Covalent Bonding and Simple Molecules
Chemistry· 2.5· 20 min read
1. Core: What is Covalent Bonding?★★☆☆☆⏱ 5 min
Covalent Bond
A chemical bond formed when two non-metal atoms share one or more pairs of electrons, resulting in electrostatic attraction between the shared electrons and the positive nuclei of both atoms.
Covalent bonding occurs only between non-metal elements, as both atoms need to gain electrons to achieve a stable full outer electron shell (noble gas configuration). No electrons are transferred, so no charged ions are formed during this process.
Explain why two hydrogen atoms form a covalent bond instead of transferring electrons.
- 1
- Each hydrogen atom has 1 electron in its only shell, needing 1 more to fill its shell and reach stability.
- 2
- Neither hydrogen atom can easily lose its single electron, as this requires a very large input of energy.
- 3
- Instead, both atoms share their single electron to form a shared pair: each atom now has access to 2 electrons, a full outer shell.
Exam tip:
Always explicitly state that covalent bonding occurs between non-metals in exam answers to secure full marks.
2. Core: Dot-and-Cross Diagrams for Simple Molecules★★★☆☆⏱ 7 min
Dot-and-cross diagrams are used to represent electron arrangement in covalent molecules. Electrons from one atom are drawn as dots, and electrons from the other as crosses, to show the origin of each shared electron. Only outer shell electrons are shown in IGCSE diagrams.
Draw the outer electron shell for each atom in the molecule
Count the number of electrons each atom needs to fill its outer shell (usually 8, except hydrogen which needs 2)
Arrange the atoms so each shared electron pair counts towards the outer shell of both bonded atoms
Draw non-bonding (lone) pairs of electrons on each atom as well as shared pairs
Draw a dot-and-cross diagram for a water molecule (H₂O).
- 1
- Oxygen has 6 outer electrons, needing 2 more to fill its shell. Each hydrogen has 1 outer electron, needing 1 more.
- 2
- Place the oxygen atom in the centre, with the two hydrogen atoms on either side.
- 3
- Share one pair of electrons between oxygen and each hydrogen atom: this gives each hydrogen 2 electrons, and oxygen has access to 8 electrons total.
- 4
- Draw the remaining 4 electrons on oxygen as two separate lone pairs.
Draw a dot-and-cross diagram for a hydrogen chloride molecule (HCl).
- 1
- Chlorine has 7 outer electrons, needing 1 more to fill its shell. Hydrogen has 1 outer electron, needing 1 more.
- 2
- Share one pair of electrons between the hydrogen and chlorine atoms: this gives hydrogen 2 electrons and chlorine a full outer shell of 8 electrons.
- 3
- Draw the remaining 6 electrons on chlorine as three separate lone pairs.
Exam tip:
You do not need to draw inner electron shells for dot-and-cross diagrams in 0620 exams, only the outer shell of each atom.
3. Core: Properties of Simple Molecular Covalent Substances★★☆☆☆⏱ 4 min
Simple molecular covalent substances have characteristic properties directly caused by their structure:
Low melting and boiling points: most are liquids or gases at room temperature
Do not conduct electricity in any state (solid, liquid, dissolved)
Usually insoluble in water, but soluble in organic solvents like ethanol
Explain why methane (CH₄) has a boiling point of -161°C.
- 1
- Methane is a simple covalent molecule.
- 2
- The covalent bonds inside each methane molecule are very strong, but weak intermolecular forces between individual methane molecules require very little energy to overcome.
- 3
- When methane boils, only the weak intermolecular forces break, not the strong covalent bonds, so the boiling point is very low.
4. Extended: Intermolecular Forces and Molecular Size★★★★☆Extended only⏱ 4 min
Intermolecular Forces
Weak attractive forces between individual covalent molecules, much weaker than the covalent bonds inside each molecule.
As the size of a covalent molecule increases (i.e. the number of atoms per molecule and relative molecular mass increases), the strength of the intermolecular forces between molecules also increases. This means more energy is needed to overcome these forces, so melting and boiling points are higher for larger simple covalent molecules.
