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Between the transition metals and the staircase of metalloids lies a small, often-overlooked set of elements. They’re definitely metals — shiny, conductive, malleable — but weaker and softer than their neighbours to the left. They’re called the post-transition metals, and sometimes, less kindly, the “poor metals”.
Which elements?
Definitions vary more for this category than almost any other. On our table, the post-transition metals are:
| Element | Density (g/cm³) | Melting point (°C) |
|---|---|---|
| Aluminium | 2.70 | 660 |
| Gallium | 5.91 | 30 |
| Indium | 7.31 | 157 |
| Tin | 7.29 | 232 |
| Thallium | 11.8 | 304 |
| Lead | 11.34 | 327 |
| Bismuth | 9.81 | 271 |
Some tables also include zinc, cadmium and mercury (group 12), polonium, or the predicted properties of superheavy elements like nihonium and flerovium. Classification is a convention, not a law of nature — see metals, nonmetals and metalloids.
What they have in common
- Low melting points for metals. Gallium melts in your hand; tin and lead melt easily on a stove. Compare iron at 1,538 °C.
- Softness. Lead and indium can be scratched with a fingernail.
- More covalent character in their compounds than transition metals, and oxides that are often amphoteric (reacting with both acids and bases — aluminium oxide and hydroxide are classic examples).
- Filled d subshells. Their d electrons are locked in a full shell and don’t contribute much to metallic bonding, which is why the bonding is weaker than in the transition metals.
Many also show the inert pair effect: the heavier ones (thallium, lead, bismuth) prefer oxidation states two lower than their group number, because their outer s electrons are held unusually tightly. Thallium is often +1, lead +2, bismuth +3.
Aluminium: the exception that rules the world
Aluminium is far from “poor” in practical terms. It’s the most abundant metal in the Earth’s crust, light (a third the density of steel), corrosion-resistant thanks to a thin, tough oxide film, and an excellent conductor. It’s used in drinks cans, foil, aircraft, window frames, cars and overhead power lines (see best conductor of electricity). Recycling aluminium uses only about 5% of the energy needed to make it from ore.
Gallium and indium: electronics
- Gallium is combined with arsenic and nitrogen to make gallium arsenide and gallium nitride semiconductors, found in LEDs (blue and white LEDs depend on gallium nitride), laser diodes, fast chargers and solar cells. It was one of Mendeleev’s predicted elements (“eka-aluminium”), discovered in 1875.
- Indium, as indium tin oxide (ITO), forms the transparent conductive layer in touchscreens and flat-panel displays.
Tin and lead: the old metals
Humans have used both for thousands of years. Tin made bronze possible; lead made Roman plumbing. Today tin is used in solder and tin-plated steel, and lead mostly in car batteries and radiation shielding. Lead is toxic, and its use in petrol, paint and pipes has been phased out in most countries. See the carbon group.
Thallium and bismuth: poison and cure
Thallium is extremely toxic — its compounds were once used as rat poison and, notoriously, by murderers, because they’re tasteless and the symptoms look like other illnesses. Its use today is limited and tightly controlled.
Bismuth, right beside it, is one of the least toxic heavy metals — it’s in stomach medicines. It’s increasingly used as a non-toxic replacement for lead. See the nitrogen group.
Quick answers
Why are they called post-transition metals? Because they come after the transition metals in each row of the periodic table.
Are post-transition metals weak? Mechanically they’re softer and lower-melting than most transition metals, and more chemically “in between” metals and nonmetals.
Is aluminium a post-transition metal? On many tables, including ours, yes — though some classify it simply as a “metal” of group 13.
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