Reactive nonmetals

The elements you are made of

About 96 per cent of the mass of the human body is just four elements: oxygen, carbon, hydrogen and nitrogen. Add phosphorus and sulfur and you have the six elements that build almost every molecule in living things, from DNA and proteins to fats and sugars. Selenium, the seventh member of this group, is needed in tiny amounts for enzymes that protect cells from damage.

Sharing rather than giving

Metals tend to give away electrons. Reactive nonmetals do the opposite: they have nearly full outer shells and complete them by gaining electrons or, more often, by sharing them in covalent bonds. Two atoms sharing a pair of electrons hold each other tightly, which is why these elements form discrete molecules such as O₂, N₂, H₂O and CO₂.

Carbon is the champion. With four electrons to share, it can bond to itself in chains, rings and sheets of almost unlimited length, giving rise to the millions of compounds studied in organic chemistry.

One element, many forms

Several reactive nonmetals exist as different structural forms, called allotropes, with startlingly different properties.

• Carbon: diamond, the hardest natural material; graphite, soft and slippery; graphene and fullerenes
• Oxygen: the O₂ we breathe and ozone, O₃, which shields us from ultraviolet light
• Phosphorus: white phosphorus glows and ignites in air; red phosphorus is stable enough for match boxes
• Sulfur: yellow crystals, or a rubbery brown form when molten sulfur is poured into water

Cycles that sustain the planet

These elements move constantly between air, water, rock and living things. Plants pull carbon dioxide from the air and turn it into sugars; animals and decay return it. Bacteria fix nitrogen from the air into forms plants can use, and since the early 20th century the Haber process has done the same industrially, more than doubling the food the world can grow. Human activity now shifts these cycles on a global scale, most visibly by adding carbon dioxide to the atmosphere.