Actinides

Fifteen radioactive heavyweights, from the uranium in reactors to elements that exist for minutes at a time.

No other family of elements has changed history the way the actinides have. Uranium and plutonium powered the first nuclear weapons and still power nuclear reactors. A speck of americium sits in the smoke detector on your ceiling. And beyond plutonium stretches a series of elements that do not exist in nature at all, made atom by atom in laboratories.

The actinides are the fifteen elements from actinium to lawrencium. They form the lower of the two rows parked beneath the main table, in period 7. Every single one of them is radioactive. That shared instability, more than any chemical trait, is what defines the family.

What they have in common

Like the lanthanides above them, the actinides fill an f subshell, in this case the 5f. Their atoms are large and heavy, and they are all metals: silvery, dense and fairly reactive, tarnishing in air and reacting with hot water and acids. Uranium is about nineteen times as dense as water, and neptunium and plutonium are denser still.

Radioactivity is the trait they all share. The nuclei of actinide atoms are so large that the forces holding them together are stretched to the limit, and sooner or later they break apart, emitting radiation. Some thorium and uranium isotopes survive for billions of years, which is why those two are still found in rocks, while the heaviest actinides last only hours or minutes.

Some actinide isotopes can also undergo fission, splitting into two smaller nuclei when struck by a neutron and releasing a burst of energy and more neutrons. This chain reaction is the basis of nuclear power and nuclear weapons.

How they change across the row

The early actinides do not behave like the lanthanides at all. Their 5f electrons are less deeply buried than the 4f electrons of the lanthanides, so they can take part in bonding. As a result thorium, protactinium, uranium, neptunium and plutonium show a wide range of oxidation states. Uranium forms ions with charges of plus three, four, five and six, and its compounds can be green, yellow, orange, red or black depending on the state.

From americium onwards the f electrons settle deeper into the atom, and the elements start to behave more like the lanthanides, mostly forming plus-three ions with similar chemistry.

Melting points are irregular. Thorium melts at about 1750 degrees Celsius, uranium at about 1132 degrees, but neptunium and plutonium melt at only about 639 degrees. For the elements beyond curium, most physical properties are estimated from a few atoms or predicted from theory rather than measured on a solid sample.

Where you meet them

Uranium is the actinide you are most likely to have benefited from. Enriched in its fissile isotope, it fuels the nuclear power stations that supply electricity in many countries. The steady decay of uranium isotopes is also used to date rocks and to work out the age of the Earth. Thorium was once used in the glowing mantles of gas lamps and is being studied as an alternative reactor fuel.

Plutonium is produced inside reactors from uranium. Its main uses are in nuclear weapons and in the power sources of long-distance space probes, where the heat from its decay is turned into electricity far from the Sun. Americium is the one actinide found in ordinary homes: a tiny amount in a smoke detector ionises the air so that smoke particles can be detected. Actinium and a few other isotopes are being trialled in targeted cancer treatments, where their short-range radiation can destroy tumour cells while sparing nearby tissue.

Odd ones out

Thorium is the exception to the family's variable chemistry. It behaves almost entirely as a plus-four metal, more like zirconium and hafnium in group 4 than like uranium. It is also the least radioactive actinide in practical terms, because its main isotope decays so slowly. Uranium and thorium are the only actinides that occur in nature in useful quantities; actinium, protactinium and traces of neptunium and plutonium exist only where those two are decaying.

Everything from americium onwards is synthetic. The elements beyond fermium cannot be made in weighable amounts at all, only as a few atoms at a time in particle accelerators, so their chemistry has been studied by tracking single atoms. Lawrencium, at the end of the row, is the actinide most often argued over. Its outer electron arrangement does not quite match the rest of the family, and some chemists prefer to place it with the transition metals in group 3 instead. Wherever it is drawn, it closes the row, and the next element, rutherfordium, begins the superheavy transition metals.

The numbers

Melting points rise down the list, from Actinium at 1227 °C to Lawrencium at 1627 °C.

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

Did you know?

• Every actinide is radioactive, though some isotopes of thorium and uranium last for billions of years.
• Uranium is about nineteen times as dense as water, and neptunium and plutonium are denser still.
• Americium is the only actinide found in ordinary homes, inside smoke detectors.
• Plutonium metal has six different solid forms between room temperature and its melting point.
• Uranium compounds can be green, yellow, orange, red or black depending on their oxidation state.
• Only uranium and thorium occur in nature in useful amounts; everything beyond plutonium is made artificially.

Frequently asked questions

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