Giant structures and small molecules.

Step 1 of 7

One element, four solids

Diamond and graphite, the stuff in a pencil, are both made of nothing but carbon atoms. The only difference between them is how the atoms are joined. Different forms of the same element are called allotropes.

In diamond each carbon atom is held by covalent bonds to several neighbours, each of those to more, and so on through the whole crystal. There are no separate molecules: the whole crystal is one giant covalent structure. Melting diamond means breaking a great many covalent bonds, so it is very hard and stands up to heat. Count how many neighbours each atom is bonded to.

Step 2 of 7

Sheets

In graphite the atoms lie in flat sheets of hexagons. Within a sheet the bonds are covalent and strong, but between the sheets act only weak forces. Predict what happens when you slide the top layers one whole step.

Step 3 of 7

A non-metal that conducts

In graphite each atom is bonded to only three neighbours, so one of its four outer electrons takes no part in a bond. These electrons move freely along the sheet, and thanks to them graphite conducts electricity, although carbon is a non-metal. That is why graphite is used for electrodes and in batteries.

Step 4 of 7

A molecule of carbon

A buckyball, C₆₀, is something else: a molecule of $60$⁠ carbon atoms. The bonds within the ball are covalent and strong, but between one ball and the next act only weak forces. That is how molecular substances are built: water, carbon dioxide, oxygen, sugar. When they melt or boil, the molecules move apart, but they stay whole. So molecular substances have much lower melting and boiling points.

Step 5 of 7

Structure decides

Structure	Particles	What holds it	Melting point	Conducts?
Ionic	ions	the pull between opposite charges	high	only melted or dissolved
Metallic	positive ions and electrons	a sea of electrons	usually high	yes
Giant covalent	atoms	covalent bonds through the crystal	very high	no (except graphite)
Molecular	molecules	weak forces between molecules	low	no

Step 6 of 7

One sheet

Graphene is a single sheet of graphite, one atom thick. Each of its atoms is bonded to its neighbours just as in graphite. Count how many neighbours each carbon atom in the sheet has.

Step 7 of 7

The repeat

A sheet of graphene is a net of hexagons. How far do you have to move for everything to look exactly the same?