Explain why iodine (I₂) has a much higher boiling point than chlorine (Cl₂).
- 1
- Both iodine and chlorine are simple diatomic covalent molecules.
- 2
- An iodine molecule is larger than a chlorine molecule, with more electrons and a higher relative molecular mass.
- 3
- The intermolecular forces between iodine molecules are stronger than those between chlorine molecules.
- 4
- More energy is required to overcome the stronger intermolecular forces in iodine, so it has a higher boiling point.
Exam tip:
Never state that covalent bonds break when a simple molecular substance melts or boils: this is a very common exam mistake that loses marks. Only the weak intermolecular forces break.
5. Common Pitfalls
Wrong move:
Stating that simple covalent compounds conduct electricity when molten
Why:
Simple covalent compounds have no free ions or delocalised electrons to carry electrical charge, even when liquid
Correct move:
State that simple covalent compounds do not conduct electricity in any state, including molten or dissolved in water
Wrong move:
Claiming covalent bonds break when a simple molecular substance boils
Why:
Boiling only overcomes weak intermolecular forces between molecules; strong covalent bonds inside each molecule remain intact
Correct move:
Explicitly state that only intermolecular forces break during melting/boiling of simple covalent substances
Wrong move:
Drawing dot-and-cross diagrams without showing lone pairs of electrons
Why:
Lone pairs are part of the outer electron shell of bonded atoms and are required for full marks in exam diagrams
Correct move:
Always show all outer shell electrons, including non-bonding lone pairs, in dot-and-cross diagrams
Wrong move:
Stating covalent bonding occurs between metals and non-metals
Why:
Covalent bonding only occurs between non-metal atoms; bonding between metal and non-metal is ionic
Correct move:
Clearly distinguish covalent (non-metal + non-metal) and ionic (metal + non-metal) bonding in all answers
Wrong move:
(Extended only) Claiming intermolecular forces are stronger than covalent bonds
Why:
Intermolecular forces are many times weaker than intramolecular covalent bonds, which is why simple covalent substances have low boiling points
Correct move:
Always specify that intermolecular forces are much weaker than covalent bonds in extended answers
6. Quick Reference Cheatsheet
Concept | Core Tier Requirement | Extended Tier Requirement |
|---|---|---|
Covalent Bond Definition | Define as shared electron pair between non-metals, with electrostatic attraction to positive nuclei | Same as core + compare strength vs intermolecular forces |
Dot-and-Cross Diagrams | Draw for H₂, Cl₂, HCl, H₂O, NH₃, CH₄ | Same as core + O₂, CO₂ (and larger simple molecules as specified in questions) |
Simple Molecule Properties | List low melting/boiling point, no conductivity, solubility rules | Same as core + explain effect of molecular size on melting/boiling point |
7. Frequently Asked
Do simple covalent compounds conduct electricity?
Almost all simple covalent compounds do not conduct electricity in any state (solid, liquid, dissolved) as they have no free charged particles (ions or delocalised electrons) to carry electrical charge.
How many covalent bonds can a non-metal atom form?
The number of covalent bonds an atom forms equals the number of electrons it needs to gain to fill its outer shell: e.g. oxygen needs 2 electrons to fill its outer shell, so it forms 2 covalent bonds.
Going deeper
- study_guideIonic Bonding (0620 Unit 2)
- practice_packBonding Past Paper Questions
What's Next
Now that you have mastered covalent bonding and simple molecules, you can progress to other bonding types in CIE IGCSE Chemistry 0620 Unit 2. Extended tier candidates should next study giant covalent structures and metallic bonding, before moving on to chemical formula writing and mole calculations. Core tier candidates can proceed to practice naming compounds and writing balanced chemical equations, before starting the unit on chemical reactions. Make sure to practice dot-and-cross diagrams repeatedly, as they are a frequent high-mark question topic in both Paper 2 and Paper 4 exams.