Introduction
So far you've met ionic bonding, where electrons are transferred completely from a metal to a non-metal. But what happens when two non-metal atoms meet? Neither one is willing to give up its electrons completely — instead, they share them. This is the basis of the covalent bond, one of the most important ideas in chemistry, since it explains how molecules like H₂O, CO₂, and even DNA hold together.
What is a covalent bond?
A covalent bond forms when two atoms share one or more pairs of electrons. Each shared pair is attracted simultaneously to both positive nuclei, and it's this mutual attraction that holds the atoms together (Talbot, p.143).
Think of the simplest example, H₂:
- Two hydrogen nuclei (protons) each contribute one electron.
- The shared pair sits between the nuclei.
- Both nuclei are attracted to this same pair, pulling the atoms together.
A simple electrostatic picture shows that when the electron pair sits exactly between the two nuclei, the attraction is stronger than if the electrons were unevenly placed — this is why sharing electrons stabilises the system (Talbot, p.143).
Single, double, and triple bonds
Sometimes one shared pair isn't enough for both atoms to achieve a full outer shell (a stable octet). In these cases, atoms share more than one pair:
- Single bond – one shared pair (e.g. H–H, C–C)
- Double bond – two shared pairs (e.g. O=O in O₂)
- Triple bond – three shared pairs (e.g. N≡N in N₂)
As the number of shared pairs increases, the nuclei are pulled closer together and held more tightly. This means:
- Bond length decreases from single → double → triple
- Bond enthalpy (strength) increases from single → double → triple
Worked Example 1: Compare C–C, C=C b