Radioactivity

The process of emission of α, β, γ radiations when an unstable nucleus becomes stable is called radioactivity or radioactive disintegration.

Properties of α-Particles

1. They are helium nuclei.
2. They are positively charged.
3. They are deflected in electric and magnetic field.
4. They have less penetrating power.
5. They can ionise the gas through which they pass.

Properties of β-Particles

1. They are fast moving electrons.
2. They are negatively charged.
3. They are deflected in electric and magnetic field.
4. The penetrating power is more than α-particles.
5. They have less ionising power than α-particles.

Properties of γ-Rays

1. They are electromagnetic waves.
2. They are chargeless.
3. They are not deflected in electric and magnetic field.
4. They have high penetrating power.
5. The ionising power is less than α and β-particles.

Laws of Radioactivity

1. The radioactivity process is not affected by external factors like pressure, temperature, humidity etc.
2. Either α-particle is emitted or β-particle is emitted but not both simultaneously.

When α-Particle is Emitted

When α-particle is emitted, atomic number decreases by 2 and mass number decreases by 4.

Example

When β-Particle is Emitted

When β-particle is emitted, atomic number increases by 1 and mass number remains unchanged / same.

Example

Radioactive Decay Law

The rate of decay of a radioactive sample at time is directly proportional to number of atoms present at that time.

If a sample contains N atoms at time t, then:

where λ is decay constant.

Let N₀ be the initial number of atoms at time t = 0.

Decay Curve

Fig: Decay curve

Half Life (T_1/2)

The time taken to decay half the number of atoms is called half-life.

Relation Between Half-Life and Decay Constant

Since:

If t = T_1/2 and N = N₀/2, then:

Mean-Life

The ratio of total life of atoms to the total number of atoms is called mean-life.

Also,

Remaining Number of Atoms after nth Half-Life

Remaining atoms after n^th half-life:

Remaining activity after n^th half-life:

Units of Radioactivity

1) Becquerel

2) Rutherford

3) Curie (Ci)