discovered by Henry Becquerel in 1896 .
Radiations Emitted by a Radioactive Element Three types of radiations emitted by radioactive elements
( i ) α-rays ( ii ) β-rays ( iii ) γ – rays
α-rays consists of α-particles , which are doubly ionised helium ion . β-rays are consist of fast moving electrons . γ – rays are electromagnetic rays .
[ When an α – particle is emitted by a nucleus its atomic number decreases by 2 and mass number decreases by 4 .
When a β -particle is emitted by a nucleus its atomic number is Increases by one and mass number remains unchanged .
When a γ – particle is emitted by a nucleus its atomic number and mass number remain unchanged
Radioactive Decay law
The rate of disintegration of radioactive atoms at any instant is directly proportional to the number of radioactive atoms present in the sample at that instant .
Rate of disintegration ( – dN / dt ) ∝ N
– dN / dt = λ N where λ is the decay constant . The number of atoms present undecayed in the sample at any instant N = N o e -λt