Give the expression for the radius of a molecule.

Write the expression for the radius of n^(th) orbit of the hydrogenic atoms and give the meanings of the symbols used.

Using Bohr's postulates of the atomic model. Derive the expression for radius of nth electron orbit, thus obtaining the expression for Bohr's radius.

What is radius of gyration? Derive an expression for the radius of gyration of n particle system.

Give the expression for the moving mass of a photon.

Give the expression for a spring force.