Covalent Bonding and Simple Molecular Structures
Covalent bonding happens between non-metals, by sharing electrons. The properties all follow from one distinction that students routinely miss: the difference between the bonds inside a molecule and the forces between molecules.
1. The definition
A covalent bond is a SHARED PAIR OF ELECTRONS between two atoms.
Atoms share so that each achieves a full outer shell.
Covalent = SHARE. Ionic = TRANSFER. Confusing the two was recorded repeatedly across both topics. If two non-metals are bonding, electrons are shared.
Covalent bonds form between NON-METALS only.
2. How many bonds each atom forms
The number of covalent bonds an atom forms equals the number of electrons it needs to fill its outer shell:
| Atom | Outer electrons | Needs | Bonds formed |
|---|---|---|---|
| Hydrogen | 1 | 1 | 1 |
| Chlorine (Group VII) | 7 | 1 | 1 |
| Oxygen (Group VI) | 6 | 2 | 2 |
| Nitrogen (Group V) | 5 | 3 | 3 |
| Carbon (Group IV) | 4 | 4 | 4 |
Hydrogen needs only 2 electrons to fill its shell — so it forms ONE bond. Recorded errors had hydrogen needing 7, and hydrogen’s requirement misunderstood twice more. Its first shell holds a maximum of 2.
Oxygen needs TWO more electrons, not one. Also recorded — and it is why oxygen forms two bonds, or one double bond.
Every other atom needs 8 outer electrons. A recorded error denied that oxygen needs 8 for stability.
Check the group number. A recorded error placed nitrogen in Group VI instead of Group V. The group number gives the outer electrons, which gives the bonds.
3. Single, double and triple bonds
Single bond — one shared pair (H₂, HCl, H₂O, CH₄) Double bond — two shared pairs (O₂, CO₂) Triple bond — three shared pairs (N₂)
Carbon dioxide has DOUBLE bonds: O=C=O. Drawing single bonds for CO₂ was a specific recorded error — carbon needs 4 bonds and each oxygen needs 2, which only works with two double bonds.
Confusing single and double bonds was recorded twice. Count what each atom needs before you draw.
4. Dot-and-cross diagrams
Conventions:
- Use dots for one atom’s electrons and crosses for the other’s
- Show shared pairs in the overlap between the two atoms
- Show all outer-shell electrons, including lone pairs
- Check every atom ends up with a full outer shell
Dot-and-cross diagrams can earn 3 marks, and tutors flagged that they must show complete sharing — every atom’s outer shell filled, with the shared pairs clearly in the overlap.
Common molecules to practise: H₂, Cl₂, HCl, H₂O, NH₃, CH₄, CO₂, O₂, N₂.
Count electrons carefully. An incorrect electron count for sulfur was recorded — use the periodic table.
5. Simple molecular substances
Simple molecular substances consist of small, separate molecules.
Two very different forces are involved, and this is the crux of the topic:
INSIDE each molecule: STRONG covalent bonds (intramolecular) BETWEEN molecules: WEAK intermolecular forces
Confusing intramolecular with intermolecular forces was recorded, and it makes the properties impossible to explain.
Low melting and boiling points
Simple molecular substances have low melting and boiling points because only the WEAK INTERMOLECULAR FORCES between molecules need to be overcome — NOT the strong covalent bonds within them.
This is the phrase that earns the marks, and tutors flagged it explicitly. Say “weak intermolecular forces between molecules” — the covalent bonds themselves are not broken when the substance melts or boils.
Larger molecules have stronger intermolecular forces, so melting and boiling points increase with molecular size.
Poor electrical conductivity
Simple molecular substances do not conduct, because the molecules are neutral — there are no free ions or free electrons to carry the charge.
Solubility
Many are insoluble in water but soluble in organic solvents.
6. Structure vs bonding
Exam questions distinguish these, and students routinely blur them.
BONDING is the type of force holding particles together — ionic, covalent or metallic. STRUCTURE is the arrangement — simple molecular, giant ionic, giant covalent or giant metallic.
“Metallic structure” is not the same as “metallic bond”. A recorded error swapped them, and confusion about the difference between structure and bonds was recorded separately.
A full answer names both: for example, “sodium chloride has ionic bonding and a giant ionic lattice structure”.
Don’t mention intermolecular forces for a giant covalent structure — a recorded error. Giant structures like diamond have no separate molecules, so there is nothing between them.
7. Identifying the bonding type
| Elements involved | Bonding | Structure |
|---|---|---|
| Metal + non-metal | ionic | giant ionic lattice |
| Non-metal + non-metal | covalent | usually simple molecular |
| Metal only | metallic | giant metallic |
Use the properties as evidence: high melting point and conducts when molten → ionic; low melting point and doesn’t conduct → simple molecular; conducts when solid and is malleable → metallic.
8. Mistakes that cost marks
Saying electrons are transferred in a covalent bond.
Giving hydrogen 8 electrons instead of 2.
Saying oxygen needs one electron instead of two.
Drawing single bonds in CO₂.
Wrong group number for an element.
Incomplete dot-and-cross diagrams.
Saying covalent bonds break on melting — the intermolecular forces do.
Omitting the word “intermolecular” in a melting point explanation.
Confusing structure with bonding.
Mentioning intermolecular forces in a giant covalent structure.
Frequently asked questions
What is a covalent bond? A shared pair of electrons between two atoms.
Which elements form covalent bonds? Non-metals with non-metals.
How many bonds does carbon form? Four.
How many electrons does hydrogen need? Two — so it forms one bond.
What kind of bonds does CO₂ have? Two double bonds — O=C=O.
Why do simple molecular substances have low melting points? Only the weak intermolecular forces between molecules need overcoming — not the strong covalent bonds.
Do covalent bonds break when a substance melts? No — only the intermolecular forces.
Why don’t they conduct electricity? The molecules are neutral — no free ions or electrons.
What’s the difference between structure and bonding? Bonding is the type of force; structure is the arrangement of particles.
How do I tell ionic from covalent? Metal + non-metal is ionic; two non-metals is covalent.
Quick revision checklist
- I can define a covalent bond as a shared pair
- I know covalent bonding is between non-metals
- I know how many bonds each common atom forms
- I know hydrogen needs 2 electrons and others need 8
- I can identify single, double and triple bonds
- I know CO₂ has double bonds
- I can draw complete dot-and-cross diagrams with lone pairs
- I know the difference between intramolecular and intermolecular forces
- I can explain low melting points using weak intermolecular forces
- I know covalent bonds are not broken on melting
- I can explain poor conductivity
- I can distinguish structure from bonding and name both
- I don’t mention intermolecular forces for giant structures
These notes cover covalent bonding and simple molecular structures in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and data booklet for your own exam series.
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