(a) What is meant by ionisation energy? Write its value for hydrogen atom.

Ionization energy is the minimum energy required to remove an electron from an isolated atom of an element.

It is the energy required to excite an electron from energy level $\displaystyle n = 1$ to $\displaystyle n = \infty$ from an isolated atom of an element.

The ionization energy for hydrogen atom is $\displaystyle 13.6$ eV.

OR(b) Mass defect is the difference between the actual mass of the nucleus and the sum of the masses of its nucleons.

Greater the mass defect, greater will be the binding energy and the nucleus will be more stable.