Metals and Non-metals

All the elements found in nature can be broadly divided into two large groups: metals and non-metals. They are told apart first by their physical properties — that is, properties we can observe without changing the substance chemically, such as appearance, hardness, and conductivity. Most metals share a set of typical physical properties, while non-metals generally show the opposite.

Metals usually have these physical properties: they have a shiny surface called metallic lustre (e.g. polished gold and silver); they are hard (except sodium and potassium, which are soft); they are malleable, meaning they can be beaten into thin sheets without breaking; they are ductile, meaning they can be drawn into thin wires; they are good conductors of heat and electricity; they are sonorous, meaning they produce a ringing sound when struck; and most metals have high melting and boiling points. Familiar metals include iron, copper, aluminium, gold, and silver.

Non-metals generally show the opposite physical properties: they are usually dull (not lustrous), soft or brittle (solids break easily into pieces), and are poor conductors of heat and electricity (they are insulators). They are not malleable or ductile, and they are not sonorous. Non-metals can be solids (like sulphur and carbon), liquids (bromine is the only liquid non-metal), or gases (like oxygen, nitrogen, and hydrogen). Examples of non-metals include oxygen, carbon, sulphur, and chlorine.

There are some interesting exceptions that make the picture richer. Mercury is the only metal that is liquid at room temperature; sodium and potassium are metals soft enough to be cut with a knife. Among non-metals, graphite (a form of carbon) is a good conductor of electricity, and diamond (another form of carbon) is the hardest known natural substance. Knowing these typical properties and exceptions lets us identify whether a material is a metal or a non-metal and helps explain why each is used as it is.

Besides their physical properties, metals are recognised by how they behave in chemical reactions — their chemical properties. Metals react with oxygen, water, and acids in characteristic ways, though different metals react with different vigour. Studying these reactions helps us understand corrosion, the extraction of metals, and many everyday processes.

Reaction with oxygen: Metals react with oxygen (usually on heating) to form metal oxides. For example, magnesium burns in air with a bright white flame to form magnesium oxide, and copper forms a black coating of copper oxide on heating. Importantly, metal oxides are basic in nature — they turn red litmus blue and react with acids. Some metal oxides, like those of aluminium and zinc, can behave as both acid and base and are called amphoteric, but the general rule is that metal oxides are basic.

Reaction with water: Metals react with water (or steam) to form metal hydroxides or oxides and release hydrogen gas, but the vigour varies greatly. Very reactive metals like sodium and potassium react violently even with cold water, so they must be stored under kerosene oil to keep them away from air and moisture. Less reactive metals like iron react only slowly or with steam, and some metals like gold do not react with water at all.

Reaction with acids: Most metals react with dilute acids to form a salt and release hydrogen gas, which burns with a "pop" sound. For example, zinc reacts with dilute hydrochloric acid to give zinc chloride and hydrogen. However, very unreactive metals like gold and silver do not react with dilute acids. The differing reactivity of metals is summarised in the reactivity series, an arrangement of metals from most reactive (like potassium and sodium) to least reactive (like silver and gold). A more reactive metal can displace a less reactive metal from its salt solution. These chemical properties explain why some metals tarnish or corrode quickly while others, like gold, stay shiny for centuries.

Just as metals have characteristic chemical properties, non-metals behave in their own typical ways, which are often the opposite of metals. Comparing the two helps complete our understanding of how elements react and explains many everyday observations, such as why burning sulphur or carbon produces acidic gases.

Reaction with oxygen: Non-metals react with oxygen to form non-metal oxides. The crucial difference from metals is that non-metal oxides are acidic (or neutral) in nature — they turn blue litmus red and form acids when dissolved in water. For example, carbon burns in oxygen to form carbon dioxide, which dissolves in water to give carbonic acid; sulphur burns to form sulphur dioxide, which forms sulphurous acid in water. (A few non-metal oxides, such as water and carbon monoxide, are neutral.) This is exactly why the gases released by burning fuels can cause acid rain.

