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

(N/A) $(i)$ Aluminium $(Al)$ has a valency of $3$ (Group $13$) and Bromine $(Br)$ has a valency of $1$ (Group $17$). The formula is $AlBr_{3}$.
$(ii)$ Sodium $(Na)$ has a valency of $1$ (Group $1$) and Oxygen $(O)$ has a valency of $2$ (Group $16$). The formula is $Na_{2}O$.
$(iii)$ Silicon $(Si)$ has a valency of $4$ (Group $14$) and Oxygen $(O)$ has a valency of $2$ (Group $16$). The formula is $SiO_{2}$.
$(iv)$ Phosphorus $(P)$ has a valency of $5$ (Group $15$) and Chlorine $(Cl)$ has a valency of $1$ (Group $17$). The formula is $PCl_{5}$.