What is the oxidation state of gallium?

The valency (oxidation state) of phosphorus in $H_3PO_4$ is

Which of the properties remains unchanged on descending a group in the periodic table?

What is the oxidation state of sulfur in cyclic sulfur trioxide $(S_3O_9)$?

Predict the formula of the stable binary compounds that would be formed by the combination of the following pairs of elements:
$(a)$ Lithium and oxygen
$(b)$ Magnesium and nitrogen
$(c)$ Aluminium and iodine
$(d)$ Silicon and oxygen
$(e)$ Phosphorus and fluorine
$(f)$ Element $71$ and fluorine

Explain the periodicity of valence or oxidation states and explain the oxidation states of oxygen in $Na_2O$ and $OF_2$.