Post-transition metals: post-transition metals periodic table facts

What is a post-transition metal?

A post-transition metal is a metallic element in the p-block that sits after the d-block transition metals and before the metalloids. In practice that means the metals of groups 13 to 16 in periods 3 to 6, tucked under the diagonal dividing line that separates metals from nonmetals. Students working through isotope tables can check element 58, cerium for the stable isotope list and the standard atomic weight explained.

Because they sit right on the metal-nonmetal border, they keep the shine and electrical conduction you expect from a metal while borrowing several habits from the nonmetals next door. The name is purely a position label: in chemistry it means the metallic elements that come after the transition block in each row of the periodic table.

Physically, these metals are soft or brittle, have low mechanical strength, and melt at far lower temperatures than iron, nickel or tungsten. A pure tin rod about a centimetre thick bends under your thumb, and a lump of gallium turns liquid on a hot summer afternoon.

In inorganic chemistry they are all main group elements, and their chemistry leans covalent: they react with both acids and alkalis, and in strongly basic solution they form anionic species such as aluminates, stannates and bismuthates. Several also form half-metal compounds with very electropositive metals, which is why chemists describe the group as metals with one foot in nonmetal territory.

Which elements are post-transition elements

In chemistry, most sources agree on a core set of eight post-transition elements and argue about the rest. The undisputed core is aluminium, gallium, indium and thallium in group 13, tin and lead in group 14, bismuth in group 15, and polonium in group 16. Some authors drop aluminium because it has no filled d shell beneath its outer shell, and some move polonium to the metalloids, but most lists, including the one below, keep all eight. When comparing barium, calcium and magnesium in the s-block, readers are pointed to the pure radium for group comparisons.

Read down the melting point column and the pattern is obvious: only aluminium clears 600 °C, and gallium melts below body temperature. Mass per unit volume climbs as you move down each group, which is why thallium and lead feel so heavy in the hand despite being soft. The edges of the group are fuzzy, though, and three border zones cause most of the disagreement in the chemistry literature.

Group 12: zinc, cadmium and mercury

Whether zinc, cadmium and mercury count as post-transition metals depends on where you think the transition metals stop. The IUPAC Red Book calls groups 3 to 12 the d-block but notes that the group 12 elements are often left out of the transition series, and the IUPAC Gold Book definition of a transition metal (an atom with an incomplete d sub-shell) excludes them, because their d electrons stay full in every common compound. Surveys of chemistry textbooks have found them filed under transition and main group headings in roughly equal numbers.

Their behaviour supports the post-transition reading. Zinc and cadmium are soft, pack in a slightly distorted version of the close-packed hexagonal structure, form amphoteric hydroxides, and dissolve in strong alkali as zincates and cadmates. Mercury is the extreme case: the only metal that is liquid at room temperature, with the weakest metallic bonding of any metal.

Copper, silver, gold and platinum at the edge

A few authors push the line further left. Copper, silver and gold, the coinage metals of group 11, have the relatively low melting points and high electronegativity typical of the group, and in their +1 valence state their compounds behave much like those of thallium(I).

Platinum in group 10 is occasionally added for its softness and covalent complexes, although it melts at about 1,768 °C. Most references still treat all four as transition metals, because copper(II) and gold(III) chemistry shows classic partly filled d shells, and gold is a true noble metal that never tarnishes in air.

Metalloids, astatine and the superheavy elements

On the right-hand side the border runs into the metalloids. Germanium, arsenic, antimony and tellurium are usually classed as metalloids, and selenium as a nonmetal, yet older texts list some of them as post-transition metals because they look metallic and conduct. Astatine is a special case: no one has ever seen a visible sample, but relativistic modelling predicts a metallic crystal, which would make astatine a post-transition metal on paper.

The superheavy elements 112 to 118 are candidates too. Copernicium, nihonium, flerovium, moscovium, livermorium, tennessine and even oganesson have only been made a few atoms at a time, so their chemistry is predicted rather than measured. Calculations suggest copernicium and flerovium may be volatile, mercury-like metals, while nihonium and moscovium should favour a +1 state much like thallium.

Why the poor metals are less metallic

The weaker metallic character of the group comes down to how tightly each atom holds its outer electrons. Moving left to right across a period, every extra proton pulls harder on the electron cloud, so atoms shrink, electronegativity rises and fewer bonding pairs are free to join the shared sea that makes a metal a metal. Inorganic chemistry explains the whole classification with this one trend, sharpened by two effects that are unique to the heavier rows.

