Clasically, an electron can be in any orbit around the nucleus of an atom. Then what determines the typical atomic size? Why is an atom not, say , thousand times bigger than its typical size? The question had greatly puzzled Bohr before he arrived at his famous model of the atom. To stimuluate what he might well have done before his discovery, let us play with the basic constants of nature and see if we can get a quantity with the dimensions of length that is roughly equal to known size of an atom (~~10^(-10)m). (a) Construct a quantity with the dimensions of length form the fundamental constant e,m_e and c. Determine its numerical value. (b) you will find that the length obtained in (a) is many orders of magnitude smaller than the atomic dimensions. Further, it involves c. But energies of atoms are mostly in non-relativistic domain where c is not expected to play any role. This is what may have suggested Bohr to discard c and look for something else to get the right atomic size. Now, the Planck's constant h had already made its appearence elsewhere. Bohr's great insight lay in recognising that h,m_e and e will yield the right atomic size. Construct a quantity with the dimensions of length form h, m_e and e and confirm that its numerical value has indeed the correct order of magnitude.