Metals, Non-metals and Metalloids: The Three Big Families

Where metals, non-metals and metalloids sit on the periodic table, how their properties and bonding differ, why silicon matters, and why hydrogen is odd.

Before you learn about groups, periods and blocks, the periodic table can be split into just three broad families: metals, non-metals and metalloids. Roughly three quarters of the elements are metals, about twenty are non-metals, and a thin diagonal band of six or so metalloids sits between them. Knowing which family an element belongs to tells you a surprising amount about how it looks, how it behaves and how it bonds.

Where each family sits

Look at any periodic table and imagine a zig-zag line running down and to the right, starting between boron and aluminium and stepping down between silicon and germanium, arsenic and antimony, tellurium and polonium. Chemists call this the staircase line. Metals sit to the left of it and below it, which covers the vast majority of the table: groups 1 and 2, the whole transition metal block in the middle, the lanthanides and actinides at the bottom, and the metals such as aluminium, tin and lead that spill into the right-hand groups.

Non-metals sit in the top right corner: carbon, nitrogen, oxygen, fluorine, phosphorus, sulfur, chlorine, selenium, bromine, iodine, plus the six noble gases in the final column. Hydrogen is also a non-metal, even though it is drawn on the far left, and we will come back to that oddity at the end.

The metalloids are the elements that touch the staircase line itself: boron, silicon, germanium, arsenic, antimony and tellurium. Some textbooks add polonium and astatine, but those two are so rare and radioactive that the list of six is the one most people use.

Physical properties compared

Metals are good conductors of both heat and electricity, because their outer electrons are not tied to any one atom. They are malleable (they can be hammered into sheets) and ductile (they can be drawn into wires), because the layers of atoms can slide past each other without the structure breaking. Most are solid at room temperature with high melting points. Tungsten, the metal in old-fashioned light bulb filaments, melts at around 3400 degrees Celsius, higher than any other element except carbon. Mercury is the well-known exception, a liquid at room temperature, and gallium is close behind, melting in the palm of your hand at about 30 degrees.

Non-metals are almost the opposite. They are poor conductors, they are brittle if solid, they tend to be dull rather than shiny, and their melting and boiling points are usually low. Eleven of them are gases at room temperature, bromine is a liquid, and the solids such as sulfur and iodine crumble rather than bend. There are exceptions here too: carbon in the form of graphite conducts electricity, and carbon in the form of diamond is the hardest natural substance known, with an extremely high melting point.

Metalloids sit in between, often looking metallic but behaving like non-metals. Silicon has a grey, mirror-like shine yet shatters like glass. Most importantly, metalloids are semiconductors: they conduct electricity a little, and how much they conduct can be controlled by temperature, light or by adding tiny amounts of other elements. That single property is the basis of the entire electronics industry.

Chemical properties and bonding

The three families differ chemically because of what their atoms do with their outer electrons.

Metal atoms have only a few outer electrons and lose them easily to form positive ions. In a lump of pure metal, those loose electrons form a shared "sea" that holds the positive ions together. This is metallic bonding, and it explains conductivity (the electrons can flow) and malleability (the sea flows around the ions when they move). When metals react with non-metals, they hand electrons over completely to form ionic compounds such as sodium chloride and magnesium oxide. Metal oxides are generally basic; they neutralise acids.

Non-metal atoms have nearly full outer shells and gain or share electrons instead. Between themselves they form covalent bonds, sharing pairs of electrons to make molecules such as water, carbon dioxide and oxygen gas. Because the forces between those molecules are weak, most non-metals have low melting points. Non-metal oxides tend to be acidic; sulfur dioxide dissolving in rainwater to make acid rain is the classic example.

Metalloids can go either way depending on what they are reacting with. Silicon forms a giant covalent structure like diamond, which is why it is hard and has a high melting point of around 1400 degrees Celsius. Its oxide, silicon dioxide (sand and quartz), is weakly acidic. Antimony behaves more like a metal and can form positive ions, while boron is firmly covalent. This mixed behaviour is exactly why they are called metalloids: "metal-like" rather than metal.

Why silicon runs the modern world

Pure silicon is a poor conductor. But add a few atoms of phosphorus per million, which brings an extra electron each, and it starts to conduct through those spare electrons. Add boron instead, which brings one electron fewer, and it conducts through the gaps left behind. Put a region of each type side by side and you have a junction that lets current pass one way but not the other. That is a diode. Arrange three regions and you have a transistor, a switch with no moving parts.

A modern processor contains billions of these switches, each carved out of a single slice of very pure silicon. Germanium was used for the first transistors, but silicon won the volume race because it is cheap, abundant (it is the second most common element in the Earth's crust after oxygen) and forms a stable, protective oxide layer on its surface. Solar panels rely on the same physics: light knocks electrons loose in doped silicon, and the junction pushes them in one direction to make a current.

Everyday examples

You handle all three families daily, often without noticing.

• Metals: copper wiring in the walls, aluminium drinks cans, steel (mostly iron) in cutlery and car bodies, zinc coating on galvanised fences, gold and silver in jewellery, lithium in phone batteries, titanium in hip replacements.
• Non-metals: oxygen and nitrogen in every breath, carbon in pencil leads and in every living cell, chlorine used to disinfect swimming pools, sulfur in matches and in vulcanised rubber tyres, iodine in antiseptic, helium in party balloons.
• Metalloids: silicon in computer chips, solar panels and glass, boron in heat-resistant borosilicate glassware, germanium in some camera lenses and fibre-optic cables, antimony in flame-retardant plastics and in lead-acid car batteries, tellurium in a type of thin-film solar cell.

Try it on Periodic Bubbles. On the bubbles view, each element is coloured by its category, so the three families and the staircase between them jump out as soon as you start popping. Pop silicon or antimony and compare their properties with a metal on one side and a non-metal on the other.

Comparison table

Hydrogen, the awkward exception

Hydrogen sits at the top of group 1, directly above lithium and sodium, and like them it has a single outer electron. On paper that makes it look like a metal. In reality it is a colourless, non-conducting gas made of two-atom molecules, which is about as non-metallic as an element can get.

The reason is that hydrogen's one electron is also its only electron, sitting in a shell that holds a maximum of two. It can lose that electron to form a positive ion, like a metal, but it can also gain one to fill the shell and form a negative hydride ion, like a halogen. Most of the time it simply shares, forming covalent bonds in water, methane and almost every organic molecule. Some tables float hydrogen on its own above the main body, and a few place it above fluorine. Under extreme pressure, deep inside planets like Jupiter, hydrogen is thought to become a metallic liquid, which shows that the metal versus non-metal boundary is really about conditions as much as identity.

For exam purposes, hydrogen is a non-metal. Just be ready to explain why its position on the table does not match.

Key takeaways

• Metals sit left of and below the staircase line, non-metals sit in the top right, and metalloids lie along the line itself.
• Metals conduct, bend and form positive ions through metallic and ionic bonding; non-metals are poor conductors, brittle, and bond covalently.
• Metalloids are semiconductors, and doping silicon with tiny amounts of phosphorus or boron is the basis of every microchip and solar panel.
• Metal oxides are generally basic and non-metal oxides generally acidic, a useful pattern for predicting reactions.
• Hydrogen is drawn in group 1 but is a non-metal; its single electron lets it behave like both families.