Nonmetals (reactive nonmetals)

Where the nonmetals sit

Apart from hydrogen, which sits at the top of group 1, the reactive nonmetals are in the upper right of the periodic table: carbon in group 14, nitrogen and phosphorus in group 15, and oxygen, sulfur and selenium in group 16. They have four to six outer electrons, as in oxygen ([He] 2s² 2p⁴), while hydrogen has just one (1s¹). Their high ionization energies and electronegativities mean they tend to gain or share electrons rather than lose them.

Physical properties

At room temperature hydrogen, nitrogen and oxygen are colorless gases made of two-atom molecules (H₂, N₂ and O₂), while carbon, phosphorus, sulfur and selenium are solids. Most nonmetals have low melting and boiling points and conduct heat and electricity poorly. Carbon is the big exception: diamond is the hardest natural material and graphite conducts electricity.

Several nonmetals exist as allotropes, different forms of the same element, such as diamond and graphite, oxygen (O₂) and ozone (O₃), and white and red phosphorus.

Chemical behavior and trends

With metals, nonmetals gain electrons to form negative ions such as O²⁻, S²⁻ and N³⁻. With each other they share electrons in covalent bonds, forming molecules such as water (H₂O), carbon dioxide (CO₂), methane (CH₄) and ammonia (NH₃). Nitrogen gas is very unreactive because of the strong triple bond in N₂.

Oxygen supports burning: methane burns as CH₄ + 2O₂ → CO₂ + 2H₂O, and sulfur burns to sulfur dioxide, S + O₂ → SO₂. Nonmetal oxides are usually acidic, which is why sulfur dioxide and nitrogen oxides cause acid rain. Nonmetallic character increases across a period and decreases down a group, so gray selenium is already a semiconductor.

Nonmetals in life and industry

Oxygen, carbon, hydrogen and nitrogen make up about 96% of the mass of the human body, and phosphorus and sulfur are essential parts of DNA and proteins. Selenium is a trace nutrient but toxic in larger doses. Hydrogen is the most abundant element in the universe, and air is about 78% nitrogen and 21% oxygen.

Nitrogen and hydrogen are combined in the Haber process to make ammonia for fertilizers (N₂ + 3H₂ ⇌ 2NH₃). Oxygen is used in hospitals and steelmaking, sulfur is turned into sulfuric acid, the most produced industrial chemical, and carbon is the basis of fuels, plastics and steel.