The soft, low-melting metals wedged between the d-block and the metalloids

Post-transition metals

Aluminium, gallium, indium, tin, lead, bismuth and their neighbours. They are metals, but poor ones — softer, weaker and lower-melting than the transition metals, with a stubborn tendency to behave covalently. Between them they gave us cans, solder, touchscreens and plumbing.

Members
12
Group
13–16 (p-block)
Configuration
ns² np¹⁻⁴
Radioactive
5

Highlighted in the periodic table

Every member

The 12 post-transition metals

13 Al Aluminum 26.982 u Once more precious than gold, now the most-used metal after iron — because someone worked out how to electrolyse it cheaply. solid 31 Ga Gallium 69.723 u A metal that melts in your hand at 29.8 degrees C and is the foundation of blue LEDs and 5G radios. solid 49 In Indium 114.818 u Soft enough to leave a mark on paper, and the transparent conductor in every touchscreen. solid 50 Sn Tin 118.71 u Bronze's other half: tin turned the Stone Age into the Bronze Age. solid 81 Tl Thallium 204.38 u Colourless, tasteless and lethal — thallium was the poisoner's element until detection caught up. solid 82 Pb Lead 207.2 u Dense, soft and endlessly useful — and quietly responsible for centuries of chronic poisoning. solid 83 Bi Bismuth 208.98 u The heaviest element that is very nearly stable, and the one that grows spectacular rainbow crystals. solid 84 Po Polonium 209 u Intensely radioactive: a gram of polonium-210 emits enough alpha particles to glow blue and self-heat to 500 degrees C. radioactive solid 113 Nh Nihonium 284 u The first element discovered and named by an Asian research team. radioactive synthetic solid 114 Fl Flerovium 289 u Predicted to sit in an "island of stability" where superheavy nuclei live far longer. radioactive synthetic liquid 115 Mc Moscovium 288 u Its decay chain provided crucial confirming evidence for the discovery of nihonium. radioactive synthetic solid 116 Lv Livermorium 293 u Made by fusing calcium-48 with curium-248, and decays within milliseconds. radioactive synthetic solid

What defines them

Three things that make a post-transition metal

01

Metals with an inferiority complex

They conduct and shine like metals, but their bonding is noticeably more directional than the d-block's. Melting points collapse: gallium melts at 30 °C, tin at 232 °C, lead at 327 °C.

02

The inert pair effect

Heavy p-block metals often prefer an oxidation state two lower than their group suggests, because the outer s pair becomes reluctant to bond. Thallium prefers +1 over +3, and lead prefers +2 over +4 — which is why Pb(IV) compounds are strong oxidisers.

03

Amphoteric oxides

Aluminium, zinc, tin and lead oxides react with both acids and bases. That in-between behaviour is exactly what you expect from elements sitting on the boundary between metal and nonmetal.

Periodic trends

How they change across the family

Metallic character increases down each group and decreases across each period. Aluminium is the most reactive and most useful; bismuth is the heaviest element with a nucleus that is very nearly stable.

In the world

What they are used for

Aluminium

The second most-used metal after iron: aircraft, cans, window frames, power lines. Light, corrosion-resistant, endlessly recyclable, and cheap only because of the Hall-Héroult electrolysis process.

Tin

Solder, bronze, and the coating that stops "tin" cans from rusting. Tin-plating steel was the breakthrough that made preserved food practical.

Lead

Batteries, radiation shielding and, historically, everything from plumbing to petrol. Its uses have been steadily withdrawn as its toxicity was recognised.

Gallium & indium

Gallium arsenide and gallium nitride power LEDs, laser diodes and high-frequency radio. Indium tin oxide is the transparent conductor in every touchscreen.

Bismuth

A non-toxic replacement for lead in shot, solder and cosmetics, and the active ingredient in stomach remedies.

Handling

Safety

Lead is a cumulative neurotoxin with no safe exposure level, particularly in children. Thallium is lethal in small doses and was long the poisoner's element of choice. Polonium is intensely radioactive. Aluminium, tin and bismuth are, by contrast, among the least troublesome metals in ordinary use.

Worth knowing

  • Gallium melts at 29.8 °C — hold a piece and it becomes a puddle in your palm.
  • Below 13 °C, ordinary white tin slowly crumbles into grey powder. "Tin pest" has destroyed organ pipes and military buttons.
  • Bismuth crystals form iridescent stair-stepped hoppers because the crystal edges grow faster than their faces.

Common questions

About the post-transition metals

Why are they called "poor metals"?

It is an informal older name reflecting how badly they perform as metals compared with the transition metals: softer, weaker, lower-melting, and more covalent in their bonding. "Post-transition metals" is the modern, less judgemental term for the same set.

Why does aluminium not corrode when iron does?

It does corrode — instantly. The difference is that aluminium oxide forms a dense, adherent film a few nanometres thick that seals the surface and stops further attack. Iron oxide is flaky and porous, so it falls away and exposes fresh metal underneath. Anodising deliberately thickens aluminium's protective layer.

Is bismuth really radioactive?

Technically yes. Bismuth-209 was shown in 2003 to undergo alpha decay with a half-life of about 2×10¹⁹ years — a billion times the age of the universe. For every practical purpose it is stable, and it is routinely used as a safe substitute for lead.

Keep going

The other nine families

6 Alkali metals Li Na K Rb Cs Fr 6 Alkaline earth metals Be Mg Ca Sr Ba Ra 38 Transition metals Sc Ti V Cr Mn Fe Co 7 Metalloids B Si Ge As Sb Te At 7 Nonmetals H C N O P S Se 5 Halogens F Cl Br I Ts 7 Noble gases He Ne Ar Kr Xe Rn Og 15 Lanthanides La Ce Pr Nd Pm Sm Eu 15 Actinides Ac Th Pa U Np Pu Am