Identify the cations and anions present (if any) in the following compounds:
$(a)$ $CH_3COONa$
$(b)$ $NaCl$
$(c)$ $H_2$
$(d)$ $NH_4NO_3$

State whether the following statement is true or false:
Iron shows valencies of $Fe^{2+}$ and $Fe^{3+}$.

How many atoms are present in a $(i)$ $H_2S$ molecule and $(ii)$ $PO_4^{3-}$ ion?

Compute the difference in masses of $10^{3} \, \text{mol}$ each of magnesium atoms and magnesium ions. (Mass of an electron $= 9.1 \times 10^{-31} \, \text{kg}$)

$(a)$ Define atomicity.
$(b)$ Give an example for an element which is monoatomic and a polyatomic molecule.

Name the anion and cation which constitute the molecule of aluminium oxide.