(i) The electron-dot structure of sodium (Na):

The electron-dot structue of oxygen(O):

The electron-dot structue of magnesium(Mg):

(ii) (a) Formation of Na₂O

Na₂O is sodium oxide. Sodium atom has one electron in its outermost shell whereas oxygen atom has six electrons in its outermost shell. In the formation of sodium oxide, two sodium atoms transfer their two outermost electrons to an oxygen atom. Thus, sodium atoms form two sodium ions (2Na⁺) by losing two electrons while oxygen atom forms an oxide ion (O^2-) by gaining two electrons.

Sodium ions and oxide ion, being oppositely charged, attract each other and are held together by strong electrostatic forces of attraction to form the ionic sodium oxide compound 2Na⁺O^2- or Na₂0.

(b) Formation of MgO

MgO is magnesium oxide. Magnesium atom has two electron in its outermost shell whereas oxygen atom has six electrons in its outermost shell. In the formation of magnesium oxide, magnesium atom transfer its two outermost electrons to an oxygen atom. Thus, magnesium atom forms magnesium ion (Mg²⁺) by losing two electrons while oxygen atom forms an oxide ion (O^2-) by gaining two electrons.

Magnesium ion and oxide ion, being oppositely charged, attract each other and are held together by strong electrostatic forces of attraction to form the ionic magnesium oxide compound Mg²⁺ O^2- or MgO.

(iii) (a) The ions present in sodium oxide (Na₂O) compound are sodium ions (Na⁺) and oxide ions (O^2-).

(b) The ions present in magnesium oxide (MgO) compound are magnesium ions (Mg²⁺) and oxide ions (O^2-).