Diamond and graphite are both pure carbon — why is one the hardest natural material and the other soft enough to write with?
It's the bonding arrangement.
It's the bonding arrangement. In diamond every carbon is covalently bonded to four others in a rigid 3-D network, so it's extremely hard and doesn't conduct (no free electrons). In graphite carbon forms flat layers of hexagons with a spare delocalised electron each; the layers are held only by weak forces, so they slide (soft, lubricating) and the free electrons let graphite conduct electricity.
Key point
Diamond = rigid 3-D covalent network (hard, insulator); graphite = sliding layers with free electrons (soft, conductor).
Common mistake
thinking two forms of the same element must have the same properties.
Memory tip
same atoms, different arrangement (allotropes) → totally different properties.
Tap to watch or see it right here — the app stays open (it only opens a quick search if nothing embeddable is found).