non-metal
One of a set of elements (around 20 in total) with certain physical and chemical properties opposite to those of metal elements. The division between metal and non-metal elements forms the simplest division of the periodic table of the elements. Common physical properties are that non-metals have low electrical conductivity, are brittle when solid, or are gases or liquids. Exceptions include graphite, a form of carbon, which is a good electrical conductor.
In structure the non-metals are very diverse. Hydrogen (H), oxygen (O), nitrogen (N), fluorine (F), chlorine (Cl), and the noble gases (rare gases), helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radioactive radon (Rn), are gases. Only bromine (Br) is a liquid at room temperature, and the rest are solids.
Non-metals are the chemical opposites of metals. Metals form positively charged ions or cations; non-metals form negatively charged ions or anions. The exceptions are the chemically unreactive noble gases (rare gases), although xenon does react under certain conditions. Non-metals are electronegative, which means that they are able to gain electrons when bonding with metals. Apart from the noble gases, the non-metal elements have incomplete outer electron shells, and so try to gain enough electrons to fill them. The noble gases do not react because they already have complete outer electron shells. The type of bonding where ions are formed is known as ionic bonding. Non-metals may also share electrons with other non-metal elements to complete their outer shell; this type of bonding is known as covalent bonding.
Reaction with oxygen Non-metals react with air or oxygen to form acidic or neutral oxides. The oxides may dissolve in water to form acids. An example is the main oxide of carbon, carbon dioxide, which forms when carbon, or carbon compounds such as fossil fuels, burn in air. The equation for carbon burning is: |
| carbon + oxygen → carbon dioxide |
| The carbon dioxide in the air dissolves in rain water to make natural rain water slightly acidic. The acid that forms is called carbonic acid, and is also what makes soda water and other fizzy drinks slightly acidic. The reaction is reversible: |
| carbon dioxide + water ⇌ carbonic acid |
| An important industrial reaction is the formation of sulphuric acid by the contact process. Sulphur is first burned in air to form sulphur dioxide, which is then further reacted with a catalyst to form sulphur trioxide. The sulphur trioxide is made into sulphuric acid. Sulphur dioxide is also a pollutant formed from the burning of coal in power stations. The sulphur dioxide is oxidized high in the atmosphere to form the very acidic sulphuric acid that is a cause of acid rain. |
| The formation of sulphur dioxide in air is: |
| sulphur + oxygen → sulphur dioxide |
Reaction with alkalis Non-metals do not react with acids but some do react with alkalis. The halogens (Group 7 of the periodic table) react with sodium hydroxide to form bleach-type compounds. The reaction with chlorine is used to form household bleach: |
| chlorine + sodium hydroxide → sodium hypochlorite + sodium chloride + water, or |
| Cl2 + 2NaOH → NaOCl + NaCl + H2O |
Oxidizing properties Non-metals may show oxidizing properties; they are able to gain electrons from other substances and become reduced. For example, chlorine will oxidize bromide ions to form bromine the element: |
| chlorine + sodium bromide → sodium chloride + bromine, or |
| Cl2 + 2NaBr → 2NaCl + Br2 |
Biological function The non-metals play an essential role in the science of life. Carbon is the only element that can form long chains of atoms, such as those found in proteins. Other non-metal elements are also necessary for life, such as nitrogen and sulphur which are found in many proteins. Phosphorus, in the form of calcium phosphate, is essential for healthy bone formation. Other elements are needed in trace quantities for good health; for example, iodine is needed for a healthy metabolism. Lack of iodine leads to the condition known as goitre (enlargement of the thyroid gland). Fluorine, in the form of fluoride, is needed for healthy teeth, and fluoride is sometimes added to drinking water (fluoridation). |
Commercial use Non-metals have a number of commercial and industrial uses. Diamond, a form of carbon, is the hardest natural substance known. Diamonds are used extensively in jewellery and industrial diamonds are used for cutting and drilling. Silicon, found in the same group as carbon, is a borderline non-metal. A semiconductor of electricity, it is used in transistor circuits such as those found in computers, the so-called ‘silicon chips’. |
Periodic table In the periodic table of the elements, the metals lie on the left-hand side of the table and the non metals lie in the top right-hand corner of the table. The dividing line between metal and non-metal starts at boron (B) and continues down diagonally to astatine (At). |
