Write the chemical formulae of the compounds formed by the following ions:
$(a)$ $Mg^{2+}$ and $S^{2-}$
$(b)$ $Cu^{2+}$ and $OH^-$
$(c)$ $Al^{3+}$ and $Br^-$
Name the compounds formed in each case.

(N/A) To write the chemical formula,we use the criss-cross method where the valency of one ion becomes the subscript of the other.
$(a)$ For $Mg^{2+}$ and $S^{2-}$:
- The charges are $+2$ and $-2$. Since the magnitudes are equal,they cancel out to form a $1:1$ ratio.
- Chemical Formula: $MgS$
- Name: Magnesium sulphide
$(b)$ For $Cu^{2+}$ and $OH^-$:
- The valency of $Cu$ is $2$ and $OH$ is $1$. By criss-crossing,we get $Cu(OH)_2$.
- Chemical Formula: $Cu(OH)_2$
- Name: Copper$(II)$ hydroxide
$(c)$ For $Al^{3+}$ and $Br^-$:
- The valency of $Al$ is $3$ and $Br$ is $1$. By criss-crossing,we get $AlBr_3$.
- Chemical Formula: $AlBr_3$
- Name: Aluminium bromide