Giant covalent structures
A giant covalent structure (also called a giant molecular lattice) consists of a vast number of atoms all joined together by strong covalent bonds. Unlike simple molecules (like H_2O or CO_2), there are no discrete, separate molecules. Instead, the entire crystal is essentially one huge molecule.
Because covalent bonds are very strong and there are millions of them holding the structure together, giant covalent structures typically have:
- Very high melting and boiling points (lots of energy is needed to break the bonds).
- Hardness (in rigid lattices like diamond).
- Poor electrical conductivity (usually, because electrons are fixed in place), with specific exceptions like graphite.
- Each carbon atom forms 4 covalent bonds with 4 other carbon atoms.
- The atoms are arranged in a rigid, tetrahedral lattice.
- There are no free electrons or ions to carry charge.
Why is it hard?
The rigid 3D network of strong covalent bonds makes diamond extremely hard. It cannot be scratched by other materials (except another diamond).
Why does it not conduct electricity?
All four outer-shell electrons of each carbon atom are used in bonding. There are no delocalised (free) electrons to move and carry an electric current.
- Each carbon atom forms 3 covalent bonds with 3 other carbon atoms.
- This creates flat, hexagonal layers of carbon atoms.
- The fourth outer-shell electron of each carbon atom is delocalised (free to move) between the layers.
Why is it soft and slippery?
The layers are held together by weak intermolecular forces. These weak forces allow the layers to slide over each other easily when force is applied. This makes graphite soft and useful as a lubricant.
Why does it conduct electricity?
The delocalised electrons are free to move through the structure. These mobile charge carriers allow graphite to conduct electricity, even though it is a non-metal.
Silicon(IV) oxide (also known as silica or silicon dioxide) has a giant covalent structure similar to diamond, but with alternating atoms.
The Connectivity (Crucial Detail):
- Each silicon atom is bonded to 4 oxygen atoms.
- Each oxygen atom is bonded to 2 silicon atoms.
This creates a continuous 3D lattice. The formula SiO_2 represents the ratio of atoms in the giant structure, not a discrete molecule.
Properties:
- High melting point: Many strong covalent bonds must be broken.
- Hard and brittle: Rigid lattice structure.
- Does not conduct electricity: All electrons are fixed in covalent bonds; no free charge carriers.
Structural Similarity:
Both diamond and silicon(IV) oxide have giant covalent lattices where every atom is bonded to four others in a rigid 3D network.
- In diamond: Each C is bonded to 4 C atoms.
- In SiO_2: Each Si is bonded to 4 O atoms (and each O bridges two Si atoms).
Property Similarity:
Because both structures consist of a giant network held together by strong covalent bonds, they share similar physical properties:
- Very high melting points.
- Hardness (though SiO_2 is brittle).
- Insulators (do not conduct electricity).
The Correction: CO_2 and SiO_2 have very different structures despite similar formulas. CO_2 consists of small, separate molecules held by weak forces. SiO_2 is a giant covalent lattice. The silicon atom is larger than carbon and can form 4 bonds with oxygen, creating a continuous network. Always remember: Silicon(IV) oxide is a hard solid with a high melting point.
Mistake 2 (Electrode): Saying 'graphite conducts electricity' without mentioning it is inert. For electrodes, both properties are often required: it must conduct current AND not react with the electrolyte.
Why examiners accept this: Examiners look for specific connectivity details. Simply saying 'it has a giant structure' is often insufficient for full marks. You must specify the bonding ratios.
Correct Phrasing: 'Each silicon atom forms 4 covalent bonds with oxygen atoms, and each oxygen atom forms 2 covalent bonds with silicon atoms.'
Example: Q: Explain why SiO_2 has a high melting point. A: It has a giant covalent structure where many strong covalent bonds must be broken.
Why examiners accept this: The markscheme typically awards one mark for conductivity and one mark for chemical stability. Mentioning only one may result in a lost mark if the question implies 'two reasons'.
Correct Phrasing: 'Graphite conducts electricity due to delocalised electrons, and it is inert (unreactive) at high temperatures.'
Example: Q: Give two reasons why graphite is used for electrodes. A: 1. It conducts electricity. 2. It is chemically inert.
- Cutting tools: Diamond is extremely hard due to its rigid giant covalent lattice. 2. Drill bits: Same reason; it does not wear down easily.