An atomic number is the number of protons that the atom of a particular element has.
For example, take carbon. Carbon has 6 protons. It's atomic number is 6.
MgCl2 = 1Mg + 2Cl = 1(24.3) + 2(35.45) = 95.2g/1mole
7.50moles MgCl2 x 95.2g MgCl2 = 714g MgCl2