Transition metals

Iron, copper, gold and their relatives: the hard, colourful, useful metals that fill the middle of the table.

The wide central block of the periodic table, from group 3 through to group 12, holds the transition metals. These are the metals most people picture when they hear the word metal: iron, copper, silver, gold, nickel, titanium and their relatives. They are strong, shiny, dense and mostly slow to react, which is exactly why civilisation has been built out of them.

Chemists argue about the edges of the family, but the usual definition is an element whose atoms, or whose common ions, have a partly filled d subshell of electrons. That shared electron arrangement gives the group its personality. A handful of synthetic superheavy elements sit in the same columns, with properties predicted rather than measured.

What they have in common

Transition metals are typically hard, strong and dense, with high melting and boiling points. Tungsten melts at about 3422 degrees Celsius, the highest melting point of any metal, which is why it was chosen for the glowing filaments in old light bulbs. Osmium and iridium are the densest elements known, each more than twenty-two times as dense as water.

They are good conductors of heat and electricity, with copper and silver the best of all. Most react only slowly with air and water, and some, such as gold and platinum, hardly react at all.

Two chemical habits set them apart from metals in groups 1 and 2. First, they can form ions with several different charges. Iron forms both iron(II) and iron(III) ions, and manganese ranges from plus two up to plus seven. Second, their compounds are often brightly coloured: copper compounds are usually blue or green, and manganese gives the deep purple of potassium manganate(VII). The colours come from electrons in the d subshell absorbing particular colours of light. Many transition metals are also excellent catalysts, substances that speed up reactions without being used up.

How they change across the block

Moving across a row of transition metals, each new element adds a proton to the nucleus and an electron to an inner d subshell rather than to the outer shell. Because the outer shell barely changes, neighbouring elements are far more alike than neighbours in the rest of the table. Atomic size changes only slightly across the block, which is why so many of these metals mix freely to form alloys such as steel, brass and bronze.

Melting points and hardness tend to peak in the middle of each row, where the d subshell is about half full and there are the most electrons available for metallic bonding. That is why chromium and tungsten are so tough while scandium and zinc are soft.

Going down a column, the metals become heavier and denser. Scandium at the top of group 3 has a density of only about three times that of water. Two rows below, in the same region of the table, hafnium and tantalum are more than four times denser than that.

Where you meet them

Iron, turned into steel, holds up buildings, bridges, ships and cars. Copper carries electricity through every wire in your home. Zinc coats steel to stop it rusting, in a process called galvanising, and pairs with copper to make brass. Nickel and chromium make stainless steel stain less. Titanium is light, strong and does not corrode, so it is used for aircraft parts and artificial hips.

The precious metals are transition metals too. Gold and silver are prized for jewellery and coins, but silver also has the best electrical conductivity of any element, and gold is used in electronics because it never tarnishes. Platinum, palladium and rhodium sit inside the catalytic converter of a petrol car, turning harmful exhaust gases into less harmful ones. Many transition metals are also essential to life in tiny amounts: iron carries oxygen in blood, cobalt sits at the heart of vitamin B12, and zinc and copper help enzymes work.

Odd ones out

Mercury is the obvious rebel. It is the only metal that is liquid at room temperature, melting at about minus 39 degrees Celsius. Its neighbours in group 12, zinc and cadmium, are also low melting and form only one type of ion, with a charge of plus two. Because their d subshells are completely full, some chemists do not count group 12 as true transition metals at all.

Technetium is another oddity. Every one of its isotopes is radioactive, so almost none survives in nature, and it was the first element to be made artificially rather than dug out of the ground. The synthetic elements from rutherfordium onwards complete the block on paper, but they exist for only fractions of a second and their chemistry has been tested one atom at a time.

The numbers

Melting points rise down the list, from Scandium at 1541 °C to Rutherfordium at 2127 °C.

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

Did you know?

• Tungsten melts at about 3422 degrees Celsius, the highest melting point of any metal.
• Mercury is the only metal that is liquid at ordinary room temperature.
• Osmium and iridium are the two densest elements known, each more than twenty-two times as dense as water.
• Copper compounds are usually blue or green, and the colour comes from electrons in the d subshell absorbing light.
• Technetium was the first element to be produced artificially, because it has no stable isotopes.
• Silver is the best electrical conductor of any element, with copper a close second.

Frequently asked questions

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