Nuclear charge and the d-block contraction

From period 4 onward, the post-transition metals sit directly after a filled d shell, and in period 6 also after a filled 4f shell. d and f electrons screen the outer shell poorly (screening power falls in the order s > p > d > f), so the effective nuclear charge felt by the outer shell is higher than you would expect. The result is the d-block contraction after the first transition row and the lanthanide contraction after the 4f series: atomic radii are smaller, ionisation energies are higher, and the cations that form are small and strongly polarizing, which pushes bonds toward covalency.

Relativistic effects in period 6

Thallium, lead, bismuth and polonium get an extra push from relativistic effects. In atoms this heavy, the innermost shells move at a sizeable fraction of the speed of light, which contracts the 6s orbital and binds it more tightly, so the 6s pair becomes reluctant to bond: the inert pair effect.

If you know the electron configuration of lead, [Xe]4f¹⁴5d¹⁰6s²6p², you can read its stable oxidation state straight off it: lose the 6p pair, keep the 6s pair, and you get Pb²⁺. The same effect gives mercury, the borderline group 12 member, its liquid state, and it helps keep the melting points of thallium and lead low.

Properties of post-transition metals

Because the group is defined by position rather than one shared trait, its members do not all follow every trend. Still, the properties of post-transition metals cluster tightly enough that inorganic chemistry courses teach them as a family, and each one below shows up across most of the core eight elements.

Melting points, softness and density

Low melting points are the group's most useful trait. Gallium liquefies at 29.8 °C, indium at 156.6 °C and tin at 231.9 °C, compared with 1,538 °C for iron. Lead has a low melting point by engineering standards too, which is why it was cast into pipes and shot long before furnaces could handle steel.

Softness comes from the same weak bonding: on the Mohs scale indium and thallium sit near 1 and lead and tin near 1.5, so you can mark them with a fingernail. Hardness is low, yet most of the group is ductile and malleable, and indium is plastic enough to cold-weld when two clean pieces are pressed together.

Their structures show the strain of sitting near the nonmetals. Aluminium and lead adopt a close-packed face-centred cubic crystalline structure, but the lead atoms sit unusually far apart.

Gallium, indium and tin adopt distorted, low-symmetry arrangements with fewer nearest neighbours than a typical metal. Electrical conductivity and thermal conductivity are generally lower than in the transition metals, aluminium being the big exception, and bismuth is one of the poorest conducting metals known.

Covalent bonding, amphoteric oxides and Zintl phases

Post-transition metals hold their outer shell tightly enough that sharing electron pairs often beats giving them away, so covalent bonding runs through many of their compounds. Aluminium chloride exists as Al₂Cl₆ dimers in the vapour, tin(IV) chloride is a fuming liquid, and organolead chemistry produced the tetraethyl lead once added to petrol. The same directional bonding is why their crystals are less regular than those of the transition metals.

Most of the group forms amphoteric oxides, which react with both acids and bases. Aluminium's oxide dissolves in hydrochloric acid to give Al³⁺ ions and in sodium hydroxide to give aluminate ions, and the oxides of gallium, tin(II) and lead(II) behave the same way, giving gallates, stannites and plumbates in strong alkali. Put simply, the oxide accepts protons from an acid, while its hydrated form acts as one of the proton donors when it meets a strong base.

They also form Zintl phases, salt-like intermetallic compounds made with very electropositive metals such as sodium, potassium or barium. In NaTl, sodium gives up its outer electron to thallium, which then builds a diamond-like network of Tl⁻ anions. Zintl phases sit between true alloys and ionic salts, and they are a favourite topic in solid-state inorganic chemistry precisely because they show these metals behaving like nonmetals.

Filled d orbitals and wide oxidation states

Every post-transition metal from gallium onward carries a completely filled d subshell under its valence shell. The filled d orbitals stay out of bonding, so the chemistry happens in the outer s and p orbitals. When these atoms form cations they empty the p orbital first and the s pair second, giving two common oxidation states two units apart.

That gap explains the familiar pairs: Tl(I) and Tl(III), Sn(II) and Sn(IV), Pb(II) and Pb(IV), Bi(III) and Bi(V). Down each group the lower oxidation state becomes more stable, so tin(II) is a reducing agent while lead(IV) is a strong oxidiser.

Aluminium is the outlier with essentially one oxidation state, +3, and a strongly negative electrode potential. The transition metals still show a wider spread overall, but this two-step pattern makes post-transition chemistry far easier to predict.

