Give difference between relative atomic mass and relative molecular mass.

What is the valency of underlined element in the given compounds?
$\left(\mathrm{i}\right) \underline{{\mathrm{Ca}}_3}{\mathrm{N}}_2 \left(\mathrm{ii}\right) \underline{\mathrm{Zn}}\mathrm{SO}4 \left(\mathrm{iii}\right) \underline{\mathrm{C}}{\mathrm{Cl}}_4$. 

Write chemical formulae of the following compounds.
(a) Aluminium nitride
(b) Ammonium phosphate
(ii) Name an element which shows a variable valency. Write the formulae of its two chloride.