The laws of radioactivity are brodally divided into two parts
(1) the qualitative law and
(2) the quantitative law
1. The Rutherford – Soddy’s Law is the qualitative law. It tells the nature of daughter nucleus obtained in a ratioactive transformation.
(a) A particular radioactive nuecleus can emit either an αor a β particle but both can never be emitted simuitaneously.
(b) When a radioactive nucleus \(^A_Z X\) emits an α–particle its mass number decreases by 4 and atomic number by 2; i.e.
When a radioactive nucleus emits a β –particle, the mass number remains same and atomic number increases or decreases by one depending on whether the β –particle emitted is an electron or a positron.
2. The Exponential Law
This is quantative law. It relates the number of radioactive atoms initially with the number of atoms present at t = t.
Hence it becomes obvions that radioactive disintegration are governed by the laws of probability. The phenomenon can only be described statistically.
Let N0 be the number of radioactive atoms at the instant t = 0 and let N be their number at a latter instant t.
Let λ represent the probability that one of these atoms will disintegrete in a unit time. λ is known as disintegration constant.
Hence the probable number of disintegration per unit time is λ N. Thus we have
which means that radioactive disintegrations are governed by an exponential law as illustrated in the figure. λ the disintegration constant, is a characterstics of a particular radioactive material.
