When calcium reacts with chlorine, calcium chloride is formed. The balanced chemical reaction of this equation is `Ca + Cl_2 -> CaCl_2` .

One mole of calcium reacts with one mole of chlorine gas to give one mole of calcium chloride. One mole of a substance contains 6.023*10^23 formula units. The molar mass of calcium is 40 g/mole. 5.3 g of calcium contains `5.3/40 ~~` 0.1325 moles of calcium.

This is equivalent to 0.1325*6.023*10^23 ~~ 7.98*10^22 atoms of calcium. For each atom of calcium one formula unit of calcium chloride is created when it reacts with excess chlorine gas. This gives the number of formula units of calcium chloride created when 5.3 g of calcium reacts with excess chlorine as 7.98*10^22.