Diamond: four covalent bonds lock every carbon atom into one giant structure
- Diamond is one of the structural forms of the element carbon, whose chemical symbol is C\text{C}C.
- A covalent bond is a shared pair of electrons between two atoms.
- In diamond, each carbon atom forms four strong covalent bonds with four neighbouring carbon atoms.
- This uses all four outer-shell electrons of every carbon atom.
- These covalent bonds point in all directions and repeat endlessly to build a giant covalent structure.
- A giant covalent structure is one huge lattice of atoms joined by covalent bonds, not a collection of small separate molecules.
Key Idea
- Diamond is a giant network of carbon atoms, not a set of separate carbon molecules.
- Every carbon atom is held in that network by four strong covalent bonds.
Hardness: strong bonds in every direction make diamond extremely difficult to deform
- Diamond is very hard, so it strongly resists scratching and changes of shape.
- Every carbon atom is locked in a fixed position by four strong covalent bonds.
- Scratching or deforming diamond would mean breaking many strong covalent bonds at once, which needs a great deal of force.
- There are no weak forces in the lattice to let atoms slide, so the whole framework stays rigid.
Example
Diamond-tipped cutting tools and drills can cut and grind hard materials because diamond resists scratching and deformation.
Melting point: breaking a whole network of covalent bonds needs a huge energy transfer
Definition
Melting point
The melting point is the temperature at which a substance changes from a solid to a liquid.
- Diamond has a very high melting point, among the highest of any substance.
- Melting diamond means breaking many strong covalent bonds right through its giant structure.
- Because these covalent bonds are strong and so numerous, a large amount of energy and a very high temperature are needed.
Common Mistake
- Do not explain diamond's melting point using intermolecular forces, because diamond contains no separate molecules.
- The forces overcome when diamond melts are the strong covalent bonds within its giant structure.
Electrical conductivity: diamond has no charged particles free to move
Definition
Outer-shell electron
An electron in the highest occupied electron shell of an atom.
- Diamond does not conduct electricity, whether solid or molten.
- All four outer-shell electrons of each carbon atom are held in covalent bonds.
- This leaves diamond with no delocalised electrons and no free ions to move through the lattice.
- Electrical conduction needs charged particles that are free to move, and diamond has none.
Note
- To explain why diamond does not conduct, link its four covalent bonds per carbon atom to the absence of delocalised electrons.
- Saying only that diamond has no free electrons does not fully connect its bonding to its conductivity.
Exam technique
Explaining diamond: link every property back to its structure and bonding
- For hardness, say that each carbon atom forms four strong covalent bonds in a rigid, three-dimensional giant structure.
- For the very high melting point, say that many strong covalent bonds must be broken, which needs a large amount of energy.
- For the lack of electrical conductivity, say that all four outer-shell electrons are used in bonds, so there are no delocalised electrons to carry charge.
Self review
- How many covalent bonds does each carbon atom form in diamond?
- Why is diamond very hard?
- Why does diamond have a very high melting point?
- Why does diamond not conduct electricity, even when molten?
