Bonding and structure, Year 10: diamond, graphite and graphene

4

提示Every one of carbon's outer electrons is used.

为什么Each carbon atom is joined to four others, and each of those to four more, building a rigid three-dimensional giant structure.

Every atom is held by four strong covalent bonds in a giant structure

提示To scratch it you would have to tear atoms out of a rigid network.

为什么The bonds run in all directions through the whole crystal, so there are no weak points and no layers that can slide. Nothing can be moved without breaking strong covalent bonds.

It has no delocalised electrons

提示Everything in each atom's outer shell is busy holding the structure together.

为什么Carbon has four outer electrons and in diamond all four are used in covalent bonds. With no free electrons and no ions, there is nothing that can move to carry charge.

Covalent bonds

提示When ice melts the H2O molecules stay whole; diamond has no molecules to keep whole.

为什么Ice is made of small molecules held to one another by weak intermolecular forces, which are easily overcome. Diamond is a giant structure, so its atoms can move only when strong covalent bonds are broken.

Their atoms are bonded and arranged in different ways

提示Same building blocks, but not the same blueprint.

为什么In diamond each atom makes four bonds in a rigid three-dimensional network. In graphite each atom makes three bonds, giving flat layers with free electrons. The properties of a material come from its structure and bonding, not only from which atoms it contains.

3

提示One of carbon's four outer electrons is left out of the bonding.

为什么Each carbon atom is bonded to three others in the same flat layer. The fourth outer electron from each atom is delocalised.

One electron from each carbon atom is delocalised and free to move

提示There are four outer electrons available, yet fewer bonds than that are made.

为什么Only three of each atom's four outer electrons are used in bonds. The fourth is free to move, and moving electrons carry charge. In this respect graphite is like a metal.

There are no covalent bonds between its layers, so the layers slide

提示Think of what happens when you drag a pencil across paper.

为什么Within a layer the atoms are strongly bonded, but one layer is held to the next only weakly. The layers slide over each other and rub off easily, which is why graphite is used in pencils and as a lubricant.

hexagonal

提示It is the same shape as the cells of a honeycomb.

为什么Each carbon atom bonds to three others, which makes a flat sheet of six-sided rings, like chicken wire. The sheets are stacked on top of one another.

The strong covalent bonds within each layer must be broken

提示Sliding sheets past one another is easy; pulling a single sheet to pieces is not.

为什么Graphite is a giant covalent structure. Softness comes from the layers sliding, which needs no bonds to break, but melting means separating the atoms inside the layers, and those are held by strong covalent bonds.

Graphene

提示Its name is "graphite" with the ending swapped for "-ene".

为什么Graphene is one sheet of carbon atoms in hexagonal rings. Like graphite it has delocalised electrons, so it conducts electricity, and its strong covalent bonds make it very strong for its mass. These properties make it useful in electronics and composites.

60

提示The molecule is a hollow ball; the number matches the minutes in an hour.

为什么Buckminsterfullerene, formula C60, was the first fullerene to be discovered. Its sixty carbon atoms form a hollow sphere.

A carbon nanotube

提示The prefix in its name means a billionth, a clue to how thin it is.

为什么A nanotube is like a sheet of graphene rolled into a cylinder. Nanotubes are useful in nanotechnology, in electronics and for strengthening materials.

Rings of five or seven carbon atoms

提示One fewer and one more than a hexagon has.

为什么Flat sheets can be made from hexagons alone, as in graphite. Rings of other sizes make the sheet curve, which is how a fullerene closes up into a hollow shape.

Electronics, nanotechnology or strengthening materials (any one)

提示Think about what a very thin, very strong tube that conducts could be good for.

为什么Nanotubes are extremely thin, strong for their size and able to conduct. The specification names nanotechnology, electronics and materials as the areas where these properties are used.