Explain:

Duplet rule for formation of Hydrogen.

An atom $\mathrm{X}$ has three electrons more than the noble gas configuration. Write the formula of its $\left(\mathrm{X}\right)$ 

$\left(\mathrm{i}\right)$ sulphate

$\left(\mathrm{ii}\right)$ nitrate

$\left(\mathrm{iii}\right)$ phosphate

$\left(\mathrm{iv}\right)$ carbonate

$\left(\mathrm{v}\right)$ hydroxide.

What type of compounds are usually formed between metals and non-metals and why?