Metalloids (semimetals)

The staircase on the periodic table

The metalloids form a diagonal band through the p-block: boron in group 13, silicon and germanium in group 14, arsenic and antimony in group 15, and tellurium in group 16. They have three to six outer electrons in s and p orbitals, from boron ([He] 2s² 2p¹) to tellurium ([Kr] 4d¹⁰ 5s² 5p⁴). Metallic character increases down each group and decreases across each period, so the border runs diagonally.

There is no single agreed list. Some tables also include polonium, astatine, selenium or even carbon; on this site polonium is a post-transition metal, astatine a halogen and selenium a reactive nonmetal.

Part metal, part nonmetal

Most metalloids look metallic, with a shiny gray or black surface, but they are brittle. Boron, silicon, germanium and tellurium are semiconductors: they conduct better than nonmetals but much worse than metals, and their conductivity rises with temperature. Arsenic and antimony conduct more like poor metals and are often called semimetals.

Chemically they behave more like nonmetals. They usually form covalent bonds, their oxides are weakly acidic or amphoteric, and they form alloys with metals. Boron and silicon melt at 2076 °C and 1414 °C because their atoms are linked in giant covalent networks.

Typical reactions

Silicon is the second most abundant element in Earth’s crust after oxygen, but in nature it is always combined with oxygen, in sand, quartz and silicate minerals. Pure silicon is made by heating silica with carbon in an electric furnace: SiO₂ + 2C → Si + 2CO. Metalloids also react with halogens to form covalent compounds, such as liquid silicon tetrachloride (Si + 2Cl₂ → SiCl₄), which is used to make the ultrapure glass of optical fibers.

Semiconductors and other uses

The first transistor, built in 1947, used germanium, and today almost all computer chips and solar cells are made of silicon. Their conductivity is controlled by doping: adding a tiny amount of boron, which has one fewer outer electron than silicon, or of phosphorus or arsenic, which have one more.

Boron goes into heat-resistant borosilicate glass and neodymium-iron-boron magnets. Germanium is used in fiber optics and infrared lenses, arsenic in gallium arsenide LEDs, antimony in flame retardants and lead battery alloys, and tellurium in cadmium telluride solar panels. Arsenic and its compounds are highly poisonous.