Arsenic (As)
“History’s favourite poison — tasteless, odourless, ruthless — and today the silent threat inside millions of Bangladesh’s tubewells.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
The notes
Characteristics
- A group 15 metalloid — period 4, p-block, configuration [Ar] 3d¹⁰ 4s² 4p³, five valence electrons — standing chemically midway between phosphorus above it and antimony below.
- Heated in the open, grey arsenic refuses to melt and instead sublimes near 615 °C into garlic-smelling vapour that condenses back as a shiny black, mirror-like film.
- A slow cellular killer: arsenic ions sabotage the enzymes that build ATP, starving cells of their energy currency; even chronic small doses cause the skin lesions and cancers of arsenicosis.
- Utterly stealthy in water — colourless, tasteless and odourless — so a contaminated tubewell can only be caught by chemical testing; Bangladesh’s drinking-water safety line is 50 parts per billion.
- Three allotropes are known — grey, black and the soft yellow form — a shape-shifting habit it shares in spirit with phosphorus, its upstairs neighbour in the group.
The common grey form is a brittle, steel-bright metalloid that shatters like glass; the element also wears a yellow allotropic costume — soft, waxy and garlicky — which crumbles back to the grey form when light strikes it. In water it is perfectly invisible, which is exactly what makes it so dangerous.
Where you meet it
✦ Uses
- Gallium arsenide (GaAs) chips: the radio-frequency amplifiers that let a smartphone shout to a tower kilometres away run on this arsenic compound, which switches faster than silicon at high frequency.
- Arsenic trioxide (As₂O₃) — the old poison returning as medicine: sold as Trisenox, it is an approved therapy for acute promyelocytic leukaemia.
- The SONO filter: Bangladeshi-born chemist Abul Hussam’s tower of cast-iron turnings, charcoal, sand and brick chips that drags dissolved arsenic out of tubewell water — winner of the million-dollar Grainger Challenge Prize in 2007.
- Metal-workman’s whisker: a pinch of arsenic hardens the lead grids of car batteries; its older jobs — the pesticide lead arsenate and the wood preservative CCA — are now banned or heavily restricted.
◍ Everyday examples
- Red-painted tubewells across rural Bangladesh mean “do not drink” — over the arsenic limit; green or blue marks the safe ones, a colour-code that has saved countless lives.
- Ancient Egyptian artists reached for orpiment when a wall needed brilliant yellow; Victorian wallpaper dyed with arsenic green slowly poisoned its own bedrooms.
- Rice takes up arsenic from field water more eagerly than wheat or maize — one reason Bangladesh screens its paddy soils and irrigation water.
History & name
Arsenic’s two bright sulfide minerals — golden orpiment and red realgar — were painted onto papyrus and temple walls by the Egyptians and traded across the ancient world long before the element itself was understood. Around 1250 the scholar Albertus Magnus earned the credit for first isolating the metal-like substance, and for six hundred years arsenic reigned as the poisoner’s perfect choice: colourless compounds, no taste, no smell — Europe whispered its nickname, “inheritance powder”. James Marsh’s test of 1836, which could catch arsenic inside a corpse, founded forensic chemistry and dried up the poisoner’s trade. Then came the modern twist: from the 1990s Bangladesh discovered that millions of its tubewells, sunk into the delta’s grey sediments, were pouring naturally occurring arsenic into drinking cups — a mass poisoning the World Health Organization has called the largest in history.