Giant covalent structures
Giant covalent structures
- Some covalent substances are not small molecules — millions of atoms are joined into one giant covalent structure.
- The two carbon forms you must know are diamond and graphite.
Diamond vs graphite
| Diamond | Graphite | |
|---|---|---|
| bonds per carbon | 4 | 3 (in flat layers) |
| property | very hard, rigid 3-D network | layers slide (weak forces between) |
| use | cutting tools | lubricant |
| conducts? | no | yes — a free (delocalised) electron per carbon |
- Silicon(IV) oxide ($\text{SiO}_2$) is also giant covalent, like diamond — very hard, very high melting point.
Practice
Diamond is very hard because each carbon atom is:
Four strong covalent bonds per carbon make a rigid network, so diamond is extremely hard.
Practice
Graphite conducts electricity because:
Each carbon bonds to only three others; the spare outer electron is delocalised and carries charge.
Practice
Graphite is slippery and used as a lubricant because its layers can slide over each other.
Weak forces between graphite's layers let them slide, making it a good lubricant.
You've got it
Key idea
- giant covalent = millions of atoms bonded in one network
- diamond: 4 bonds → hard (cutting tools); graphite: 3 bonds, sliding layers → lubricant, and it conducts (free electrons)
- SiO₂ is diamond-like: hard, high melting point