Reaction with acids and water: Unlike most metals, non-metals generally do not react with dilute acids and do not displace hydrogen from them. Non-metals also do not usually react with water in the way reactive metals do. In terms of charge, non-metals tend to gain electrons in reactions, forming negative ions, whereas metals tend to lose electrons, forming positive ions — which is why metals and non-metals often combine to form compounds such as salts.

Between metals and non-metals lies a small group of elements called metalloids, which show intermediate properties — some metallic and some non-metallic. The most important examples are silicon (Si) and germanium (Ge) (boron and arsenic are others). Metalloids are often semiconductors, meaning they conduct electricity better than non-metals but not as well as metals. This special property makes silicon and germanium extremely valuable in making the chips and components of computers, mobile phones, and other electronic devices. Understanding non-metals and metalloids alongside metals shows how the whole range of elements provides materials for every kind of use, from acidic gases to the silicon chips of modern electronics.

Pure metals are useful, but they often lack the exact qualities we need — pure iron is comparatively soft and rusts easily, and pure gold is too soft for jewellery. To improve their properties, metals are frequently mixed with other metals or with non-metals. An alloy is a homogeneous mixture of two or more metals, or of a metal with a small amount of a non-metal. Alloying is one of the oldest and most important techniques in technology, because it lets us tailor the strength, hardness, appearance, and resistance to corrosion of a material.

Alloys generally have better properties than the pure metals from which they are made. They are often harder and stronger, more resistant to corrosion (rusting), and may have a more attractive appearance or a lower melting point, depending on what is added. This is why most metals we use in daily life are actually alloys rather than pure metals.

Several alloys are especially important. Steel is an alloy of iron with a small amount of carbon; it is far stronger and harder than pure iron and is used for construction, tools, and machinery. Stainless steel is steel with added chromium and nickel, which makes it resistant to rusting — ideal for cutlery, utensils, and surgical instruments. Brass is an alloy of copper and zinc, valued for its golden colour and used in decorative items, utensils, and musical instruments. Bronze is an alloy of copper and tin, which is hard and durable and was historically used for tools, statues, and medals.

Alloys are found everywhere around us. The pots in our kitchens, the bodies of vehicles, coins, jewellery (gold is alloyed with copper to harden it), aircraft (light, strong aluminium alloys), and electrical fittings all rely on alloys chosen for the right combination of properties. By understanding alloys, we see how mixing materials cleverly produces substances better suited to human needs than any single pure metal.

When metals are exposed to air, moisture, and other substances over time, their surfaces are slowly eaten away and damaged. This gradual eating away of a metal by the action of air, moisture, and chemicals in the surroundings is called corrosion. Corrosion is a chemical change in which the metal reacts with substances in its environment to form new compounds on its surface. It is a serious problem because it weakens metal structures and causes enormous losses of useful material every year.

Different metals corrode in different ways. The most familiar example is the rusting of iron, in which iron exposed to both water and oxygen forms a reddish-brown layer of rust (hydrated iron oxide). Other metals corrode too: silver tarnishes, forming a dull black coating when it reacts with sulphur compounds in the air; copper develops a green coating (called verdigris) over time; and even aluminium forms a thin oxide layer, though in aluminium's case this layer actually protects the metal underneath from further corrosion.

Corrosion can be prevented or slowed in several ways, mostly by keeping air and moisture away from the metal surface or by using less reactive coatings. Common methods include painting (used for gates, railings, and bridges), applying oil or grease (for tools and machine parts), galvanisation — coating iron with a layer of zinc — and electroplating, in which a thin layer of a less reactive metal such as chromium, nickel, or tin is deposited on the surface. Another important method is alloying, such as making stainless steel, which resists corrosion because of the chromium and nickel it contains.

Preventing corrosion is highly practical and economically important. It protects bridges, vehicles, ships, pipelines, machinery, tools, and household items, keeping them safe and extending their life, and it saves the huge amounts of money that would otherwise be lost to replacing corroded metal. Choosing the right prevention method for each situation — paint for large structures, galvanising for pipes and sheets, electroplating for taps and decorative fittings, and stainless steel for utensils — completes our understanding of how we make the most of metals while protecting them from being destroyed by their surroundings.