Covalent Bonding & Simple Molecular Substances

What is a covalent bond?

A covalent bond forms when two non-metal atoms share a pair of electrons. Each atom contributes one electron to the shared pair, and the shared pair is attracted to the nuclei of both atoms. This electrostatic attraction holds the atoms together.

Atoms bond because it lets them reach a full outer shell (a stable noble-gas electron arrangement). Instead of transferring electrons as in ionic bonding, non-metals share them — neither atom is willing to give electrons away, so sharing is the compromise.

Key terms

Covalent bond — a shared pair of electrons between two non-metal atoms.

Molecule — a group of two or more atoms held together by covalent bonds.

Bonding pair — the shared pair of electrons forming the bond.

Lone pair — a pair of outer electrons not used in bonding.

Showing bonds: dot-and-cross diagrams

In a dot-and-cross diagram we draw electrons from one atom as dots and electrons from the other atom as crosses. This is only to show where each electron came from — in reality all electrons are identical. We usually draw only the outer shell electrons.

The simplest molecule is hydrogen, H₂. Each hydrogen has one electron; sharing gives each a full first shell of two.

The shared pair sits in the overlap of the two atoms. A shared pair can also be drawn as a single line: H–H.

Single, double and triple bonds

Atoms can share more than one pair of electrons:

Chlorine, Cl₂ — each Cl has 7 outer electrons; they share one pair so each reaches 8. The other six outer electrons on each atom form three lone pairs.
Oxygen, O₂ — each O has 6 outer electrons and needs 2 more, so they share two pairs (a double bond).
Nitrogen, N₂ — each N has 5 outer electrons and needs 3 more, so they share three pairs (a triple bond). This triple bond is very strong, which is why nitrogen gas is so unreactive.

Watch out

Count electrons carefully. For oxygen each atom only has 6 outer electrons — if you draw a single bond, each oxygen ends up with only 7 and the diagram is wrong. You must share two pairs to give a full octet of 8.

Important small molecules

Hydrogen chloride, HCl — H shares one electron with Cl. H gets a full shell of 2; Cl gets a full shell of 8 (with three lone pairs left over).

Water, H₂O — oxygen shares one pair with each of two hydrogens. Oxygen keeps two lone pairs.

Ammonia, NH₃ — nitrogen shares one pair with each of three hydrogens, and keeps one lone pair.

Methane, CH₄ — carbon has 4 outer electrons and shares one pair with each of four hydrogens. No lone pairs remain on carbon.

Carbon dioxide, CO₂ — carbon needs 4 electrons and each oxygen needs 2, so carbon forms a double bond to each oxygen (O=C=O). Each oxygen keeps two lone pairs.

Exam tip

A reliable method for any dot-and-cross diagram:

1. Count outer electrons (from the group number) for each atom. 2. Work out how many more each atom needs to fill its shell. 3. Share enough pairs so every atom has a full outer shell — H reaches 2, others reach 8. 4. Put any leftover electrons as lone pairs.

Simple molecular substances and their properties

A simple molecular substance is made of small, separate molecules. There are two kinds of force to keep separate:

Strong covalent bonds inside each molecule (between atoms).
Weak intermolecular forces between the molecules.

These properties follow directly:

Low melting and boiling points. When the substance melts or boils, the molecules move apart but the molecules themselves stay whole. Only the weak intermolecular forces between molecules are broken — not the strong covalent bonds. Little energy is needed, so melting/boiling points are low and many simple molecular substances are gases or liquids at room temperature.
They usually do not conduct electricity. The molecules are neutral overall — there are no free ions and no free (delocalised) electrons to carry charge. This is true even when molten.
Many are insoluble in water (though some, like ammonia and HCl, do dissolve).

Watch out

When CO₂ or water boils, you break intermolecular forces, not covalent bonds. A common exam mistake is saying "the covalent bonds break" — this loses marks. The covalent bonds inside each molecule stay intact.

Ionic vs covalent: a comparison

Real world

Most everyday gases and liquids are simple molecular: the oxygen and nitrogen you breathe, the carbon dioxide you breathe out, water, and methane (natural gas). Their low boiling points are exactly why they are gases at room temperature — a useful everyday reminder that intermolecular forces are weak.