 11 become Ar-40.

By comparing the proportion of ⁴⁰K to ⁴⁰Ar in a sample of the volcanic rock, and knowing the half-life of ⁴⁰K, the date that the rock formed can be determined.

The temperature inside the earth is very high. The leakage of argon through the rock takes place at 125 °C. It creates a problem to find the actual ratio of ⁴⁰K to ⁴⁰Ar in the sample.

Rubidium-Strontium Dating:

This method is used by geologists for dating estimating the age of rocks, minerals, and meteorites from measurements of the amount of the stable isotope strontium-87 formed by the decay of β particle of the unstable isotope rubidium – 87. The method is applicable to very old rocks because the transformation is extremely slow: the half-life of rubidium-87 to is approximately 50 billion years.

The advantage of this method over potassium-argon method is that strontium formed does not diffuse through rocks as organ does.

Most minerals that contain rubidium also have some strontium incorporated when the mineral was formed, so a correction must be made for this initial amount of strontium to obtain the radiogenic increment.

In Production of Synthetic Elements:

The elements after uranium in the periodic table are called trans-uranic elements. They are prepared by nuclear transmutation in laboratories.

• When uranium ₉₂U²³⁸  isotope is bombarded by a high-speed neutron, neptunium is obtained.

Neptunium emits β particle giving rise to plutonium.

• When uranium ₉₂U²³⁸  isotope is bombarded by high-speed alpha particles, Plutonium is obtained.

Plutonium emits β particle giving rise to Armecium.

In Medicine:

• Many radioisotopes are now widely used by the medical field to detect and to cure them.
• In cancer treatment, C60 is used to destroy malignant cells. C60 gives β particles and strong ϒ rays which are used to destroy malignant cells. This treatment is called radiotherapy.
• Radioactive isotope I131 is used for the detection of a brain tumour.
• Radioactive isotope I131  is used in the detection and cure of thyroid gland diseases.
• Radioactive isotope P32 is used in the treatment of leukemia (blood cancer).
• Radioactive isotopes are used to study metabolic processes in human bodies. When used for this purpose they are called tracers.
• Radioactive isotopes are used to study the mechanism of action of drugs.
• Na24 isotope is used to examine blood circulation. An appropriate quantity of NaCI solution containing Na24 is injected into the patient’s arm. A Geiger counter is placed in contact with one of the feet. Na24 reaches the feet in about 30 minutes, if the circulation of blood is normal, while restricted circulation takes more time. In this way, the obstruction can be located by moving the Geiger counter over different parts of the body.

In Agriculture:

• Radioisotopes are used to study plant metabo­lism. e.g. Ca²⁰ and P³² are used to study plant metabolism.
• Co⁶⁰ is used for irradiation which is beneficial in the production of carrot seeds.
• S³⁵ isotope is used in the study of the effect of many fungicides on plants.
• Radiations from radioactive isotopes accelerate the growth of plants and yield of the crops.
• Sprouting of potatoes and onions is checked (avoided) by irradiation by radiations given out by a radioactive isotope.
• Proper irradiation of food and food grain increase their storage life.
• The radioactive isotope of carbon is used to study photosynthesis in plants.
• Radiations from radioisotopes are used to destroy pests and insects on plants.

The post Applications of Nuclear Chemistry appeared first on The Fact Factor.

• Nuclear forces.
• Mass defect and binding energy.
• The neutron to proton ratio (N/Z ratio).
• Odd and even number of nucleons.

Stability of Nucleus Due to Nuclear Forces:

The forces which hold the nucleons together within the nucleus are called nuclear forces. These are short-range forces. They are different from gravitation forces.

Explanation :

The nucleus of an atom, except in the case of radioactive elements is extremely stable. The nucleus on account of its stability does not take part in the chemical reaction.

There are forces between proton and proton (P-P), neutron (N-N) and even between proton and neutron (P-N). There are repulsive forces between proton and proton. In presence of such forces, the nucleons are kept together within the nucleus by such strong forces that very high energy is required to break the nucleus.

The nature of nuclear binding forces is totally different from that of gravitational and electrostatic forces. Nuclear forces are stronger than gravitational and electrostatic forces. These nuclear binding forces are independent of charge and operate over only a very short distance of range 10-15  m. Hence they are short-range forces.

Concept:

According to particle exchange theory proposed by Japanese scientist Yukawa, the nuclear forces arises from a constant exchange of particles called mesons between the nearby nucleons.

As two atoms are held together by sharing of electrons, two nucleons or nuclear particles are held together by sharing of mesons(π). According to charge carried by them, Mesons are classified into three type viz. Positive charge meson (π⁺), Negative charge meson (π^–), Zero charge meson (π⁰)

Exchange of positive charge meson (π⁺), negative charge meson (π^–) accounts for the nuclear force between protons and neutrons. Transfer of charged meson converts a proton to neutron and neutron to proton.

p        →    n   + π⁺

n      →   p   +    π^–

Mesons are exchanged between like particles such as p-p or n-n

p₁      →   p₂   +  π⁰

n₁      →   n₂   +  π⁰

Due to the constant interaction between nucleons, an exchange force is developed called nuclear force which holds nucleons together inside the nucleus of an atom with minimum potential energy. The greater the exchange force (nuclear force) the greater is the stability of the nucleus.