Types of post-transition metals, element by element

The types of post-transition metals share the trends above, but each element has its own personality once you look closer. Here's how the core eight differ, group by group.

Group 13: aluminium, gallium, indium and thallium

Aluminium is the most abundant metal in Earth's crust and the lightest of the eight core elements. The pure metal is soft, but alloyed it has an excellent strength-to-weight ratio, and a thin protective oxide layer gives it strong corrosion resistance. Its weak spot is heat: it loses about half its strength by 200 °C, which limits it wherever fire protection matters.

Gallium melts just above room temperature, but its main job is electronic. Gallium arsenide is a direct-gap semiconductor, and GaN and GaAs are the compound semiconductors behind LEDs, laser diodes and high-frequency radio chips. Indium is softer than lead and goes into indium tin oxide, the transparent conductor on nearly every touchscreen.

Thallium is the dangerous member: tasteless and soluble, it was once sold as rat poison and is now limited to photocells, infrared optics and specialty glass. Thallium chemistry is dominated by a stable +1 oxidation state that mimics potassium inside the body, which is exactly what makes thallium so toxic. All four metals form Zintl phases, from LiAl to NaTl.

Group 14: tin and lead

Tin is soft, malleable and resists corrosion, which is why it coats steel food cans and makes up most of modern electronics solders. Below 13.2 °C, though, ordinary white tin slowly converts to grey tin, a brittle diamond-structure form that takes up more volume, so tin objects left in the cold can crumble to powder.

Lead is a soft metal, dense and easy to cast, shielded by a dull protective oxide and carbonate film. People have smelted it from galena (lead sulfide) for more than 6,000 years, and its high atomic number makes it the standard shield against X-rays and gamma rays.

It is also a cumulative heavy metal poison, which is why leaded petrol, lead paint and lead water pipes have been phased out. Its chemistry is dominated by the +2 state, with Pb(IV) compounds acting as strong oxidisers.

Group 15: bismuth, the least metallic metal

Bismuth is a heavy, brittle, pinkish-silver metal with an open, layered crystalline structure and bonding that is part metallic, part covalent. It is the most diamagnetic metal and one of the poorest conductors.

Unlike lead and polonium it is one of the few heavy elements that is barely harmful, so it replaces lead in plumbing fittings, shot and free-machining steel, and it is the active ingredient in bismuth subsalicylate stomach remedies. Bismuth fuse alloys such as Field's metal melt below 65 °C, and in water its chemistry is dominated by oxyanions such as the bismuthyl ion.

Polonium and the group 16 edge

Polonium is the rarest and most dangerous of the eight elements. It is radioactive: polonium-210 decays by alpha emission with a half-life of 138 days, so a gram of it glows from its own heat.

It is made in reactors by irradiating bismuth-209, and its alpha particles are used in static eliminators and once powered spacecraft heaters. Its chemistry resembles that of tellurium, and it is a soft metal with a simple cubic crystalline structure and a preferred +4 oxidation state, sitting right where group 16 turns from nonmetal (selenium, tellurium) to metal, with astatine one step further right.

Where the basic metals are used

The uses of post-transition metals are lopsided: aluminum (the US spelling you'll see on most technical data sheets) and lead account for most of the tonnage, while gallium, indium and bismuth, the minor elements, fill small, high-value niches. Tin-based solders hold nearly every circuit board in consumer electronics together, and lead-acid batteries still start most petrol cars. In short, the basic metals do quiet, unglamorous work almost everywhere.

Post-transition metals in construction, aerospace and automotive work

In construction, aluminum shows up in window frames, curtain walls, gutters and cladding, and increasingly in building wiring as a cheaper conductor than copper. Aerospace airframes rely on aluminum alloys because every kilogram saved cuts fuel burn, and automotive plants cast aluminum into wheels, engine blocks and body panels for the same reason. Lead's biggest single market is lead-acid batteries, and tin's is circuit-board soldering for electronics manufacturing.

The low melting point trait supports a whole safety industry of its own. Many fire sprinkler heads and self-closing fire doors use fusible links, a bismuth-tin-indium alloy that melts at a set temperature and releases water or drops the door, giving passive fire detection with no electronics at all.

How post-transition metals are smelted and refined

Most of these metals, apart from aluminium and tin, are byproducts: indium is recovered from zinc refining residues, gallium from the Bayer-process liquor at alumina plants, and thallium and bismuth from lead smelting dust, so their supply tracks the output of much bigger mines rather than their own demand. Lead ore is concentrated by froth flotation and smelted to crude bullion, and refining then strips out the minor metals, with bismuth removed by the Betterton-Kroll process.

