#### Answer

a) Mg - 3 atoms ; P - 2 atoms ; O -8 atoms
b) Ba - 1 atom ; Cl - 2 atoms
c) Fe - 1 atom ; N - 2 atoms ; O -4 atoms
d) Ca - 1 atom ; O - 2 atoms ; H -2 atoms

#### Work Step by Step

a) In the formula, we can see that there are 3 atoms of Mg, 2 atoms of $P$ (1x2), and 8 atoms of $O$ (4x2)
b) In the formula, we can see that there is 1 atom of Ba and 2 atoms of $Cl$
c) In the formula, we can see that there is 1 atom of Fe, 2 atoms of $N$ (1x2), and 4 atoms of $O$ (2x2)
d) In the formula, we can see that there is 1 atom of Ca, 2 atoms of $O$ (1x2), and 2 atoms of $H$ (1x2)