Part 2: Metallic and giant covalent structures; melting trends

Explain a physical property by naming the particles, the attraction and what must move or be overcome.

Metal ions and delocalised electrons

Metallic bonding is electrostatic attraction between positive metal ions and delocalised electrons. Mobile electrons conduct electricity in solid and molten metals; positive ions are not the mobile carriers in the solid. Layers can move while metallic attraction persists, allowing malleability.

From Na to Mg to Al, higher cation charge, more delocalised electrons per atom and smaller ions generally strengthen attraction, explaining the broad rise in melting point. Do not use this simple model to assert a perfectly smooth trend across all metals.

Networks of strong covalent bonds

These giant covalent networks are insoluble in ordinary solvents because dissolving would require disruption of strong covalent bonding throughout the structure. Metals do not dissolve as intact metal lattices in water; reactive metals instead undergo chemical reactions. Many metals and giant covalent solids have high boiling points, although individual metal melting points vary widely. Graphene’s combination of strength, thinness and conductivity makes it useful in research into composites, sensors and conducting films.

Period 2 and Period 3 change structure

Period 2: Li and Be are metallic; B and C form giant covalent structures; N₂, O₂ and F₂ are simple molecules; Ne consists of single atoms. Breaking a giant covalent network takes much more energy than overcoming intermolecular attractions in small molecules.

Period 3: Na, Mg and Al are metallic; Si is giant covalent; P₄, S₈ and Cl₂ are molecular; Ar is monatomic. The sharp drop from Si to phosphorus follows a change from a giant network to molecules. Among these molecular elements, S₈ has more electrons and stronger London forces than P₄ or Cl₂, helping explain its higher melting point.

Diamond, graphite and graphene are forms of carbon, not different elements. Graphite’s softness is caused by layer sliding, while its high thermal resistance comes from strong covalent bonds within the layers. Do not call the layers individual simple molecules.

Explain a property using the particles that can move

First decide whether the solid is metallic, giant covalent or molecular. Then decide what the process requires. Melting a molecular solid mainly overcomes intermolecular attractions; it does not normally split the molecules into atoms. Melting a network requires disruption of strong covalent bonding. Electrical conduction instead requires mobile charged particles, so a high melting point alone does not establish conductivity.

In solid magnesium, delocalised electrons move through the structure under an applied potential difference. Magnesium ions vibrate about lattice positions; they do not migrate through the solid to carry the current. In a molten metal the electrons remain mobile. In a molecular solid such as sulfur, neutral molecules and localised bonding electrons do not supply comparable mobile carriers.

Why graphite is both soft and thermally resistant

Two different directions involve different interactions. Within each graphite sheet, every carbon makes three strong covalent bonds. Separating a sheet into individual atoms therefore takes much energy. Between sheets, much weaker attractions allow the layers to slide over one another, explaining softness and lubrication.

The remaining electron per carbon contributes to delocalisation within the sheet, providing mobile charge carriers. Do not locate the conducting electrons only in the gaps between layers. Graphene is one such sheet: it has no stack of layers to slide, even though it shares graphite’s in-plane bonding and delocalisation.

Worked evidence: identify a likely structure

An invented solid has a very high melting temperature, conducts electricity and can leave a soft mark on paper. High melting temperature suggests strong bonding throughout substantial parts of the solid. Conductivity requires mobile charge carriers. Softness rules out using “all giant covalent structures are hard” as a shortcut. Together, these observations are consistent with graphite’s strongly bonded conducting sheets and weak interlayer attractions.

Now compare a low-melting solid that does not conduct and forms discrete eight-atom molecules. A simple molecular model fits: weak attractions between molecules explain the low melting point. The number of atoms in a molecule does not turn it into a giant lattice.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources