Which of the following elements exhibits both positive and negative oxidation states?

In the periodic table,the valency of elements in any period...

In any period,the valency of an element with respect to oxygen:

Carbon has a valency of $2$ in $CO$ and $4$ in $CO_2$ and $CH_4$. Its valency in acetylene $(C_2H_2)$ is

Predict the formulas 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

What would be the formula of the compound formed between the following elements?
$(i)$ Aluminium and Bromine
$(ii)$ Sodium and Oxygen
$(iii)$ Silicon and Oxygen
$(iv)$ Phosphorus and Chlorine