Mass of π⁺  and π^– is 273 times that of the electron. While that of π⁰ is 264 times that of the electron. Mesons are unstable outside the nucleus.

Stability of Nucleus Due to Nuclear Binding Energy:

Mass Defect:

The difference between the calculated mass due to nucleons and the actual observed isotopic mass of the nucleus is called the mass defect. Mass defect is denoted by ‘Δm’.

Explanation: 

Considered _Z X^A isotope.  Let A be the mass number and Z be the atomic number of the element. If m_p, me and mn be the masses of a proton. electron and neutron respectively, M_i be isotopic mass then,

Thus calculated mass, M_cal     = Z ×  m_p +  ( A  –  Z  ) × m_n

Now mass defect (Δm) =   Calculated mass   –   observed mass

Δm   =   [ Z  × m_p +   ( A  -Z  ) × m_n]  –   M_i

mass of an electron is negligible. Hence it is not considered for calculation of mass defect.

The mass equivalent of mass defect is converted into energy which is given by Einstein’s energy equation.

E   = Δm ×  C²

Where C = speed of light in free space.

This energy holds the nucleons together in the nucleus called nuclear binding energy. Hence nuclear stability is proportional to the mass defect.

Nuclear Binding Energy:

The energy equivalent to mass defect, which is required to break the nucleus into its isolated nucleons is called nuclear binding energy. It is denoted by E and is measured in MeV (million electron volts) or J (joule). It is obtained by multiplying mass defect in a.m.u. by 931.Therefore,

Nuclear binding energy  = mass defect in a.m.u.  x  931

Nuclear Binding Energy Per Nucleon or Average Binding Energy:

It is the ratio of total nuclear binding energy to the total number of nucleus present in that isotope.

Nuclear Stability on the Basis of Nuclear Binding Energy:

Since binding energy is responsible for holding the nucleons in the nucleus.  Nuclear stability is proportional to the nuclear binding energy.  The more the binding energy the greater is the nuclear stability. If we plot binding energy per nucleon in MeV against mass numbers (A) for different nuclei, a curve is obtained. The curve is called binding energy curve. This curve represents the relative stability of the nuclei of the elements.

• The curve shows that the average binding energy rises sharply and then it is constant for a broad range and then it decreases slowly.
• It is observed that higher the binding energy per nucleon, greater is the stability of the nucleus.
• It is found that the stable nuclei have binding energy per nucleon between 8 and 8.8 MeV. Binding energy per nucleon for the majority of the elements lies between 8 – 8.85 Mev.
• Elements having a mass number less than 25 have low binding energy per nucleon. They show the tendency of nuclear fusion. Elements  ₂He⁴,  ₆C¹² and  ₈O¹⁶ lie above the curve, it indicates there greater stability.
• For elements having the mass number in the range, 25 to 65 binding energy per nucleon smoothly increases from 7.5 MeV to 8.8 MeV. It attains maximum value for iron. Due to extra stability iron, cobalt and Nickel form the core of the earth.
• For nuclei having the mass number in the range, 65 to 160 binding energy per nucleon is almost constant at 8.5 MeV.
• For nuclei having mass number beyond 160, the binding energy per nucleon decreases steadily and it becomes 8 MeV for ₈₃Bi209 and it reaches to 7.6 MeV for Uranium.
• Nuclei with the mass number more than 220 and having binding energy per nucleon less than 8 are highly unstable and have a tendency of fission or natural radioactivity.
• Thus the nuclei having B.E. per nucleon in a range of 8 to 8.5 MeV are stable while those having B.E. per nucleons less than 8 MeV are unstable or less stable.

The post Nuclear Stability appeared first on The Fact Factor.

Terms Used in Nuclear Chemistry:

Now let us study certain basic terms used in nuclear chemistry.

Atomic number (Z):

The number of protons present in the nucleus of an atom or the number of electrons present in an atom is called the atomic number. It is denoted by the letter ‘Z’.

Example: In a Sodium atom there are 11 protons. Hence the atomic number of Sodium is 11

Characteristics of Atomic Number:

• It is the total number of protons present in the nucleus of an atom.
• Atoms of the same element have the same atomic number.
• Isotopes have the same atomic number.
• Chemical properties of elements are periodic properties of its atomic number.

Neutron number (N):

The number of neutrons present in the nucleus of an atom is known as the 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       or            N  =   A –  Z

Example: In a Sodium atom, the total number of protons and neutrons is 23. Hence the mass number of Sodium is 23.

Characteristics of Mass Number:

• It is the total number of protons and neutrons present in the nucleus of an atom.
• Atoms of the same element can have different mass numbers.
• Isobars have the same mass number
• Chemical properties of elements are not periodic properties of its mass number.

Representation of Atom in Symbolic Form: 

Generally, every atom X is represented as

Distinguishing Between Mass Number and Atomic Number:

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 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 a 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 different atomic numbers, different mass numbers but having the same neutron number are known as isotones.
• Isotones are the atoms of different elements.
• They have 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.

Distinguishing Between Isotopes and Isobars:

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