Simple Molecules and Covalent Bonds

When non-metal atoms bond, they share pairs of electrons to complete their outer shells. A shared pair is a covalent bond: strong. Simple molecules (like $\mathrm{H_2O}$ , $\mathrm{CO_2}$ , $\mathrm{CH_4}$ ) are held to each other only by weak intermolecular forces, so they have low melting and boiling points, and they don't conduct electricity because they have no charged particles free to move.

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Section 1 of 14

Learning objectives

Describe a covalent bond as a shared pair of electrons.
Draw dot-and-cross diagrams, including double and triple bonds.
Explain the properties of simple molecular substances.

Pause and think

The O–H bonds in water are very strong. So why does water boil at only $100\,^\circ\mathrm{C}$ ?

Section 2 of 14

Core idea

Start hereSharing, not stealing

Two chlorine atoms each need one more electron. Neither will give one up, so they compromise: each puts one electron into a shared pair, and both end up with a full outer shell.

The big idea

A covalent bond is a shared pair of electrons between non-metal atoms. Bonds inside simple molecules are strong, but the forces between molecules are weak, so simple molecular substances have low melting and boiling points.

1. How many bonds?

Hydrogen: 1 bond. Halogens: 1. Oxygen: 2. Nitrogen: 3. Carbon: 4.
Each bond is one shared pair; a double bond is two pairs, a triple bond three.

Worked exampleCarbon dioxide

Work out the bonding in $\mathrm{CO_2}$ .

Goal: Bonds and lone pairs.
Strategy: C needs 4 more electrons, each O needs 2.

$\mathrm{O{=}C{=}O}$
Two double bonds give C four bonds and each O two.
$\text{each O: 2 lone pairs}$
O has 6 outer electrons; 2 are shared, 4 remain.

AnswerTwo C=O double bonds; 2 lone pairs on each oxygen.

✓ CheckEvery atom has 8 outer electrons ✓.

2. Simple molecular properties

Two kinds of force

Drawing diagram…

Boiling overcomes only the weak forces between molecules.

Bigger molecules have stronger intermolecular forces, so boiling points rise along a series such as methane ( $-162\,^\circ\mathrm{C}$ ), ethane ( $-89$ ), propane ( $-42$ ), butane ( $-1$ ).

Common misconceptionWeak bonds? Never say simple molecules have "weak bonds". The covalent bonds are strong; the forces between molecules are weak.

Key takeawayNo conduction Simple molecular substances have no ions and no delocalised electrons, so they don't conduct, even when liquid.

Pause and think

Iodine is a solid at room temperature but chlorine is a gas. Both are $X_2$ molecules. Why?

Summary

Shared pair = covalent bond.
H 1, O 2, N 3, C 4.
Weak intermolecular forces → low mp/bp; no conduction.

Covalent bond: A shared pair of electrons between two atoms.
Lone pair: A pair of outer electrons not used in bonding.
Intermolecular forces: Weak attractions between separate molecules.

HistoryLewis. In 1916 Gilbert Lewis proposed that atoms bond by sharing pairs of electrons and drew the first dot diagrams, the ancestors of today's dot-and-cross diagrams.

Fun factIce floats. Water is unusual: its molecules arrange into an open lattice when they freeze, so ice is less dense than liquid water. That's why lakes freeze from the top down and fish survive winter.

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