(a) Two energy levels of an electron in hydrogen atom are separated by 2.55 eV . Find the wavelength of radiation emitted when the electron makes transition from the higher energy level to the lower energy level.

(b) In which series of hydrogen spectrum this line shall fall?

The earth revolves around the sun in an orbit of radius $1.5 \times 10^{11} \mathrm{~m}$ with orbital speed $30 \mathrm{~km} / \mathrm{s}$. Find the quantum number that characterises its revolution using Bohr's model in this case (mass of earth $\left.=6.0 \times 10^{24} \mathrm{~kg}\right)$

Draw a graph showing the variation of potential energy of a pair of nucleons as a function of their separation. Indicate the region in which the nuclear force is (a) attractive and (b) repulsive.

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

OR

(b) Define the term, mass defect. How is it related to stability of the nucleus?