What are the dimensions of `chi`, the magnetic susceptibility? Consider an H-atom. Guess an expression for `chi` upto a constant by constructing a quantity of dimensions of `chi`, out of parameters of the atom: e, m, v, R and `mu_0`. Here, m is the electronic mass, v is electronic velocity, R is Bohr radius. Estimate the number so obtained and compare with the value of `|chi|~10^5` for many solid materials.

What are the dimenstions of chi , the magnetic susceptibility? Consider an H-atoms. Guess an expression for chi , upto a constant by constructing a quantity of dimenstions of chi , out of parameters of the atom: e, m, v, R and mu_0 . Here, m is the electronic mass, v is the electronic velocity, R is Bohr radius. Estimate the number so obtained and compare with the value of |chi| ~ 10^-5 for many solid materials.

Classically, 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 that you have learnt in the text. To simulate what he might well have done before his discovery, let us play as follows with the basic constants of nature and see if we can get a quantity with the dimensions of length that is roughly equal to the known size of an atom (~ 10^(–10)m) . (a) Construct a quantity with the dimensions of length from the fundamental constants 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 appearance 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 dimension of length from h, m_(e) , and e and confirm that its numerical value has indeed the correct order of magnitude.