Atomic number 72

Hafnium

Hf
+ phase
− phase

Shells Orbitals 4f 5d 6s

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72 electrons in 6 shells: 2, 8, 18, 32, 10, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Transition metal
Period: 6
Group: 4
Block: d
Standard atomic weight: 178.49(1) Da
Electron configuration: [Xe] 4f¹⁴ 5d² 6s²
Phase at 25 °C: solid
Melting point: 2233 °C
Boiling point: 4603 °C
Density: 13.3 g/cm³
Electronegativity (Pauling): 1.3
Discovered: 1923, Dirk Coster and George von Hevesy

Neighbours in the table

Lu Ta

Hafnium was found where a theory said it would be. Niels Bohr, reasoning from his account of how electrons are arranged, argued that element 72 would not be a rare earth. In the account of CIAAW, IUPAC’s atomic-weights commission, he expected it in zirconium ore instead. In 1923 Dirk Coster and George von Hevesy, working at Bohr’s institute in Copenhagen by CIAAW’s account, identified it in zircon from Norway by X-ray spectroscopy, Los Alamos reports. The city gave the name, from hafnia, its Latin form.

A rival had come first. The French chemist Georges Urbain claimed an element called celtium in 1911, in samples of rare earths, a claim that proved mistaken in CIAAW’s account.

Hafnium is the first element after the f block, whose members belong between lanthanum and hafnium in the periodic table, in Chem1’s account. Among hafnium’s 72 electrons is a filled 4f subshell: [Xe] 4f¹⁴ 5d² 6s², with shells of 2, 8, 18, 32, 10 and 2. Its usual oxidation state, a bookkeeping charge as if all bonds were ionic, is +4 in Jefferson Lab’s listing; Los Alamos also lists +3, +2, +1 and −2.

Los Alamos describes a ductile metal with a brilliant silver lustre. Chemically it is almost a copy of zirconium, directly above it in group 4, yet about twice as dense.

Where it is found

Jefferson Lab’s estimates on PubChem’s record put hafnium at 3.0 milligrams per kilogram of the crust and 7 × 10⁻⁶ milligrams per litre of seawater. It travels with zirconium. The US Geological Survey puts zirconium ahead of hafnium by about 50 to 1 in zircon. The same survey ties world hafnium resources to zircon and baddeleyite.

Some hafnium-176 in rocks is radiogenic, made by the beta decay of lutetium-176, whose half-life is 3.76 × 10¹⁰ years in the evaluation of the International Atomic Energy Agency (IAEA). Because that half-life is so long, IUPAC’s notes say, the ratio of the two isotopes has dated some of the oldest rocks on Earth.

Von Hevesy took part in the first separation of hafnium from zirconium. Van Arkel and de Boer made the first metal by passing tetraiodide vapour over a hot tungsten filament. Nearly all hafnium metal today comes from reducing the tetrachloride with magnesium or sodium, the Kroll process, Los Alamos says. In the United States a producer in Oregon and another in Utah make it from zirconium intermediates.

Why it matters

Hafnium goes into reactor control rods, Los Alamos says, because it absorbs slow neutrons well, is mechanically strong and resists corrosion. Its absorption cross section for thermal neutrons, a measure of how readily nuclei capture slow ones, is almost 600 times zirconium’s. Such rods serve in nuclear submarines. Zirconium, with its low cross section, clads nuclear fuel, and reactor-grade zirconium is essentially hafnium-free. Boron or cadmium-silver-indium alloys sometimes take hafnium’s place in power-plant rods, the US Geological Survey notes.

By the same survey’s reckoning, superalloys are the leading use of hafnium metal. Hafnium carbide, HfC, melts near 3890 °C, higher than any other known compound of just two elements in Jefferson Lab’s account. Its nitride, HfN, melts at 3310 °C by Los Alamos’s figure and at around 3305 °C by Jefferson Lab’s.

A vanished isotope also makes hafnium a clock. Hafnium-182 turns into tungsten-182, and its half-life, 8.9 × 10⁶ years, is far shorter than the age of the Earth or of meteorites. Tungsten isotopes in meteorites and Earth rocks still show that it was once present. Those measurements, IUPAC notes, record how the material of the early Solar System separated chemically and evolved.

Properties

Properties of hafnium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	72
Standard atomic weight	178.49(1) Da	Da	IUPAC/CIAAW 2024
Melting point	2233 °C	°C	Los Alamos: 2233 °C
Boiling point	4603 °C	°C	Los Alamos: 4603 °C
Density	13.3 g/cm³	g/cm³
Electronegativity (Pauling)	1.3
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d²
Phase at 25 °C	solid

Safety

A hafnium rod or bar is relatively inert, a mild irritant of eyes, skin and mucous membranes with no industrial poisonings on record in the US Hazardous Substances Data Bank. Fine hafnium powder is different: it can catch fire by itself in air, the hazard behind the Danger classification in about half of company reports to the European Chemicals Agency. The eyes, skin, mucous membranes and liver are its target organs for NIOSH, the US National Institute for Occupational Safety and Health. Workplaces handling the metal have that card and the PubChem Laboratory Chemical Safety Summary as references.

Questions

Why are hafnium and zirconium so hard to separate?

The two metals are chemically similar and always found mixed together, which is why pure zirconium is so hard to obtain, in Jefferson Lab's account of zirconium. Los Alamos calls the pair two of the hardest of all elements to separate. The first separation, in which von Hevesy took part, relied on crystallising double fluorides of ammonium or potassium again and again. One plain difference is density: hafnium is about twice as dense as zirconium.

Is hafnium radioactive?

Very slightly. Hafnium has five stable isotopes in Jefferson Lab's listing, and hafnium-180 is the commonest, about 35 per cent of the atoms in CIAAW's table. A sixth natural isotope, hafnium-174, makes up about 0.16 per cent. It decays by alpha emission, in the Atomic Mass Data Center (AMDC) evaluation, with a half-life of 2.0 petayears, or 2.0 × 10¹⁵ years.

What is hafnium used for besides nuclear reactors?

Superalloys take the largest share of the metal, the US Geological Survey says. Hafnium also goes into gas-filled and incandescent lamps and into alloys with iron, titanium, niobium and tantalum, Los Alamos says. In vacuum tubes it serves as a getter, which Jefferson Lab defines as a material that takes up stray gases and clears them out. Zirconium can replace hafnium in certain superalloys, the survey adds.

How much does hafnium cost?

Unwrought hafnium sold for about 778 US dollars per kilogram in 2020, by the US Geological Survey's figures. The price roughly doubled in 2022, reached 6,150 dollars in 2023 and eased to an estimated 4,600 in 2024. Between 2020 and 2023 the United States bought half its unwrought imports from Germany, with China at 21 per cent and France at 18 per cent.