Metals, nonmetals and metalloids

More than three quarters of the elements are metals. The nonmetals squeeze into the top right corner.

Think about the materials on a kitchen table: a steel knife, an aluminium can, a copper pan base, and next to them a pile of sugar and a glass of water. The first three are made of metal elements. Sugar and water are made of nonmetals: carbon, hydrogen and oxygen. You can tell them apart by touch and by look, and chemists use the same everyday clues to sort the elements into two big families, with a narrow border zone between them.

What makes a metal a metal

Metals are shiny when freshly cut, conduct electricity and heat well, and can be hammered into sheets or drawn into wires without shattering. Almost all are solid at room temperature; mercury is the one that is liquid, melting at about −39 °C. Chemically, metals tend to lose their outer electrons and become positive ions. That is why they sit on the left and in the middle of the table, where atoms have only a few outer electrons that are fairly easy to remove.

The model splits the metals into several families, following PubChem's colouring. Alkali metals (group 1 except hydrogen) and alkaline earth metals (group 2) are the most reactive. The transition metals fill groups 3 to 12 and include iron, copper, silver and gold. The post-transition metals, such as aluminium, tin and lead, sit just to the left of the staircase. The lanthanides and actinides are metals too.

Nonmetals

Nonmetals are dull or transparent, break rather than bend when they are solid, and conduct electricity poorly. Many are gases: hydrogen, nitrogen, oxygen, fluorine, chlorine and all of group 18. Bromine is a liquid, and carbon, phosphorus, sulfur, selenium and iodine are solids. In reactions nonmetals tend to gain electrons or share them, which is how they bond with each other to make molecules such as water and sugar. The model shows the halogens and noble gases in their own colours because they are such distinct families.

The staircase and the metalloids

A zigzag line runs from boron down to astatine. Elements along it, such as boron, silicon, germanium, arsenic, antimony and tellurium, are called metalloids. They look somewhat metallic but are brittle, and they conduct electricity better than nonmetals but far worse than metals. Silicon is the famous example. Its moderate conductivity can be controlled by adding tiny amounts of other elements, which is the basis of the computer chip. PubChem also lists polonium as a metalloid. Different books draw the border a little differently, especially for polonium and astatine, so do not be surprised if another table colours them differently.

Hydrogen is the odd one out. It sits above lithium in group 1 because it has one electron, but it is a gas made of H₂ molecules and behaves like a nonmetal. Its position is a matter of electron count, not of family resemblance.

If you count the coloured tiles in the model, PubChem places 91 of the 118 elements in metal families: 6 alkali metals, 6 alkaline earth metals, 38 transition metals, 11 post-transition metals, 15 lanthanides and 15 actinides. That is about 77%. Another 7 are metalloids, and the remaining 20 are split into 7 nonmetals, 6 halogens and 7 noble gases. Many of the heaviest elements have only ever been made a few atoms at a time, so their family is predicted from their position rather than measured.