Covalent Bonding and Small Molecules

When two non-metals combine, neither atom wants to give its electrons away — both need to gain electrons to fill their outer shells. The solution is to share: this is covalent bonding, the second of the three bond types in Topic C2 of OCR Gateway Science A. Covalent bonding produces the small molecules that make up so much of the world — the oxygen and carbon dioxide in the air, the water you drink, the methane that heats your home. This lesson explains how covalent bonds form, how to draw the molecules, and — most importantly for the exam — why simple molecular substances have low melting points, a property that students very often explain incorrectly.

By the end of this lesson you should be able to explain how covalent bonds form by sharing electrons, draw dot-and-cross and displayed formulae for small molecules, identify single, double and triple bonds, and explain the properties of simple molecular substances from their structure.

---

How Covalent Bonds Form

A covalent bond forms between non-metal atoms when they share a pair of electrons. Each atom contributes one electron to the shared pair, and the shared pair counts towards the outer shell of both atoms — so both can reach a full outer shell.

A covalent bond is a strong bond, because the shared pair of electrons is strongly attracted to the nuclei of both atoms, holding them together. One shared pair is a single bond (drawn as one line); two shared pairs make a double bond (two lines); three shared pairs make a triple bond (three lines).

Take hydrogen, $\text{H}_2$H2​. Each hydrogen atom has 1 electron and needs 2 for a full first shell. By sharing their electrons — one from each — the two atoms form a shared pair, and each hydrogen now "counts" two electrons. That shared pair is the covalent bond holding $\text{H}_2$H2​ together.

Exam Tip: Covalent bonding = non-metals sharing pairs of electrons, with each atom giving one electron to each shared pair, so both reach a full outer shell. The shared pair is held strongly by both nuclei — that is why the bond is strong (remember this for the low-melting-point trap below).

---

Dot-and-Cross Diagrams for Small Molecules

A dot-and-cross diagram for a covalent molecule shows the shared pair(s) in the overlap between the atoms — one dot and one cross in each shared pair, so you can see each atom contributed one electron.

Here are the molecules you should be able to draw, with the number of shared pairs (bonds):

Molecule	Formula	Bonds
Hydrogen	$\text{H}_2$H2​	one single bond
Chlorine	$\text{Cl}_2$Cl2​	one single bond
Hydrogen chloride	$\text{HCl}$HCl	one single bond
Water	$\text{H}_2\text{O}$H2​O	two single bonds
Ammonia	$\text{NH}_3$NH3​	three single bonds
Methane	$\text{CH}_4$CH4​	four single bonds
Oxygen	$\text{O}_2$O2​	one double bond
Carbon dioxide	$\text{CO}_2$CO2​	two double bonds
Nitrogen	$\text{N}_2$N2​	one triple bond

A displayed formula is a simpler way to show the same thing: each shared pair (covalent bond) is drawn as a line between the atoms. So water is H–O–H, methane is a central C with four H atoms each joined by a single line, oxygen is O=O (double bond), and nitrogen is N≡N (triple bond).

Exam Tip: A single line = one shared pair (single bond), a double line = two shared pairs (double bond, e.g. $\text{O}_2$O2​, $\text{CO}_2$CO2​), a triple line = three shared pairs (triple bond, e.g. $\text{N}_2$N2​). Count the lines to count the shared pairs.

---

Simple Molecular Substances: Structure

Substances made of small covalent molecules — like the ones above — are called simple molecular substances. Their structure has two very different kinds of force, and keeping these apart is the single most important idea in this lesson:

• Strong covalent bonds hold the atoms together within each molecule.
• Weak intermolecular forces act between separate molecules (the molecules attract one another only weakly).

This distinction explains everything about their properties. The bonds inside a molecule are strong, but the forces between molecules are weak — and it is only those weak forces that matter when the substance melts or boils.

flowchart TD
  A["Simple molecular substance"] --> B["Strong covalent bonds<br/>WITHIN each molecule"]
  A --> C["Weak intermolecular forces<br/>BETWEEN molecules"]
  C --> D["Only these weak forces<br/>are overcome on melting/boiling"]
  D --> E["Low melting and boiling points"]

---

Properties Explained by the Structure

Property	Explanation from structure
Low melting and boiling points	Melting/boiling only has to overcome the weak intermolecular forces (not the strong covalent bonds), so little energy is needed
Often gases or liquids (or low-melting solids) at room temperature	The weak intermolecular forces are easily overcome
Do NOT conduct electricity	The molecules are neutral — there are no free electrons or ions to carry charge
Boiling point rises with molecule size	Larger molecules have stronger intermolecular forces, so more energy is needed to separate them

The low melting point explanation is the classic exam trap. When a simple molecular substance melts or boils, you are only overcoming the weak intermolecular forces between the molecules. You are NOT breaking the strong covalent bonds inside the molecules — those stay intact (the molecules move apart whole). So saying "the covalent bonds are weak" or "the covalent bonds break when it melts" is wrong: the bonds are strong, but the forces between molecules are weak.

Exam Tip: This is worth a guaranteed mark: when a simple molecular substance melts, the weak intermolecular forces are overcome, not the strong covalent bonds. Never write "the covalent bonds break" or "the covalent bonds are weak" — that loses the mark every time.

Worked Example 1 — Explaining a low boiling point

Explain why methane ($\text{CH}_4$CH4​) has a very low boiling point.

Step 1 — methane is a simple molecular substance: strong covalent bonds within each molecule, weak intermolecular forces between molecules.

Step 2 — to boil it, you only have to overcome the weak intermolecular forces between the molecules.

Step 3 — these forces need little energy to overcome, so the boiling point is low. The strong covalent bonds inside the molecules are not broken.

Answer: methane boils at a low temperature because only the weak intermolecular forces between molecules are overcome; the strong covalent bonds within molecules stay intact.

Worked Example 2 — A trend in boiling point