Covalent bonding — small molecules to giant structures
Covalent bonds form between non-metal atoms that share electron pairs to fill their outer shells. Covalent compounds come in three structural types, with very different properties:
- Simple molecules — small clusters of atoms (H₂, O₂, H₂O, CO₂).
- Giant covalent structures — millions of atoms in one continuous network (diamond, graphite, silicon dioxide).
- Polymers — very long chain molecules (poly(ethene), DNA, proteins).
Simple molecules — examples
- H₂ (hydrogen): each H shares its 1 electron with the other → 2 electrons in shell 1 of both.
- Cl₂: each Cl has 7 outer electrons; share 1 pair → both have 8.
- HCl: H and Cl share 1 pair.
- H₂O: O shares with 2 H atoms (2 single bonds).
- NH₃: N shares with 3 H atoms.
- CH₄: C shares with 4 H atoms.
- O₂: 2 O atoms share two pairs (double bond) → 8 electrons each.
- CO₂: 2 double bonds (O=C=O).
- N₂: triple bond — 3 shared pairs.
Properties of simple molecular substances
- Low melting and boiling points. Many are gases or liquids at room temperature. Why? The covalent bonds within the molecule are strong, but the forces between molecules (intermolecular forces) are weak. Only the weak intermolecular forces need to be overcome to melt or boil.
- Don't conduct electricity — molecules are electrically neutral; no free charged particles.
- Often insoluble in water unless they can H-bond.
- Soft when solid (e.g. iodine).
Giant covalent structures (macromolecules)
Atoms bonded by covalent bonds in all directions through the whole structure. Three GCSE examples:
Diamond
- Each carbon bonded to 4 other carbons in a tetrahedral arrangement.
- Hard, very high melting point — many strong covalent bonds.
- Doesn't conduct — no free electrons (all 4 outer electrons used in bonding).
- Used in cutting tools and jewellery.
Graphite
- Each carbon bonded to 3 other carbons in flat hexagonal sheets.
- The 4th outer electron of each C is delocalised within the layer.
- Soft, slippery — layers slide over each other.
- Conducts electricity — delocalised electrons carry charge along layers.
- Used in pencils, lubricants, electrodes (e.g. in electrolysis).
Silicon dioxide (silica, SiO₂)
- Found in sand and quartz.
- Each Si bonded to 4 O; each O bonded to 2 Si.
- Hard, very high melting point — used in glass, sandpaper, optical fibres.
Polymers — long chain molecules
A polymer is built from many small monomer units joined by covalent bonds. Examples:
- Poly(ethene) — many ethene (C₂H₄) units link to form long carbon chains.
- DNA — biological polymer of nucleotides.
- Starch and cellulose — biological polymers of glucose.
The covalent bonds within a polymer chain are strong; the intermolecular forces between chains are weaker but still significant (because the chains are long, many forces act between them). So polymers are usually solid at room temperature with melting points higher than small molecules.
How to draw covalent bonding
Dot-and-cross: show outer shells overlapping; mark electrons with dots from one atom and crosses from the other; shared pair sits in the overlap. Stick (displayed) formula: each shared pair shown as a line. H₂O drawn as H–O–H.
⚠Common mistakes
- Saying simple molecules have weak covalent bonds. Bonds are strong; the intermolecular forces are weak. This is why m.p./b.p. is low.
- Forgetting giant covalent structures aren't molecules. They have no fixed molecular formula; they're networks.
- Saying graphite conducts because of free ions. It conducts because of delocalised electrons along the layers.
- Drawing CO₂ as C–O bonds instead of C=O. Oxygen needs a double bond to satisfy its outer shell.
Links
Builds on C1.4 (electronic structure). Compare with C2.2 (ionic) and C2.4 (metallic). Connects to C2.7 (allotropes of carbon) and C7.7 (polymers in detail).
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