Avogadro's number tells you that in one mol of element (sulfur) there are 6.022x10^23 atoms...
So,
1 mol : 6.022x10^23 = x mol : 1.2x10^2
x = 1.99 x 10^-22 mol
*Although this should be correct, you might want to check your question again. You probably misinterpret the question... you want 1.2x10^23 !! :) check it.
The ratio of sizes between the ionic radii of cations and anions in a cell influences the manner of packing for that cell thereby predicting the possible cation/anion coordination number in any compound and establishing the structure of ionic solids.
