Diamond and Graphite

Diamond and graphite are both made of carbon, but their atoms are arranged differently. This different arrangement (structure) explains why one is the hardest natural substance while the other is soft and slippery.

Giant covalent structures

A giant covalent structure is a huge network of atoms joined by strong covalent bonds. There are no small, separate molecules. Because many strong bonds must be broken to melt or scratch them, they are usually very hard and have very high melting points.

$$\text{Giant covalent} \Rightarrow \text{many strong covalent bonds} \Rightarrow \text{very high melting point}$$

Diamond

• Structure: each carbon bonds to four other carbons in a 3D tetrahedral network.
• Angles: $109.5^\circ$ between bonds.
• Properties: very hard, very high melting point, does not conduct electricity (no free electrons), good heat conductor, transparent.
• Uses: cutting tools and drill bits (very hard).

$$\text{Diamond: each C makes } 4 \text{ single bonds (tetrahedral, }109.5^\circ\text{)}$$

Graphite

• Structure: each carbon bonds to three others in flat hexagon layers (like chicken wire). Layers stack on top of each other.
• Angles: $120^\circ$ in each layer. One electron per carbon is not used in bonding and is free to move along the layer (delocalised).
• Properties: conducts electricity along layers, high melting point, soft and slippery because layers slide over each other, black and opaque.
• Uses: lubricant (layers slide) and electrodes (conducts electricity).

$$\text{Graphite: each C makes } 3 \text{ bonds (planar, }120^\circ\text{) } + 1 \text{ delocalised electron}$$

Compare diamond and graphite

• Diamond is 3D and rigid; graphite is layers that can slide.
• Diamond does not conduct electricity; graphite conducts due to delocalised electrons.
• Both have very high melting points because covalent bonds are strong.

Link to silicon(IV) oxide (silicon dioxide)

Silicon(IV) oxide has a similar giant covalent network to diamond (each Si linked to 4 O, each O to 2 Si). It is hard, has a very high melting point, and does not conduct electricity.

Common misconceptions

• Graphite is soft not because its bonds are weak, but because weak forces between layers allow sliding. Bonds inside each layer are strong.
• Graphite conducts even though it is not a metal.
• Diamond does not conduct electricity, even though it is made of carbon.