Metals, nonmetals and metalloids: what is the difference?

Where each type sits

A staircase-shaped line runs down the right side of the main table, starting at boron and stepping down to the right toward polonium and astatine. Elements to the left of the line are metals, elements to the upper right are nonmetals, and most elements along the line are metalloids.

Metals are by far the largest group: more than 80 of the 118 elements are metals, including the alkali and alkaline earth metals, the transition metals, the lanthanides and actinides, and post-transition metals such as aluminum, tin and lead. Hydrogen is the main exception to the layout. It sits at the top of group 1 but is a nonmetal.

Properties of metals

Metals are typically shiny (they have luster), good conductors of heat and electricity, and malleable and ductile, which means they can be hammered into sheets and drawn into wires. Almost all are solids at room temperature. Mercury is the only metal that is liquid at room temperature, and gallium melts at about 29.8 °C, just above room temperature.

Chemically, metals have few valence electrons and low electronegativity, so they tend to lose electrons and form positive ions such as Na⁺, Mg²⁺, Al³⁺, Fe²⁺ and Fe³⁺. Their oxides are usually basic: calcium oxide (CaO), for example, reacts with water to form calcium hydroxide, Ca(OH)₂.

Properties of nonmetals

The nonmetals include hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur and selenium, plus the halogens and the noble gases. Eleven of them are gases at room temperature (hydrogen, nitrogen, oxygen, fluorine, chlorine and the noble gases from helium to radon), bromine is a liquid, and the rest, such as carbon, sulfur and iodine, are solids.

Solid nonmetals are usually dull and brittle, and nonmetals in general are poor conductors of heat and electricity. Graphite, a form of carbon, is a notable exception because it conducts electricity. Nonmetals have high electronegativity, so they gain electrons to form negative ions such as O²⁻ and Cl⁻ when they react with metals, or share electrons in covalent bonds with each other, as in H₂O and CO₂. Their oxides tend to be acidic: carbon dioxide dissolves in water to form carbonic acid.

Metalloids: in between

Metalloids have a mix of metallic and nonmetallic properties. The six elements most often counted as metalloids are boron, silicon, germanium, arsenic, antimony and tellurium; some sources also include polonium or astatine.

Silicon looks shiny and metallic but is brittle, and it conducts electricity better than a nonmetal but far worse than a metal. That middle ground makes silicon and germanium semiconductors, the materials at the heart of computer chips and solar cells. Their conductivity can be tuned by adding tiny amounts of other elements, a process called doping.

How to tell them apart

Electronegativity is a useful guide. Metals have low values, such as cesium (0.79) and sodium (0.93). Nonmetals have high values, such as oxygen (3.44) and fluorine (3.98). The metalloids fall in between, at about 1.9 to 2.2, from silicon (1.90) to arsenic (2.18).

Metallic character increases toward the lower left of the table, so the change from metal to nonmetal is gradual rather than a sharp wall. Group 14 shows the whole range in one column: carbon is a nonmetal, silicon and germanium are metalloids, and tin and lead are metals.