Structure and properties

Ionic compounds form giant lattices with strong attractions in all directions, so they have high melting points. They conduct electricity when melted or dissolved, because the ions can move, but not when solid. Simple molecular substances, like water and carbon dioxide, have weak forces between molecules, so they have low melting and boiling points and do not conduct.

Giant covalent structures have very high melting points because many strong covalent bonds must break. Diamond is very hard; graphite is soft and slippery because its layers slide, and it conducts because each carbon has one delocalised electron. Metals conduct heat and electricity through delocalised electrons and are malleable because layers of ions slide. Alloys are harder because different-sized atoms disrupt the layers. Nanoparticles have a huge surface area to volume ratio, which makes them useful catalysts.