Chapter 2: Atoms, elements and compounds — Part 4

Topics 2.6–2.7 · Giant structures and metallic bonding

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Structure: Each carbon atom forms three strong covalent bonds to other carbon atoms in hexagonal layers. The fourth outer electron of each carbon atom is delocalised and free to move within the layers. Weak forces exist between layers, allowing them to slide over each other.
Properties and uses: Soft and slippery (layers slide) — used as a lubricant. Good conductor of electricity (delocalised electrons) — used in electrodes and pencils. High melting point (strong covalent bonds within layers).

Exam Traps

Do not say graphite is hard like diamond — weak interlayer forces make it soft and slippery.

Structure: Each carbon atom forms four strong covalent bonds in a tetrahedral arrangement, creating a rigid giant three-dimensional network. There are no free electrons.
Properties and uses: Extremely hard (strong bonds in all directions) — used in cutting tools and drill bits. Very high melting point. Does not conduct electricity (no delocalised electrons). Transparent and lustrous — used in jewellery.

Exam Traps

Do not draw diamond as layered like graphite — it is a 3D tetrahedral network with four bonds per carbon.
Do not say all carbon allotropes conduct — only graphite has delocalised electrons.

Structure: Similar to diamond: each silicon atom is bonded to four oxygen atoms and each oxygen to two silicon atoms, forming a giant covalent lattice (SiO₂).
Properties: Very hard, high melting point, and does not conduct electricity. Found naturally as quartz and sand. Used in glass and ceramics.

Definition: The attraction between positive metal ions in a lattice and a sea of delocalised electrons.
Structure: Metal atoms lose their outer electrons to form positive ions arranged in a regular lattice. The outer electrons become delocalised and move freely throughout the structure.
Electrical conductivity: Delocalised electrons are free to move and carry electric charge, so metals are good conductors of electricity (and heat).
Malleability and ductility: Metal ions can slide past each other in the lattice without breaking the metallic bond (the delocalised electrons still hold the structure together). This allows metals to be hammered into shape (malleable) and drawn into wires (ductile).

Exam Traps

Do not say metal ions move to carry current — it is the delocalised electrons that flow.
Do not confuse metallic bonding with ionic bonding — metals retain delocalised electrons, not oppositely charged ion lattices.

Definition: An alloy is a mixture of a metal with one or more other elements (usually metals or carbon). Examples: brass (copper + zinc), steel (iron + carbon).
Hardness: Alloys are often harder than pure metals because atoms of different sizes distort the regular lattice, making it more difficult for layers of ions to slide past each other.

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