I am learning about solutions and while giving an example of a liquid (solute) and a solid (solvent), my teacher told us that mercury forms an amalgam with all the metals except iron.
I want to know why this is so?
Iron does not form an amalgam readily, hence the use of iron electrodes in mercury switches. Apparently, iron does form alloys with mercury under specific conditions; see http://www.madsci.org/posts/archives/2011-04/1304143502.Ch.r.html.
By contrast, mercury spilled on gold (e.g. gold-alloy jewelry) soaks in immediately, discoloring the gold, and migrates further into the bulk metal in a few hours, thus restoring the color.
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.
Note: All metals mentioned above are of comparable size with Hg, i.e. they fulfil the amalgam formation condition.