Covalent Bonds

A covalent bond is a shared pair of electrons between two non‑metal atoms. By sharing, each atom reaches a stable outer shell like a noble gas (often called the octet). Hydrogen aims for 2 electrons; most others aim for 8.

Why atoms share

Think of two neighbours sharing a fence. Each uses the same fence, and both gardens are secure. In a covalent bond, the shared pair of electrons “belongs” to both atoms, helping each feel full on the outside shell.

Single, double and triple bonds

• Single bond: 1 shared pair (2 electrons). Example: $\mathrm{H_2}$, $\mathrm{Cl_2}$, $\mathrm{HCl}$.
• Double bond: 2 shared pairs (4 electrons). Example: $\mathrm{O_2}$, $\mathrm{CO_2}$.
• Triple bond: 3 shared pairs (6 electrons). Example: $\mathrm{N_2}$.

How many bonds does an atom usually form?

• H and Cl make 1 bond
• O makes 2 bonds
• N makes 3 bonds
• C makes 4 bonds

Examples: $\mathrm{CH_4}$ (carbon 4 bonds), $\mathrm{H_2O}$ (oxygen 2 bonds), $\mathrm{NH_3}$ (nitrogen 3 bonds), $\mathrm{HCl}$ (H 1 + Cl 1).

Dot-and-cross idea (no drawing)

Show electrons from one atom as dots and the other as crosses. A shared pair sits between the symbols of the two atoms. For $\mathrm{H_2}$: one dot and one cross together between the H atoms. For $\mathrm{H_2O}$: two shared pairs between O and each H, plus two lone pairs on O.

Properties of simple molecular substances

• Low melting and boiling points: molecules are held together by weak forces between molecules, even though the covalent bonds inside each molecule are strong.
• Poor conductors: no free ions or delocalised electrons to carry charge.

Common misconceptions

• Covalent bonding is sharing, not transferring electrons (that is ionic).
• The covalent bond inside a molecule is strong; it is the forces between molecules that are weak.
• Molecules are neutral overall; shared electrons are not “free.”