The chemical formula of calcium oxide is $\mathrm{CaO}$. What is the valency of calcium? (Hint: The valency of oxygen is two)

$\mathrm{P}, \mathrm{Q}, \mathrm{R}, \mathrm{S}$ are four elements. Their atomic numbers are $8, 17, 12$ and $16$ respectively. Find the type of chemical bond in these compounds formed by combining the following pairs of elements.

(Electronegativity values: $\mathrm{P}=3.44, \mathrm{Q}=3.16, \mathrm{R}=1.31, \mathrm{S}=2.58$)

1. $\mathrm{P}, \mathrm{R}$
2. $\mathrm{P}, \mathrm{S}$
3. $\mathrm{Q}, \mathrm{R}$