Question

why iron and platinum cannot form amalgum with mercury?

Answer 1

What actually happens in amalgam formation is that the metal ion (Mn+) is first reduced to metal atom (M), and the metal atom is then solvated by Mercury atom (Hg), to form amalgam. So, if Mercury has to form amalgam with Iron, it need to solvate Iron atoms by getting incorporated in the Iron lattice. Now, we know that, Iron is a very stable metal and it reacts very slowly (even rust formation takes many days and Iron reacts with steam not with water). And this stability of Iron can be attributed to the following reason: 

Suppose we have a Fe(II) salt solution. Now, Fe(II) on reduction will give Fe atom according to the following reaction:

Fe2+ (aq) + 2e- → Fe (s)

We know that, atomic number of Fe is 26 and the electronic configuration will be 1s2 2s2p63s2p6 4s2 3d6 which implies that outermost d-orbital has 6 electrons. Now, d-orbitals is 5 fold degenerate, so, out 5, one degenerate orbital will possess a pair of electrons and, the remaining 4 will possess one electron each. As pairing energy of Fe is small, because the octahedral splitting (Δ0) is less, all the electrons will pair up in the degenerate orbitals, giving a symmetrical electron cloud which does not allow incorporation of any other atom, because the atom (in this case, Hg which is a transition metal) approaching the Fe atom does not affect its symmetrical cloud to a much greater extent. Thus, on the basis of above reasons, it can be concluded that Fe will not form amalgam with mercury. 

Answer 2

In Fe, Co, or Ni, the metallic bonds are very strong. Hence, they don't allow mercury to diffuse into it and as such, no amalgam is possible with these metals. In contrast, Ag, Au, or Na (i.e. soft metals) can readily adsorb mercury. Hence amalgam formation is possible with these metals.

Hope it helps....