Why do atoms react? The octet idea
Noble gases (He, Ne, Ar) hardly react because their outer shell is full: 2 electrons for helium, 8 for the others. Other atoms react to reach this stable arrangement. Metals have 1–3 outer electrons and find it easier to lose them; non-metals have 5–7 and find it easier to gain.
How metals and non-metals react: electron transfer
Na (2, 8, 1) → Na+ (2, 8) + e−. Cl (2, 8, 7) + e− → Cl− (2, 8, 8). The ions attract to form NaCl.
Magnesium chloride
Mg (2, 8, 2) gives two electrons, one to each of two chlorine atoms: Mg → Mg2+ + 2e−; 2Cl + 2e− → 2Cl−. Formula MgCl2.
Magnesium oxide
Mg gives two electrons to one oxygen (2, 6): Mg2+ and O2−, formula MgO.
In an electron-dot structure you draw only the outer electrons as dots or crosses around the symbol and show the arrow of transfer. Board questions often ask you to show NaCl, MgCl2, MgO or CaO this way (3 marks).
Ionic (electrovalent) bond and ionic compounds
The compounds formed by transfer of electrons from a metal to a non-metal are ionic or electrovalent compounds. The positive ion is the cation, the negative ion the anion. The total charge of the compound is zero, which is how you work out the formula: Ca2+ + 2Cl− → CaCl2; 2Na+ + O2− → Na2O.
Properties of ionic compounds
- Physical nature: hard, brittle crystalline solids, because the ions are held by strong forces in a fixed pattern; a sharp blow shifts layers so like charges face each other and the crystal breaks.
- High melting and boiling points: a lot of energy is needed to break the strong attraction (NaCl melts at about 801 °C).
- Solubility: generally soluble in water, insoluble in kerosene and petrol.
- Conduction: do not conduct as solids (ions cannot move), but conduct when melted or dissolved in water, because the ions become free to move to the electrodes.
Key formulas and definitions
- Metal atom → cation + electrons (e.g. Na → Na⁺ + e⁻)
- Non-metal atom + electrons → anion (e.g. Cl + e⁻ → Cl⁻)
- Formula: total positive charge = total negative charge
- Key terms: octet, cation, anion, ionic/electrovalent bond, electron-dot structure
Worked examples
1. Show the formation of Na₂O by transfer of electrons.
Each Na (2, 8, 1) loses one electron → Na⁺. Oxygen (2, 6) needs two, so it takes one from each of two Na atoms → O²⁻ (2, 8). 2Na⁺ + O²⁻ → Na₂O.
2. Write the electron configurations of Mg²⁺ and Cl⁻.
Mg (12) has 2, 8, 2; losing two gives Mg²⁺ = 2, 8. Cl (17) has 2, 8, 7; gaining one gives Cl⁻ = 2, 8, 8.
3. Why does solid salt not conduct electricity but salt water does?
In the solid, Na⁺ and Cl⁻ are locked in the crystal and cannot move. In water they separate and move freely, carrying charge between the electrodes.
4. Find the formula of the compound formed by calcium and oxygen and by potassium and sulphur.
Ca²⁺ and O²⁻ balance 1:1 → CaO. S²⁻ needs two K⁺ → K₂S.
Common mistakes
- Drawing the ion with the old outer electrons: Na⁺ has no electron in the third shell.
- Writing the charge of Cl⁻ as +1 because it 'gets something': gaining an electron makes it negative.
- Saying ionic solids conduct electricity: only molten or dissolved ionic compounds conduct.
- Writing MgCl for magnesium chloride: Mg²⁺ needs two Cl⁻, so MgCl₂.