Post-transition metals

Softer, lower-melting metals that sit between the transition metals and the metalloids, from drinks cans to solder.

Between the hard transition metals and the half-metal metalloids lies a loose collection of elements that are definitely metals, but only just. They are softer, they melt more easily and they are less keen to react than most metals. Chemists call them the post-transition metals, or sometimes the poor metals, because they sit after the transition metals in each row of the table.

The core members are aluminium, gallium, indium, tin, thallium, lead, bismuth and polonium. Four synthetic superheavy elements, nihonium, flerovium, moscovium and livermorium, occupy the same columns in the bottom row and are often added to the list, though their properties are predicted rather than tested.

What they have in common

Post-transition metals have their outer electrons in a p subshell, which is why they sit in the right-hand part of the table with the non-metals. They are still metals: shiny, and good conductors of heat and electricity. But their metallic bonding is weaker than in the transition metals, so they tend to be softer and to melt at lower temperatures. Tin melts at about 232 degrees Celsius, lead at about 328 degrees and bismuth at about 272 degrees, all of which are within reach of a kitchen hob.

Their oxides often behave as both acids and bases, a property called being amphoteric. Aluminium oxide, for example, will dissolve in either a strong acid or a strong alkali.

Many of them form more than one type of ion, but the pattern is different from that of the transition metals. The charges are separated by two, such as tin(II) and tin(IV) or lead(II) and lead(IV). Their metals also resist corrosion surprisingly well, usually because a thin, tough oxide film forms on the surface and seals the metal underneath.

How they change across the block

Because this family stretches across several columns and rows, its trends run in two directions. Moving right along a row, the elements become less metallic. Aluminium and gallium are clearly metals, while bismuth and polonium are on the edge of the metalloid zone, and a step further right you leave the metals altogether.

Moving down a column, the elements become more metallic, which is the opposite of the trend across a row. In group 14, carbon is a non-metal, silicon and germanium are metalloids, and tin and lead are metals. This is why the post-transition metals form a diagonal band that widens towards the bottom of the table.

Melting points do not follow a simple rule. Gallium melts at about 30 degrees Celsius, low enough to melt in your hand, yet it boils at about 2400 degrees Celsius, giving it one of the widest liquid ranges of any element. Aluminium, directly above gallium, melts at a much higher 660 degrees. Density rises steadily down each column, with lead at more than eleven times the density of water.

Where you meet them

Aluminium is the most abundant metal in the Earth's crust and the most widely used metal after iron. It is light, strong for its weight and does not rust, so it is used for drinks cans, foil, window frames, aircraft skins and high-voltage power lines. Tin coats steel food cans to stop them corroding and goes into solder and bronze. Lead was once used for pipes, roofing, paint and petrol additives, but because it is poisonous most of those uses have been phased out. It survives in car batteries and as shielding against X-rays.

Gallium and indium are hidden inside modern electronics. Gallium compounds make the blue and white LEDs in screens and lighting, and indium tin oxide is the transparent conductive coating that lets a touchscreen sense your finger. Bismuth is the friendliest heavy metal. It is barely toxic, so it replaces lead in fishing weights, shotgun pellets and some solders, and one of its compounds is a familiar pink stomach remedy.

Odd ones out

Polonium hardly fits the everyday image of a metal at all. It is intensely radioactive and extremely rare, formed only as uranium decays in rocks. Its melting point of about 254 degrees Celsius is known, but its chemistry is studied in tiny amounts behind heavy shielding.

Thallium is another unsettling member. It is soft and easily shaped like lead, but it is far more toxic and was once used as rat poison before being banned in many countries. The four superheavy members, nihonium to livermorium, exist for seconds or less. Flerovium in particular has been predicted to be so volatile that it may behave almost like a noble gas rather than a metal, and experiments to settle the question are still being done with single atoms.

The numbers

Melting points fall down the list, from Aluminium at 660 °C to Livermorium at 436 °C.

Density in g/cm³ (g/L for gases). Values for synthetic elements are predictions.

Did you know?

• Gallium melts at about 30 degrees Celsius, so a piece of it will melt in the palm of your hand.
• Aluminium is the most abundant metal in the Earth's crust, but it was rare and expensive until an electrical method of extracting it was developed.
• Bismuth expands as it solidifies, one of very few substances that do so, and forms colourful stepped crystals.
• Lead was widely used in pipes, paint and petrol before its toxicity led to most of those uses being banned.
• Indium tin oxide is the transparent conductor that makes touchscreens work.

Frequently asked questions

Written by Anthony. Data from Periodic-Table-JSON (CC BY-SA).