The ionic bond: electrons change hands

Sodium meets chlorine

An ionic bond forms when atoms lose or gain electrons to get the electron arrangement of the nearest noble gas, a more stable structure.

Sodium (2, 8, 1) gives its outer electron to chlorine (2, 8, 7). Sodium becomes Na⁺ like neon, and chlorine becomes Cl⁻ like argon. Only outer-shell electrons take part.

The oppositely charged ions are held together by electrostatic attraction. This is an ionic, or electrovalent, bond, and the product is an ionic compound.

Calcium loses two electrons, one to each of two chlorine atoms, giving CaCl₂. The ions then surround each other in three dimensions to form a crystal lattice.

An ionic bond is formed by the complete transfer of electrons from one atom to another.

Key terms

Ion

What an atom becomes after losing or gaining electrons.

Ionic bond

Holding oppositely charged ions together.

Electrostatic attraction

The force inside an ionic bond.

Crystal lattice

How ions pack together in a solid ionic compound.

Short questions with model answers

1. Q1. Show how sodium chloride forms (Fig 3.2, p. 34).

   • Na (2, 8, 1) → Na⁺ (2, 8) + e⁻
   • Cl (2, 8, 7) + e⁻ → Cl⁻ (2, 8, 8)
   • Na⁺ + Cl⁻ → NaCl

   One electron moves from Na to Cl; Na⁺ and Cl⁻ are held by electrostatic attraction.

2. Q2. Show how calcium chloride, CaCl₂, forms (Fig 3.3, p. 35).

   • Ca (2, 8, 8, 2) → Ca²⁺ + 2e⁻
   • 2Cl + 2e⁻ → 2Cl⁻
   • Ca²⁺ + 2Cl⁻ → CaCl₂

   Two electrons move, one to each chlorine, giving CaCl₂.

Common mistakes

• ✗ In NaCl, sodium and chlorine share a pair of electrons.

  ✓ The electron is transferred completely. Sharing is a covalent bond.

• ✗ Any electron of the atom can be transferred.

  ✓ Only electrons of the outermost shell take part in a reaction (p. 35).

• ✗ Solid NaCl is made of separate NaCl pairs.

  ✓ The ions surround each other in all three directions, forming a crystal lattice.

MCQs

1. 1. What types of elements usually form ionic bonds with each other?

   1. (a) Two noble gases
   2. (b) A metal and a non-metal
   3. (c) Two identical non-metals
   4. (d) Two metals
   Show answer

   (b) Exercise p. 35: electropositive metals (Na, Ca) give electrons to electronegative non-metals (Cl, F).

2. 2. In forming CaCl₂, how many electrons does one calcium atom lose?

   1. (a) 1
   2. (b) 3
   3. (c) 4
   4. (d) 2
   Show answer

   (d) p. 35: calcium loses two electrons, one to each chlorine atom.

3. 3. What holds Na⁺ and Cl⁻ together in sodium chloride?

   1. (a) Electrostatic attraction
   2. (b) A shared electron pair
   3. (c) Hydrogen bonds
   4. (d) Gravity
   Show answer

   (a) p. 35: oppositely charged ions are held by electrostatic force of attraction.

4. 4. After sodium loses one electron, its arrangement matches which noble gas?

   1. (a) Helium
   2. (b) Argon
   3. (c) Neon
   4. (d) Krypton
   Show answer

   (c) Na⁺ is 2, 8, the same as neon, the nearest noble gas.

5. 5. How are the ions arranged in solid sodium chloride?

   1. (a) As separate NaCl molecules
   2. (b) In a straight line
   3. (c) Randomly, like a gas
   4. (d) In a three-dimensional crystal lattice
   Show answer

   (d) pp. 35–36: the ions surround each other three-dimensionally to form a crystal lattice (Fig 3.4).

Quick revision

• An ionic bond forms by the complete transfer of outer electrons from a metal to a non-metal.
• The ions reach noble-gas arrangements and attract each other electrostatically.
• Ionic solids are crystal lattices, such as NaCl and CaCl₂.