$(i)$ The representation of elements with valence electrons as dots around the elements is referred to as electron-dot structure for elements.

$(a)$ Sodium $(2,8,1)\,=\,\overset{\centerdot }{\mathop{Na}}\,$

$(b)$ Oxygen $ (2,6)\,=\,\overset{\centerdot \,\centerdot }{\mathop{:O:}}$

$(c)$ Magnesium $(2,8,2)\,=\,\overset{\centerdot \,\,\centerdot }{\mathop{Mg}}$

$(ii)$ as per image

$(iii)$ The ions present in $Na _{2} O$ are $Na ^{+}$ and $O ^{2-}$ ions and in $MgO$ are $Mg ^{2+}$ and $O ^{2-}$ ions.