(g) Covalent bonding
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
1.44Covalent bondsKnow that a covalent bond is formed between atoms by the sharing of a pair of electrons1.45Covalent electrostatic attractionsUnderstand covalent bonds in terms of electrostatic attractions1.46Covalent dot-and-cross diagramsUnderstand how to use dot-and-cross diagrams to represent covalent bonds in:• diatomic molecules, including hydrogen, oxygen, nitrogen, halogens and hydrogen halides• inorganic molecules including water, ammonia and carbon dioxide• organic molecules containing up to two carbon atoms, including methane, ethane, ethene and those containing halogen atoms.1.47Simple molecular structuresExplain why simple molecular substances are gases, liquids or low-melting solids in terms of intermolecular forces of attraction.1.48Molecular mass and boiling pointExplain why the melting and boiling points of substances with simple molecular structures increase, in general, with increasing relative molecular mass1.49Giant covalent structuresExplain why substances with giant covalent structures are solids with high melting and boiling points1.50Diamond, graphite and fullereneExplain how the structures of diamond, graphite and C60 fullerene influence their physical properties, including electrical conductivity and hardness1.51Covalent conductivityKnow that covalent compounds do not usually conduct electricity
Define a covalent bond
A covalent bond is a shared pair of electrons between two atoms.
Each atom contributes to or uses the shared pair, so the electrons count in the outer shell of both bonded atoms. One shared pair is a single covalent bond; two or three shared pairs form double or triple bonds.
| Molecule | Shared pairs between the two atoms | Bond |
|---|---|---|
| HX2 | 1 | single |
| OX2 | 2 | double |
| NX2 | 3 | triple |
Covalent bonding shares electrons; it does not transfer electrons to form ions. A line in a displayed formula represents one shared pair, not one electron.
Explain the attraction in a covalent bond
A covalent bond is the strong electrostatic attraction between a shared pair of negatively charged electrons and the positively charged nuclei of both bonded atoms.
The shared electrons lie between the nuclei and are attracted to both. This two-nucleus attraction holds the atoms together; saying only that electrons are shared describes the arrangement but not the electrostatic mechanism.
| Part | Charge | Role in the bond |
|---|---|---|
| shared electron pair | negative | attracted to both nuclei |
| nucleus of each atom | positive | attracts the shared pair |
Use nuclei in the plural. The bond is not an attraction between two nuclei, between neutral atoms as wholes, or an intermolecular force between separate molecules.
Construct covalent dot-and-cross diagrams
A covalent dot-and-cross diagram shows every outer electron and which atom it came from. A dot and a cross are only origin labels; all electrons are identical.
| Step | Check |
|---|---|
| 1 | Draw the required atoms with overlapping outer-shell regions. |
| 2 | Put one electron from each bonded atom into each shared pair. |
| 3 | Add enough shared pairs for single, double or triple bonds. |
| 4 | Add all remaining outer electrons as lone pairs and recount each atom. |
Use the same accounting for the required range: HX2, OX2, NX2, halogens and hydrogen halides; HX2O, NHX3 and COX2; and up-to-two-carbon molecules such as methane, ethane, ethene and halogen derivatives. For COX2, carbon shares two pairs with each oxygen; each oxygen also has two lone pairs.
Show only outer electrons. Do not add ionic brackets or charges to neutral molecules, and do not omit lone pairs: a correct displayed formula can still be an incomplete dot-and-cross diagram.
Explain low melting and boiling points of simple molecules
A simple molecular substance consists of separate, small molecules. Strong covalent bonds hold atoms together inside each molecule, but the forces of attraction between different molecules are weak.
Melting or boiling separates molecules by overcoming intermolecular forces. Because these forces are weak, little thermal energy is needed, so simple molecular substances are often gases, liquids or low-melting solids.
| Location | Force or bond | What happens on melting/boiling? |
|---|---|---|
| within a molecule | strong covalent bonds | remain intact |
| between molecules | weak intermolecular forces | overcome |
Do not say that covalent bonds break when a simple molecular substance melts or boils. The molecules remain chemically unchanged; only their separation and movement change.
Link molecular mass to melting and boiling point
For similar simple molecular substances, melting and boiling points generally increase as relative molecular mass increases.
Larger, heavier molecules usually have more electrons and stronger intermolecular attractions. More thermal energy is therefore needed to separate the molecules, giving a higher melting or boiling point.
| Group 7 molecule | Relative size/mass | Boiling point trend |
|---|---|---|
| FX2 | smallest | lowest |
| ClX2 | intermediate | higher |
| BrX2 | largest | highest |
The relationship is a general trend for comparable simple molecules, not a claim of direct proportionality. The change is in intermolecular forces; the covalent bonds inside each molecule are not broken during boiling.
Explain high melting points of giant covalent structures
A giant covalent structure is a continuous network of atoms joined by many strong covalent bonds. It is not made of separate molecules.
To melt or boil the substance, many strong covalent bonds throughout the network must be broken. This requires a large amount of thermal energy, so giant covalent substances are solids with high melting and boiling points.
| Structure | Particle unit | Attraction overcome on melting | Energy needed |
|---|---|---|---|
| simple molecular | separate molecules | weak intermolecular forces | little |
| giant covalent | continuous atom network | many strong covalent bonds | large |
Do not use intermolecular forces to explain a giant covalent melting point: there are no separate molecules. 'More energy' is incomplete unless it is linked to breaking many strong covalent bonds.
Connect carbon structures to their properties
Diamond, graphite and CX60 fullerene contain only carbon, but their atoms are connected differently. Structure controls whether strong bonds extend through the solid, whether layers can slide, and whether electrons can carry charge through the sample.
| Form | Structure and bonding | Hardness | Electrical behaviour |
|---|---|---|---|
| Diamond | each C bonded to 4 others in a rigid 3-D giant network | very hard because strong bonds hold every direction | does not conduct; no delocalised electrons |
| Graphite | each C bonded to 3 others in hexagonal layers; weak attractions between layers | soft because layers slide | conducts; one delocalised electron per C can move through the structure |
| CX60 fullerene | separate hollow molecules; each C bonded to 3 others; weak forces between molecules | soft compared with diamond because molecules separate or move more easily | poor conductor as a molecular solid because charge cannot move freely from molecule to molecule |
Diamond and graphite have high melting points because many strong covalent bonds in their giant structures must be broken. CX60 has a much lower melting point because melting overcomes weak forces between its molecules, not the covalent bonds within each cage.
Graphite layers are sheets of atoms, not molecules, so do not call the attractions between them intermolecular. Delocalised electrons—not ions or moving carbon atoms—explain graphite's conductivity.
Explain why covalent compounds usually do not conduct
Covalent compounds do not usually conduct electricity because they do not usually contain charged particles that are free to move through the substance.
Their electrons are held in covalent bonds or localized around atoms, and their molecules are neutral. Giant covalent compounds such as silicon dioxide also lack mobile ions or delocalised electrons, so charge cannot flow through the structure.
| Possible charge carrier | Typical covalent compound |
|---|---|
| mobile ions | absent |
| delocalised electrons moving through the whole structure | absent |
The word 'usually' matters: a covalent substance may form ions when it reacts with water, and graphite is a covalently bonded element with mobile delocalised electrons. Neither case changes the general rule for covalent compounds themselves.