Valency, Radicals and Chemical Formula

What is the valency of ${\mathrm{Ca}}^{2+}$?

What is meant by bivalent?

The correct chemical formula of aluminium oxide is

The valency of iron in ${\mathrm{Fe}}_2{\mathrm{O}}_3$ is

${\mathrm{F}}^{-}, {\mathrm{Cl}}^{-}, {\mathrm{K}}^{+} \mathrm{and} {\mathrm{Na}}^{+}$ are the examples of 

The chemical formula of sodium chlorate is ${\mathrm{NaClO}}_3$ then the chemical formula of sodium hypochlorite is 

The formula of compound represents

Oxygen, nitrogen and platinum are respectively

Define valency and variable valency.

The valency of nitrogen in dinitrogen oxide$\left({\mathrm{N}}_2\mathrm{O}\right)$ is