Metals, non-metals and metalloids

Metals and non-metals

No single property is enough on its own. Graphite (carbon) conducts electricity but is a non-metal; mercury is a liquid but is a metal. Chemists look at the whole set of properties, and especially at how the element reacts.

Metalloids such as silicon and germanium are in between. They look shiny like metals but are brittle like non-metals. Most importantly, they are semiconductors: they conduct electricity a little, and how much can be controlled by temperature, light, or by adding tiny amounts of other elements. That control is what makes computer chips, solar cells and LEDs possible. A single phone chip contains billions of tiny silicon switches.

Group 1: the alkali metals

Lithium, sodium, potassium, rubidium, caesium and francium are soft, silvery metals with low melting points and low densities. They tarnish within seconds in air, so they are stored under oil. They all react with water to make hydrogen gas and an alkaline solution of the metal hydroxide (which is where the family name comes from):

sodium + water → sodium hydroxide + hydrogen

Their reactivity increases down the group. Lithium fizzes gently, sodium melts and skates, potassium bursts into a lilac flame, and rubidium and caesium explode. You'll see why in 2D: it comes down to how easily the single outer electron can be lost.

Australia mines more lithium than any other country, mostly from a rock called spodumene in Western Australia. Almost all of it goes into the lithium-ion batteries in phones, laptops and electric cars.

Group 2: the alkaline earth metals

Beryllium, magnesium, calcium, strontium, barium and radium are harder and denser than the alkali metals and less reactive, but reactivity still increases down the group. Magnesium barely reacts with cold water but burns in air with a blinding white light, which is why it is used in flares and fireworks. Calcium reacts steadily with cold water. Calcium compounds are everywhere: limestone, marble, shells, coral, bones and teeth are all built from them.

The transition metals

The central block holds most of the metals people use every day: iron, copper, zinc, nickel, chromium, titanium, silver, gold and platinum. Compared with groups 1 and 2 they are:

• harder, stronger and denser, with high melting points (tungsten melts at 3422 °C, which is why it was used in light bulb filaments; mercury is the exception, liquid at room temperature)
• much less reactive: gold and platinum stay shiny for thousands of years
• good at forming coloured compounds: copper sulfate is bright blue, rust (iron oxide) is red-brown, chromium compounds are green, orange or yellow
• often useful as catalysts, which speed up reactions without being used up. The catalytic converter in a petrol car uses platinum, palladium and rhodium.

Iron, cobalt and nickel are also the only common elements that are strongly magnetic at room temperature.

Other metals

To the right of the transition metals are metals such as aluminium, gallium, indium, tin and lead. Aluminium is light and doesn't corrode, because it forms a thin, tough oxide layer. Gallium melts at 29.8 °C, so a piece will melt in your hand. Indium, mixed with tin and oxygen, makes a coating that is both transparent and able to conduct electricity, which is what makes touchscreens work.

Group 17: the halogens

Fluorine, chlorine, bromine and iodine are coloured, poisonous non-metals. They show the change of state down a group very clearly:

• fluorine: pale yellow gas, the most reactive element of all
• chlorine: yellow-green gas
• bromine: dark red-brown liquid that gives off a choking brown vapour
• iodine: shiny grey-black solid that turns straight into a purple vapour when warmed

For the halogens, reactivity decreases down the group, the opposite of group 1. Halogens react with metals to make salts (the name means "salt former"): sodium and chlorine make sodium chloride. Their compounds are useful even though the elements are dangerous: chlorine disinfects drinking water and pools, fluoride in water and toothpaste strengthens teeth, and iodine solutions kill germs on skin.

Group 18: the noble gases

Helium, neon, argon, krypton, xenon and radon are colourless gases that hardly react at all, because their outer shells are already full. For decades chemists thought they could not form any compounds, until in 1962 Neil Bartlett made a compound of xenon with platinum and fluorine. Their lack of reactivity is exactly what makes them useful:

• helium for balloons and airships (it can't catch fire, unlike hydrogen) and, as a liquid at −269 °C, to cool the magnets in MRI scanners
• neon in advertising signs, where electricity makes it glow red-orange
• argon to protect hot metal from air during welding, and between the panes of double-glazed windows
• radon is radioactive, and comes out of rocks into some homes (1E)

Comparing metals in acid

Metals higher in reactivity react faster with dilute acid, giving off hydrogen gas:

metal + hydrochloric acid → metal chloride + hydrogen