Metalloids

The in-between elements that look like metals, behave like non-metals and power every computer chip.

Every phone, laptop and games console depends on an element that is neither a proper metal nor a proper non-metal. Silicon looks shiny and grey like a metal, but it is brittle and conducts electricity only grudgingly. That half-way behaviour is exactly what engineers need to build transistors, and it is the defining feature of the metalloids.

The metalloids form a diagonal staircase running from boron at the top left to astatine at the bottom right of the p block: boron, silicon, germanium, arsenic, antimony, tellurium and astatine. This staircase marks the boundary between metals on the left and non-metals on the right. Not everyone draws the family the same way; some leave out astatine and others add polonium.

What they have in common

Metalloids have a metallic appearance but non-metallic chemistry. Most are grey, shiny solids, yet they shatter rather than bend, and they are poor conductors of heat compared with true metals. Their most useful shared property is that they are semiconductors. A semiconductor conducts electricity a little, and the amount can be controlled by warming it, shining light on it or adding tiny amounts of other elements.

Chemically they sit on the fence. Their oxides are usually weakly acidic or amphoteric, meaning they can react with both acids and bases. They tend to share electrons and form covalent bonds rather than losing electrons to form simple positive ions. Boron and silicon in particular build up huge networks of atoms bonded to one another, which gives them very high melting points. Boron melts at about 2076 degrees Celsius and silicon at about 1414 degrees.

When metalloids react with metals they act as the non-metal partner, accepting electrons. When they react with non-metals such as oxygen or chlorine they act as the metal partner.

How they change down the staircase

Metallic character increases as you go down and to the left. Boron, at the top of the staircase, is closest to being a non-metal: it is very hard, very high melting and forms only covalent compounds. Silicon and germanium are the textbook semiconductors, with germanium slightly more metallic and lower melting at about 938 degrees Celsius.

Arsenic, antimony and tellurium look increasingly like metals. Antimony melts at about 631 degrees Celsius and tellurium at about 450 degrees, values that are ordinary for metals. Yet all three remain brittle and their chemistry still involves mostly covalent bonding, so they stay in the metalloid camp.

Density rises down the staircase, from boron at roughly twice the density of water to tellurium at more than six times. Astatine, at the bottom, is a radioactive element so rare that its physical properties are estimated rather than measured. Its predicted melting point of around 300 degrees Celsius and its expected metallic sheen are what place it among the metalloids in many tables, though others treat it as a halogen.

Where you meet them

Silicon is the second most abundant element in the Earth's crust after oxygen, and most of it is locked up in sand, quartz, clay and rock as silicon dioxide and silicates. Purified into large single crystals, it becomes the wafers on which microchips and solar cells are built. Melted with other ingredients it becomes glass.

Boron turns up in borosilicate glass, the heat-resistant glass of laboratory beakers and cookware, and in the cleaning product borax. Germanium was the material of the very first transistors and is still used in infrared lenses and fibre-optic cables. Arsenic is famously poisonous, but combined with gallium it makes fast electronic chips and the red LEDs in older displays. Antimony strengthens lead in car batteries and goes into flame retardants. Tellurium is added to steel and copper to make them easier to machine, and cadmium telluride is one type of thin-film solar panel.

Odd ones out

Arsenic stands out for its reputation. It has been used as a poison for centuries and still contaminates drinking water in some parts of the world where it leaches from rocks. Its grey metallic form does not melt under normal pressure at all; heat it in an open container and it turns straight into a vapour, a process called sublimation.

Astatine is the most extreme case of an element that is hard to classify. It is one of the rarest natural elements, present in the Earth's crust only in vanishing traces as a product of radioactive decay. Chemically it behaves partly like iodine above it and partly like a metal, which is why it can appear on both the metalloid and halogen pages of this site. Its synthetic neighbour tennessine, made in only a few atoms, is predicted to lean even further towards metallic behaviour.

The numbers

Melting points fall down the list, from Boron at 2076 °C to Astatine at 302 °C.

Density in g/cm³ (g/L for gases). Values for synthetic elements are predictions.

Did you know?

• Silicon is the second most abundant element in the Earth's crust, mostly as sand and rock.
• Boron melts at about 2076 degrees Celsius, higher than iron, despite being far lighter.
• Grey arsenic does not melt at normal pressure; it sublimes straight from solid to vapour.
• The first transistors were made from germanium before silicon took over.
• Astatine is so radioactive and so rare that its melting point is estimated, not measured.
• Metalloids conduct electricity better when warmed, the opposite of metals.

Frequently asked questions

Written by Anthony. Data from Periodic-Table-JSON (CC BY-SA).