Covalent Bonding — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions

Formation of covalent bonds when non-metal atoms react with each other, they share outer electrons so each reaches a full shell. The shared pair is attracted electrostatically to both positive nuclei, which makes the bond strong.

H₂ has one shared pair; H₂O has two O–H bonds plus two lone pairs on O; CH₄ has four C–H bonds; NH₃ has three N–H bonds and one lone pair; HCl has one bond and three lone pairs on Cl.

O₂ (O=O double bond), CO₂ (two C=O double bonds), N₂ (N≡N triple bond), C₂H₄ (one C=C double bond).

Simple molecular structures strong covalent bonds within each molecule but weak intermolecular forces between molecules. Melting or boiling only overcomes the weak forces, so little energy is needed – many are gases or liquids at room temperature.

Larger molecules have stronger intermolecular forces, so their boiling points are higher (e.g. down the halogens, or along the alkanes).

Giant covalent structures every atom is joined to others by covalent bonds in a continuous network. Melting requires breaking many strong bonds, so melting points are very high and they are insoluble in water.

Diamond each carbon forms four bonds in a rigid tetrahedral network – very hard, no free electrons. Graphite: each carbon forms three bonds in hexagonal layers; weak forces between layers let them slide (soft, lubricant); one delocalised electron per atom allows conduction. C₆₀ is a simple molecule with weak intermolecular forces, so it is soft with a relatively low sublimation point.

Covalent vs ionic in a covalent bond, electrons are shared between non-metal atoms and no ions form. In ionic bonding, electrons are transferred from a metal to a non-metal, making oppositely charged ions.

Bonding and non-bonding electrons the shared electrons in a bond are a bonding pair. Outer electrons not used in bonds are non-bonding (lone) pairs. Usually each atom gives one electron to each bond, which is drawn as a line, e.g. Cl–Cl.

Cl₂: one shared pair, and each Cl has three lone pairs. Ethane, C₂H₆: one C–C bond and six C–H bonds, no lone pairs. A double bond such as O=O shares four electrons.

Why bigger molecules boil higher molecules with a larger relative molecular mass have more electrons, so the intermolecular attractions between them are stronger and more energy is needed to separate them.

Graphite melting point graphite is soft because its layers slide, but it still has a very high melting point. Melting it means breaking the many strong covalent bonds within the layers.

C₆₀ structure buckminsterfullerene is a hollow sphere of 60 carbon atoms arranged in 12 pentagons and 20 hexagons. Fullerenes can carry drugs inside the cage, hold catalysts on their large surface and act as lubricants.

C₆₀ conductivity each carbon in C₆₀ has a spare electron, but these electrons can only move within one molecule, not from molecule to molecule. So solid C₆₀ does not conduct electricity, unlike graphite.

Covalent insulators having no free ions or electrons, simple covalent substances do not conduct even when molten. Plastics are used to coat electrical wires for this reason.