(a) Ionisation energy:
Ionisation energy is the minimum amount of energy which is to be supplied to an atom in its ground state so that it gets converted into an ion i.e., the minimum energy requires to shift an electron from n=1 to n=∞.
The value of ionization energy for Hydrogen is 13.6 eV.
OR
(b) Mass defect:
Mass defect is the difference of total mass of all the nucleons present in a nucleus and the rest mass of the nucleus.
Mass defect =∆m=Zmp+(A−Z)mn−M
As the mass defect increases, binding energy of the nucleus increases. This means the stability of the nucleus increases as mass defect increases.