2.5 Simple molecules and covalent bonds
- Syllabus
- 0620–2026–2027
- Topic
- 2.5
- Level
- —
Learning objectives
2.5.1Covalent bond is formed when a pair of• State: a covalent bond is formed when a pair of electrons is shared between two atoms leading to noble gas electronic configurations2.5.2Formation of covalent bonds in simple• Describe formation of covalent bonds in simple molecules, including H2, Cl2, H2O, CH4, NH3 and HCl. Use dot-and-cross diagrams to show the electronic configurations in these and similar molecules2.5.3Simple molecular compound properties• Describe simple molecular compound properties from structure/bonding: low melting/boiling points and poor electrical conductivity2.5.4Formation of covalent bonds in simple• Describe formation of covalent bonds in simple molecules, including CH3OH, C2H4, O2, CO2 and N2. Use dot-and-cross diagrams to show the electronic configurations in these and similar molecules2.5.5Simple molecular compound properties• Explain simple molecular compound properties: low melting/boiling points due to weak intermolecular forces, and poor electrical conductivity
Define a covalent bond
A covalent bond is formed when a pair of electrons is shared between two atoms, giving each atom access to a noble-gas electronic configuration.
The shared pair is attracted to both nuclei and counts in the outer shell of both bonded atoms. Hydrogen reaches a full first shell with two electrons; many other simple non-metal atoms reach eight outer electrons.
A covalent bond is the shared pair itself, not a transfer of electrons and not the weaker attraction between separate molecules.
Draw single covalent bonds in simple molecules
One single covalent bond contains one shared pair of electrons. In a dot-and-cross diagram, use a different symbol for electrons originating from each bonded atom and count the shared pair for both atoms.
| Molecule | Bond pattern | Non-bonding outer electrons |
|---|---|---|
| H₂ | H–H; one shared pair | none |
| Cl₂ | Cl–Cl; one shared pair | three lone pairs on each Cl |
| HCl | H–Cl; one shared pair | three lone pairs on Cl |
| H₂O | two O–H single bonds | two lone pairs on O |
| NH₃ | three N–H single bonds | one lone pair on N |
| CH₄ | four C–H single bonds | none on C |
Show outer-shell electrons only when instructed. Every H should count two electrons around it; C, N, O and Cl should count eight from bonding pairs plus lone pairs.
Lone-pair electrons are not shared and must stay on their atom. Do not place brackets or ionic charges around a covalent molecule.
Describe simple molecular properties
A simple molecular substance consists of small, discrete molecules with strong covalent bonds inside each molecule.
| Property | Description |
|---|---|
| melting point | low |
| boiling point | low |
| electrical conductivity | poor; normally does not conduct in solid or liquid form |
Simple molecular describes separate molecules, not a giant covalent network. The covalent bonds within a molecule are not described as weak.
Draw single, double and triple covalent bonds
A single bond has one shared pair, a double bond has two shared pairs and a triple bond has three shared pairs of electrons.
| Molecule | Connectivity and bonding | Lone pairs to show |
|---|---|---|
| CH₃OH | H₃C–O–H; five single bonds | two on O |
| C₂H₄ | H₂C=CH₂; four C–H single bonds and one C=C double bond | none on C |
| O₂ | O=O; one double bond | two on each O |
| CO₂ | O=C=O; two double bonds | two on each O |
| N₂ | N≡N; one triple bond | one on each N |
Start from the displayed connectivity, place the required shared pair(s) between bonded atoms, then add lone pairs until each H has two outer electrons and each C, N or O has eight.
Keep dots and crosses consistent with electron origin. A double bond must show four shared electrons between the same two atoms; a triple bond must show six.
Bond order counts shared pairs, not lines elsewhere in the structural formula. Do not add lone pairs to carbon in the listed neutral molecules after its octet is complete through bonding.
Explain simple molecular properties
Melting or boiling a simple molecular substance separates molecules from one another; it does not break the covalent bonds inside each molecule.
| Property | Structure-and-bonding explanation |
|---|---|
| low melting and boiling points | forces between molecules are weak, so little energy is needed to overcome them |
| poor electrical conductivity | the molecules are uncharged and there are no mobile ions or delocalised electrons to carry charge |
The general name for the weak attractions between separate molecules is intermolecular forces. Specific types of intermolecular force are not required by this syllabus objective.
Covalent bonds within molecules are strong; intermolecular forces between molecules are much weaker. This difference is why a molecular substance can melt without its molecules decomposing.
Do not explain a low boiling point by saying covalent bonds are weak. The forces overcome during boiling are intermolecular forces.