The covalent bond: sharing electron pairs

Sharing instead of giving

Some atoms lower their energy by sharing electrons instead of transferring them. As two atoms approach, each nucleus attracts the other atom's electrons, while like charges repel.

The atoms settle where the energy is lowest, and a stable molecule forms. A covalent bond is formed by mutually sharing an electron pair provided by the bonded atoms.

One shared pair is a single bond (H–H). If each atom shares two electrons, it is a double bond (O=O); if each shares three, a triple bond (N≡N).

Between identical atoms the pair is shared equally: a non-polar bond, as in H₂ and Cl₂. Between different atoms it bends towards the more electronegative one: a polar bond, as in HCl and water.

Count the shared pairs between two atoms: one pair is single, two pairs double, three pairs triple.

Key terms

Covalent bond

Joining non-metal atoms into molecules.

Double bond

Atoms that share two electrons each.

Triple bond

Atoms that share three electrons each.

Polar covalent bond

Bonds between atoms of different electronegativity.

Short questions with model answers

1. Q1. How are the bonds in carbon dioxide formed? (p. 38)

   • C: 4 outer electrons; O: 6 outer electrons
   • C shares 2 with each O; each O shares 2 with C
   • O=C=O

   Carbon shares its four electrons with two oxygens: two double bonds, O=C=O.

2. Q2. Is the bond in HCl polar or non-polar? (p. 39)

   • H and Cl are different atoms
   • Cl is more electronegative
   • Hδ+–Clδ−

   Polar covalent: the pair is pulled towards chlorine.

Common mistakes

• ✗ A double bond shares two electrons.

  ✓ Each atom gives two, so two pairs (four electrons) are shared.

• ✗ The bond in Cl₂ is polar because chlorine is very electronegative.

  ✓ Both atoms are identical and pull equally, so the bond is non-polar.

• ✗ A covalent bond makes ions, like NaCl.

  ✓ No electron leaves; the pair is shared, so neutral molecules form.

MCQs

1. 1. Which has a double covalent bond?

   1. (a) Ethane
   2. (b) Methane
   3. (c) Ethylene
   4. (d) Acetylene
   Show answer

   (c) Exercise MCQ (x), p. 50: ethylene (ethene, C₂H₄) has C=C (Fig 3.6).

2. 2. How many electrons are shared in the N≡N triple bond?

   1. (a) 3
   2. (b) 6
   3. (c) 2
   4. (d) 8
   Show answer

   (b) Each nitrogen shares three electrons: three pairs, six electrons.

3. 3. Which molecule has a non-polar covalent bond?

   1. (a) HCl
   2. (b) H₂O
   3. (c) NH₃
   4. (d) Cl₂
   Show answer

   (d) p. 39: a bond between two identical atoms is non-polar.

4. 4. What type of elements usually form covalent bonds?

   1. (a) Non-metals with non-metals
   2. (b) Metals with non-metals
   3. (c) Noble gases
   4. (d) Metals with metals
   Show answer

   (a) Exercise p. 37: all the molecules in Figs 3.5–3.6 are made of non-metals (H, C, N, O, Cl).

5. 5. In a water molecule, where does each shared pair lean?

   1. (a) Towards hydrogen
   2. (b) Towards oxygen
   3. (c) Nowhere: it is shared equally
   4. (d) It moves fully to oxygen
   Show answer

   (b) p. 39: the bonds in water are polar; the pair bends towards oxygen, the more electronegative atom, but stays shared.

Quick revision

• A covalent bond is a shared electron pair; each atom gives one electron to it.
• One, two or three shared pairs make single, double or triple bonds.
• Identical atoms share equally (non-polar); different atoms share unequally (polar).