Half Life – We have already studied that radioactive elements continuously emit radiations, due to which they change into new elements. For example if an a particle is emitted by an element
. The atomic mass number of the newly produced element remains the same, but its charge number increases by one unit. It is due to the fact that protons and neutrons are present inside the nucleus, which continuously change into one another. When a neutron emits a B – particle, it changes into a proton. So the charge of the new element is greater by one unit.
In such a reaction, the original element is called the parent element and the newly formed element is called the daughter element. The number of breaking atoms is proportional to the number of atoms lift in that element.
This means that the number of initially breaking atoms is large, which decreases with time.
An indefinite time is required for completing the breaking process of atoms in radioactive elements.
The half life of an element is that time during which the numbers of atoms of that element are reduced to one half.
For example, if the half life time of a radioactive element is T, then at the end of this time the number of atoms in this element remains one half, after a time 2T, the number of atoms remains 25% and after time 3T, the number of atoms are reduced to 12.5% of the initial number.
Example 18.1: the half life of krypton is 3. 16 minutes. Out of 100 g of krypton, how much will be left after 9.48 minutes?
Solution:
Total quantity of krypton = 100 g
Half life of krypton = T = 3.16 minutes
The remaining quantity of krypton after first half life,
The remaining quantity of krypton after second half life,
The remaining quantity of krypton after third half life,
Stable and unstable nuclides
Nuclei which do not emit radiations naturally are called stable nuclei. Generally, most of the nuclei whose atomic number are from 1 to 82 are stable nuclei, but the elements whose atomic number is greater than 82, are naturally unstable. These elements, depending upon their characteristics, emit, all the time, different types of strong radiations and they continuously change from one type of element to another. The stable elements can also be changed into unstable from by bombarding them with neutrons. Such elements are called radio isotopes.
