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Group 15 holds two elements your body can’t live without — nitrogen and phosphorus — and one of history’s most notorious poisons, arsenic. Below them come antimony and bismuth, and at the bottom, the superheavy moscovium. Together they’re known as the pnictogens (from the Greek for “to choke”, a nod to nitrogen’s ability to suffocate).
Five outer electrons
Each has five outer electrons (ns² np³). They can gain three electrons to form −3 ions (as in nitrides and phosphides), share electrons in covalent bonds, or use up to all five in bonding, giving oxidation states from −3 to +5.
From gas to metal (from our element data)
| Element | Type | State at 25 °C | Melting point (°C) | Electronegativity |
|---|---|---|---|---|
| Nitrogen | Nonmetal | Gas (N₂) | −210 | 3.04 |
| Phosphorus | Nonmetal | Solid (white P) | 44 | 2.19 |
| Arsenic | Metalloid | Solid | sublimes at ~615 | 2.18 |
| Antimony | Metalloid | Solid | 631 | 2.05 |
| Bismuth | Metal | Solid | 271 | 2.02 |
The shift from nonmetal to metal down the group is clear: nitrogen is a gas of small molecules, while bismuth is a shiny, brittle metal.
Nitrogen: abundant but locked away
Nitrogen gas makes up about 78% of the air. But the N₂ molecule is held together by a triple bond, one of the strongest bonds known, which makes it remarkably unreactive. Plants are surrounded by nitrogen but can’t use it directly.
Nitrogen has to be fixed — converted into ammonia or nitrates — before living things can use it. Nature does this through lightning and nitrogen-fixing bacteria (many living in the root nodules of peas and beans). Since the early 20th century, the Haber–Bosch process has done it industrially, combining nitrogen from air with hydrogen to make ammonia for fertilizer. Roughly half of the nitrogen in the proteins of people alive today is estimated to have passed through a Haber–Bosch plant. See Le Chatelier’s principle for the chemistry.
Nitrogen also appears in explosives (TNT, nitroglycerin), because forming N₂’s strong triple bond releases a great deal of energy very quickly.
Phosphorus: the element of energy and DNA
Phosphorus was discovered in 1669 by Hennig Brand, a German alchemist who boiled down large quantities of urine looking for the philosopher’s stone. What he got instead glowed in the dark — the name means “light-bearer” in Greek. It was the first element discovered by a known individual.
Phosphorus has several allotropes:
- White phosphorus — waxy, toxic, catches fire spontaneously in air, and glows as it slowly oxidizes. It’s stored under water.
- Red phosphorus — much more stable; used on the striking strip of safety matchboxes.
- Black phosphorus — the most stable form, layered like graphite.
In living things phosphorus is essential: it forms the backbone of DNA and RNA, it’s in ATP (the molecule cells use to store and transfer energy), and calcium phosphate makes up bones and teeth. Phosphate fertilizers (the “P” in NPK) are mined from phosphate rock, a finite resource.
Arsenic: the poison
Arsenic compounds were notorious poisons for centuries — tasteless, easily hidden, and once hard to detect. The development of the Marsh test in 1836, which could detect tiny traces of arsenic, changed forensic science. Today the main concern is natural arsenic in groundwater, which affects the drinking water of millions of people in parts of South and Southeast Asia and elsewhere. Arsenic is also used in small, controlled amounts in the semiconductor gallium arsenide, and arsenic trioxide is a highly effective treatment for one form of leukaemia.
Antimony and bismuth
Antimony has been used since ancient times — antimony sulfide was an eye cosmetic (kohl) in ancient Egypt. Today it’s a flame retardant and hardens lead in batteries.
Bismuth is unusual for a heavy metal: it has very low toxicity. It’s in stomach remedies (bismuth subsalicylate) and has replaced lead in some solders, shot and fishing weights. Bismuth crystals grown in the lab form striking iridescent, stair-stepped “hopper” shapes. Technically bismuth is very slightly radioactive — see radioactive elements.
Quick answers
What are the pnictogens? Another name for group 15: nitrogen, phosphorus, arsenic, antimony, bismuth and moscovium.
Why is nitrogen gas so unreactive? Its N≡N triple bond takes a great deal of energy to break.
Which group 15 elements are metalloids? Arsenic and antimony.
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