Non-metals

Properties of carbon

'Carbon' comes from the Latin word for charcoal and is the only non-metal in group IV. The rest are metalloids or metals. The atomic number for carbon is 6, which means carbon has two electrons in its first shell and four electrons in its outermost shell.

Carbon is versatile because it can form strong bonds with itself and/or other elements in different structures, such as in a long chain or a ring compound. If a carbon atom is unable to bond with another element to complete its outer shell, it will form multiple bonds with other carbon atoms.

Allotropes

Allotropes are different molecules of an element. In each allotrope, the molecule comprises the same element but has a different number of atoms and/or a different structural form, with different properties.

Naturally occurring allotropes of carbon include amorphous carbon (charcoal), graphite and diamond. Burning substances containing carbon, like wood, produces amorphous carbon. The word 'amorphous' means 'no shape', suggesting that charcoal particles have no definitive structure.

Graphite, a shiny grey-black solid, is structured hexagonally across layers. Each carbon atom forms covalent bonds with three others, but because carbon atoms can form up to four bonds, each layer contains roaming electrons, which means graphite can conduct electricity. Of the three bonds, only a single bond exists between the layers of atoms, giving graphite its brittle, slippery quality. Some common uses for graphite include the 'lead' of pencils, as a heat-resistant lubricant (due to its high melting point) and as part of a battery, because of its electrical conductivity. See image 1.

Diamond is the hardest naturally occurring substance known to chemists. Although diamond is also made of carbon, its structure differs from graphite as it forms bonds with four other atoms in a tetrahedral (four-sided) structure. This accounts for the properties of diamond - its hardness and its extremely high melting point (3550°C) - because a lot of energy is required to break the bonds. Clear diamonds have an aesthetic use as gemstones in jewellery, but we also use diamonds on cutting tools and as heat conductors in electronic devices. See image 2.

The above diagrams only represent a fraction of the overall structure. Both graphite and diamond structures can exist in an infinite network, which means that neither of these molecules contains a definitive number of atoms. It also means that it is possible to see a carbon molecule. A crystal of diamond is a giant molecule that contains billions of carbon atoms.

Finally, three scientists - Richard Smalley, Robert Curl and Harold Kroto - discovered one other allotrope of carbon, containing 60 atoms C₆₀), after firing a laser at some graphite. The atoms in this allotrope form a sphere using networks of pentagons and hexagons, much like a soccer ball. Called buckminsterfullerene, after the similar-looking geodesic domes by architect Richard Buckminster Fuller, this molecule is known by its nickname, bucky balls. Carbon atoms that take this formation are referred to as fullerenes, and the number of atoms in each fullerene molecule vary from 20 to 540 (C₂₀ to C₅₄₀). Scientists also hope to develop uses for fullerene nanotubes, tiny tubes of carbon, lighter and stronger than steel, which can conduct electricity without loss of energy. See image 3.

Other non-metals

Typically, non-metals are not good conductors of heat or electricity and are solid or gaseous in standard laboratory conditions. The solids tend to be brittle and dull in colour while most of the gases are colourless.

Hydrogen is the lightest element and a versatile gas due to its unstable outer shell. This gives it the ability to act as a group I (willing to lose one electron) or group VII (willing to gain one electron) element during reactions.

The other non-metals in groups IV to VI include the gases nitrogen and oxygen, which are both abundant in the Earth's atmosphere. Nitrogen is an essential plant nutrient often used in fertiliser. Most organisms need oxygen for respiration in order to function and grow. Importantly, oxygen forms compounds with most other elements in a process called oxidisation.

The solid non-metals include phosphorous and sulphur, both used in fertilisers and in a number of industrial processes, and selenium, which has antioxidant properties.

Halogens all have seven valence electrons, which means that they are hungry for one electron to fill and stabilise their outer shell. Halogens are the most reactive non-metals, often forming ionic salts with metals in group I (see the chapter on 'Acids & Bases'). Fluorine and chlorine are gases, while bromine is a liquid and iodine and astatine are solids in standard laboratory conditions.

As the name suggests, the noble gases of group VIII are all gaseous in standard laboratory conditions. These elements all have a stable outer shell (two electrons for helium, eight for all the others) and therefore tend not to react with other elements. For a long time noble gases were also known as inert gases but some of them, particularly krypton and xenon, do form compounds.See animation 1.