OCR GCSE Chemistry A (J248) — Elements, compounds and mixtures
Every time you write with a pencil, you are leaving trails of carbon atoms on the paper, yet a diamond—made of the exact same element—can cut through solid rock. This dramatic difference comes down to how their atoms are arranged.
A covalent bond forms when non-metal atoms share pairs of electrons to achieve a full, stable outer shell. In some substances, these bonds do not just form small, simple molecules.
Instead, they extend continuously in every direction to create a giant covalent structure (often called a macromolecular structure by OCR). These are immense 3D networks of atoms all held together by a repeating lattice of strong covalent bonds.
Because breaking these countless strong bonds requires a massive amount of energy, giant covalent structures share several general properties:
Diamond is an allotrope of carbon. Inside its structure, every single carbon atom shares electron pairs with exactly four other carbon atoms.
This specific connectivity creates a rigid, 3D tetrahedral lattice geometry (with bond angles of approximately ). Because the atoms are locked into this fixed, rigid 3D framework, diamond is exceptionally hard. Furthermore, diamond is a strict electrical insulator; all four outer-shell electrons of every carbon atom are involved in strong covalent bonds, meaning there are no free electrons to carry a charge.
Graphite is another allotrope of carbon, but its atoms are arranged very differently. Each carbon atom forms strong covalent bonds with only three other carbon atoms.
This arrangement creates flat 2D hexagonal layers of carbon rings. While the covalent bonds within these layers are incredibly strong, the layers themselves are held together only by weak intermolecular forces. These weak forces allow the layers to slide over one another easily, making graphite soft and slippery.
Because each carbon atom only uses three of its four outer electrons for bonding, the fourth electron becomes a delocalised electron. These delocalised electrons are free to move throughout the layers, which makes graphite an excellent conductor of electricity.
Silicon dioxide (found in sand and quartz) is a compound rather than a pure element, but it shares a very similar giant covalent lattice to diamond. Its empirical formula is , which tells us the simplest ratio of atoms in the lattice.
In this giant 3D tetrahedral structure:
Just like diamond, the fixed 3D network makes silicon dioxide very hard, gives it a very high melting point, and ensures it acts as an electrical insulator since no delocalised electrons are present.
When asked to compare these substances, you must state both their structural similarities (how the bonds form) and their precise differences (the arrangement and connectivity of atoms).
| Feature | Diamond | Graphite | Silicon Dioxide () |
|---|---|---|---|
| Similarity | Atoms share electron pairs | Atoms share electron pairs | Atoms share electron pairs |
| Connectivity (Bonds per atom) | 4 bonds per carbon atom | 3 bonds per carbon atom | 4 per silicon, 2 per oxygen |
| Arrangement | 3D Tetrahedral lattice | 2D Hexagonal layers | 3D Tetrahedral lattice |
| Electrical Conductivity | Insulator | Conductor | Insulator |
| Hardness | Very hard | Soft and slippery | Very hard |
A student is given a sample of diamond and a sample of graphite. Explain why the graphite sample conducts electricity while the diamond sample acts as an insulator. (3 marks)
Step 1: State the connectivity and electron arrangement in graphite.
Step 2: Explain the effect of this on conductivity.
Step 3: Contrast this exact mechanism with diamond.
Students often state that strong covalent bonds are 'overcome' when melting giant structures, but mark schemes specifically require you to say the bonds are 'broken'.
Never mention intermolecular forces when discussing diamond or silicon dioxide; these weak forces only exist between the layers in graphite.
When a 6-mark question asks you to 'compare' diamond and graphite, you must explicitly state the exact number of bonds (4 vs 3) and the specific geometry (tetrahedral vs hexagonal layers).
Do not confuse with — silicon dioxide is a giant covalent lattice with high melting points, whereas carbon dioxide is a simple molecule held together by weak intermolecular forces.
Covalent bond
A strong chemical bond formed when non-metal atoms share pairs of electrons.
Giant covalent structure
A structure consisting of a huge, variable number of non-metal atoms all linked by many strong covalent bonds in a regular 3D lattice.
Macromolecular
Another term used by exam boards to describe a giant covalent lattice.
Allotrope
Different structural forms of the same element in the same physical state (e.g., diamond and graphite are both forms of solid carbon).
Tetrahedral
A molecular shape where a central atom is bonded to four others, positioned at the corners of a triangular pyramid.
Hexagonal layers
The 2D arrangement of carbon atoms in graphite, where each atom forms part of a flat, six-sided ring.
Intermolecular forces
Weak forces of attraction that exist between molecules, or in the case of graphite, between the flat 2D layers of carbon atoms.
Delocalised electron
An electron that is not associated with a specific atom or covalent bond and is free to move through an entire structure.
Empirical formula
The simplest whole-number ratio of atoms of each element in a compound.
Put your knowledge into practice — try past paper questions for Chemistry A
Covalent bond
A strong chemical bond formed when non-metal atoms share pairs of electrons.
Giant covalent structure
A structure consisting of a huge, variable number of non-metal atoms all linked by many strong covalent bonds in a regular 3D lattice.
Macromolecular
Another term used by exam boards to describe a giant covalent lattice.
Allotrope
Different structural forms of the same element in the same physical state (e.g., diamond and graphite are both forms of solid carbon).
Tetrahedral
A molecular shape where a central atom is bonded to four others, positioned at the corners of a triangular pyramid.
Hexagonal layers
The 2D arrangement of carbon atoms in graphite, where each atom forms part of a flat, six-sided ring.
Intermolecular forces
Weak forces of attraction that exist between molecules, or in the case of graphite, between the flat 2D layers of carbon atoms.
Delocalised electron
An electron that is not associated with a specific atom or covalent bond and is free to move through an entire structure.
Empirical formula
The simplest whole-number ratio of atoms of each element in a compound.
