Understanding covalent bonds and drawing molecules Revision Notes | GCSE AQA Higher Chemistry

1. Why Atoms Share Electrons

Picture two people trying to complete a jigsaw puzzle. Each has some pieces, but neither has enough to finish alone. The solution? Share pieces to complete the picture together. Atoms do something similar with electrons.

Non-metal atoms need to fill their outer electron shells to become stable. When they can't steal electrons (like metals do), they share them instead. This sharing creates covalent bonds - some of the strongest connections in chemistry.

2. Drawing Molecules - The Dot and Cross Method

Before we can understand how molecules work, we need to see their electron structure. Dot and cross diagrams are like X-rays for molecules - they show us exactly where every electron lives.

The rules are simple: dots represent electrons from one atom, crosses from another. Shared pairs sit between the atoms, whilst lone pairs stay with their original atom. This visual method makes bonding crystal clear.

3. Simple Molecules - Hydrogen to Methane

Let's build molecules step by step, starting with the simplest and working up to more complex structures. Each molecule follows the same principle: atoms share electrons to fill their outer shells.

Worked example: Drawing Hydrogen Molecule (H₂)

Draw the dot and cross diagram for hydrogen gas, H₂

1. Each H atom has 1 electron in outer shell — Hydrogen is in Group 1, so has 1 outer electron
2. Each H needs 1 more electron to fill shell (2 total) — Hydrogen's outer shell is full with 2 electrons
3. H• ×H becomes H•×H — The shared pair (•×) counts for both atoms
4. H—H with shared pair shown as •× between them — This represents one covalent bond

Answer: H₂ has one covalent bond formed by sharing one pair of electrons

Now let's tackle oxygen, which needs two bonds. Oxygen atoms have 6 outer electrons each, so need 2 more to reach the stable number of 8.

Worked example: Drawing Oxygen Molecule (O₂)

Draw the dot and cross diagram for oxygen gas, O₂

1. Each O atom has 6 electrons in outer shell — Oxygen is in Group 6
2. Each O has 2 lone pairs: ••O•• and ××O×× — 4 electrons arranged as 2 pairs on each atom
3. ••O•× וO×× — Two pairs shared = double bond
4. Each O now has 8 electrons around it — 2 lone pairs + 2 shared pairs = 8 electrons

Answer: O₂ has a double covalent bond (O=O) with two lone pairs on each oxygen

4. Complex Molecules - Ammonia and Methane

The real test comes with molecules where one atom bonds to multiple others. Ammonia (NH₃) and methane (CH₄) show how central atoms can form several bonds simultaneously.

Worked example: Drawing Ammonia (NH₃)

Draw the dot and cross diagram for ammonia, NH₃

1. N has 5 outer electrons, each H has 1 — Nitrogen is in Group 5, hydrogen in Group 1
2. N with 5 electrons: ••N••• — Nitrogen will be the central atom
3. Three H atoms each contribute 1 electron — H×, H×, H× ready to bond
4. N shares 3 electrons with 3 H atoms — N now has 8 electrons (3 bonds + 1 lone pair)

Answer: NH₃ has three single covalent bonds and one lone pair on nitrogen

Methane takes this further - carbon forms four identical bonds. This creates a perfectly symmetrical molecule that's the basis of all organic chemistry.

5. Properties of Covalent Compounds

Understanding bonding helps predict properties. Covalent compounds behave very differently from ionic ones because of how their electrons are arranged.

Strong bonds within molecules, but weak forces between molecules. This explains why water boils at 100°C (breaking intermolecular forces) but doesn't decompose until much higher temperatures (breaking covalent bonds).

6. Exam Technique - Getting Full Marks

Drawing molecular diagrams in exams requires precision. Examiners look for specific features, and missing any one can cost marks.

Command word 'Draw': Show all outer electrons as dots and crosses. Shared pairs must be clearly between atoms. Don't forget lone pairs - they're often worth separate marks.

Common exam mistakes: Forgetting lone pairs on oxygen in water, miscounting electrons in nitrogen compounds, or drawing ionic structures for covalent molecules. Practice the standard molecules until they're automatic.

Exam precision note: In Understanding covalent bonds and drawing molecules - Covalent bonding - Chemical bonds, ionic, covalent and metallic, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

Students need to recognise more than just hydrogen and methane in covalent dot-and-cross diagrams. Chlorine forms Cl2 with one shared pair and three lone pairs on each atom. Nitrogen forms N2 with three shared pairs and one lone pair on each nitrogen atom. Hydrogen chloride forms HCl with one shared pair and three lone pairs on the chlorine atom. These examples show why covalent bonding is about shared pairs of electrons.