Radioactivity

Chemical Kinetics of Class 12

Henri Becquerel in 1896 discovered that rays emitted from Uranium compounds had the properties of affecting photographic plates, ionising air and penetrating through substances. The spontaneous emission of such radiations is known as radioactivity and the elements emitting these radiations are called radioactive elements.

Rutherford and Soddy (1903) gave an ingenious interpretation of the radioactive processes and the origin of α and β emissions on the basis of neutron-proton theory.

(i) The atomic nucleii of radioactive elements are unstable.

(ii) The nucleii of radioactive elements are undergoing a process of disintegration forming atoms of new elements called daughter elements which are distinct in physical and chemical properties from parent elements.

(iii) α and β particles are ejected from the nucleus.

(iv) γ-rays are secondary products of atomic disintegration.

Radioactivity is found to be of three types α, β and γ radiations.

α particles	β particles	γ ray
1.	It carries two units positive charge and four units mass.	It carries unit negative charge with no mass.	They are electromagnetic waves with very short wave length.
2.	It is represented as helium nucleus. or He ions (He++)	It is represented as	It is represented as
3.	They have high ionising power.	Ionisation power is less than that of α particle.	Ionisation power is low.
4.	The velocity of α particle is 2 × 109 cm s⁻¹.	Velocity is less than that of light. It varies from 2.36 to 2.83 × 1010 cm s⁻¹	Velocity is the same as that of light. viz. 3 × 1010 cm s⁻¹.
5.	They have low penetrating power.	Penetration power is 100 times that of α particle	Penetration power is 100 times that of β particles.
6.	The range i.e. the distance upto which radioactive properties can be shown is very small, 8 − 12 cm.	Range is more than that of α particle.	Range is more.
7.	They cause luminescence on zinc sulphide screen.	Very little effect on zinc sulphide screen.	Very little effect on zinc sulphide screen.
8.	When an α particle is emitted, atomic number decreases by 2 units and mass number by 4 units.	When a β particle comes out, atomic no increases by one unit, mass number remains unchanged.	There is no change in atomic number or mass number when a γ-ray is emitted.
9.	Deflected in magnetic field.	Deflected to a greater extent in magnetic field.	Not deflected in magnetic field.

TYPES OF RADIOACTIVE DECAY

Types of decay	Radiation	Equivalent process	Nuclear change		Usual Nuclear condition
Types of decay	Radiation	Equivalent process	At Numbers	Mass Number	Usual Nuclear condition
(i) Alpha emission (α)		⎯	– 2	– 4	Z > 83
(ii) Beta emission (β)			+1	0	too large
(iii) Positron emission (β+)			– 1	0	too small
(iv) Electron capture	X-rays		––	––	too small
(v) Gamma emission (γ)	γ	⎯	0	0	Excited nucleus

Radioactivity

CAUSE OF RADIOACTIVITY

Except in the case of ordinary hydrogen all other nuclide contain both neutrons and protons. A look at the stable nuclides show that the ratio N/P (neutron/proton) in them is either equal to 1 or more than 1. The ratio is ≈ 1 in all the light stable nuclides up to calcium Radioactivity and thereafter the ratio is greater than 1 for heavy nuclides.

Radioactivity

The stable nuclides lie within the shaded area which is called the region or zone of stability. All those nuclides falling outside this zone are invariably radioactive and unstable in nature. Nuclides falling above the stability zone have an excess of neutrons while those lying below have more protons. Both of these cause instability. These nuclides attain stability by making adjustment in the N/P ratio.

THEORY OF RADIOACTIVE DISINTEGRATION

Rutherford and Soddy, in 1903, postulated that radioactivity is a nuclear phenomenon and all the radioactive changes are taking place in the nucleus of the atom. They presented an interpretation of the radioactive process and the origin of radiations in the form of a theory known as theory of radioactive disintegration.

The disintegration process may proceed in one of the following two ways:

(a) α-particle emission: When an α-particle Radioactivity is emitted from the nucleus of an atom of the parent element, the nucleus of the new element, called daughter element, possess atomic mass or atomic mass number less by four units and nuclear charge or atomic number less by 2 units, because α-particle has mass of 4 units and nuclear charge of two units.

Parent element Radioactivity Daughter element

Atomic mass W W - 4

Atomic number Z Z - 2

Radioactivity

(b) β-particle emission: β particle is merely an electron which has negligible mass. Whenever a β-particle is emitted from the nucleus of a radioactive atom, the nucleus of the new element formed, possess the same atomic mass but nuclear charge is increased by one unit over the parent element. β - particle emission is due to the result of decay of neutron into proton and electron.

Radioactivity