Choosing a metal: toxicity, temperature and safer substitutes

If you're picking one of these metals for a job, work through three questions in order:

1. Toxicity: lead, thallium, cadmium and polonium carry serious health risks, and lead lingers in the environment, so rules such as the EU RoHS directive push makers toward lead-free solders of tin, silver and copper.
2. Temperature: a part that must melt on cue suits bismuth or indium alloys, while a part that must stay solid suits aluminum or tin.
3. Strength: low mechanical strength is the group's main weakness, so structural use is limited mostly to alloyed aluminum.

Lead zirconate titanate, used in sensors and gas lighters, is one of the last large uses of the metal, and the search for lead-free piezoelectric ceramics is part of the same phase-out.

B-subgroup metals and other names for the group

Few corners of the periodic table carry as many names as this one. The term post-transition metals goes back at least to Horace Deming's 1940 textbook Fundamental Chemistry, which applied it to the elements from copper to germanium, silver to antimony and gold to polonium.

The older label B-subgroup metals comes from pre-1988 group numbering, in which groups IB to VIIB ran from the coinage metals to the halogens; some authors even split them into pseudo metals (groups 12 and 13) and hybrid metals (arsenic, antimony, bismuth, tellurium, polonium and astatine). Earlier inorganic chemistry texts simply counted the whole block among the transition metals of low melting point.

Poor metals, other metals and p-block metals

Poor metals is the informal name you'll find on older school charts, a nod to weak metallic behaviour and a low place in the activity series. Other metals is what many printed periodic table posters use, simply because the group didn't fit the alkali, alkaline earth or transition labels. p-block metals is the most precise name: it covers the metals of groups 13 to 16 and makes the shell logic explicit, though it includes aluminium, which has no filled d shell under its outer shell.

Frontier, fusible, soft and heavy metals

Frontier metals

The chemically weakest metals beside the metal and nonmetal boundary, stressing that they occupy frontier territory.

Fusible metals

A materials-handbook label for zinc, cadmium, mercury, gallium, indium, thallium, tin, lead, antimony and bismuth, the ingredients of low-melting alloys.

Soft metals

Groups 11 to 15 plus platinum, the source of brass, bronze and sterling silver.

Heavy metals of low melting point

A category on the classic Sargent-Welch chart that runs from zinc through polonium.

Ordinary, peculiar and less typical metals

Physicists draw a different line. Abrikosov and Gray call these the ordinary metals, as opposed to transition metals with unfilled inner shells, while Slater split metals into ordinary and peculiar metals, the latter having directional bonding near the nonmetals. Habashi's less typical metals class bundles groups 11 and 12 with gallium, indium, thallium, tin and lead.

Metametals, borderline metals, base metals and semimetals

Metametals (zinc, cadmium, mercury, indium, thallium, tin and lead) are ductile but show distorted structures. Borderline metals is Parish's term for gallium, indium, thallium, germanium and tin, all with non-standard structures.

Base metals, in Mingos's sense, are metals that need oxidizing acids to dissolve. Semimetals, finally, is a loose term sometimes applied to gallium, bismuth and mercury for their incomplete metallic character, though it more often means the metalloids.

Chemically weak metals versus their neighbours

Rayner-Canham and Overton call the metals nearest the staircase line chemically weak metals, a list that adds beryllium and magnesium to aluminium, gallium, tin, lead, bismuth and polonium. Comparing the group with its neighbours shows where that weakness comes from.

Post-transition metals compared with transition metals

The transition metals to the left have partly filled d subshells, which give them high melting temperatures, real structural strength, coloured compounds and many oxidation numbers. Post-transition metals have that subshell full, so they melt lower, bend more easily, mostly form colourless compounds, and switch between just two common charges. That fault line is also why the group 12 elements sit so awkwardly between the two families.

Compared with metalloids

Metalloids such as boron, silicon, germanium, arsenic and tellurium sit on the staircase itself. They look metallic but are semiconducting rather than true conductors, and their oxides lean acidic, so much of their chemistry is that of nonmetals. Post-transition metals are true metals: their conductivity falls as temperature rises, and they have clearly higher boiling points than the metalloids in the same period.

Compared with light metals

Light metals are defined by low density alone (lithium, magnesium, aluminum, titanium), so the overlap with this group is just aluminum. Thallium and lead, at more than 11 g/cm³, are firmly heavy. Post-transition metals are defined by position and chemistry instead, so the two labels answer different questions.