Some elements exist in several different structural forms, called allotropes. Each allotrope has different physical properties.

For more information on the Visual Elements image see the Uses and properties section below.



A vertical column in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

A horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.

Elements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp (s), principal (p), diffuse (d), and fundamental (f).

Atomic number
The number of protons in an atom.

Electron configuration
The arrangements of electrons above the last (closed shell) noble gas.

Melting point
The temperature at which the solid–liquid phase change occurs.

Boiling point
The temperature at which the liquid–gas phase change occurs.

The transition of a substance directly from the solid to the gas phase without passing through a liquid phase.

Density (g cm−3)
Density is the mass of a substance that would fill 1 cm3 at room temperature.

Relative atomic mass
The mass of an atom relative to that of carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists, the value given is the abundance weighted average.

Atoms of the same element with different numbers of neutrons.

CAS number
The Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems.

Fact box

Group Melting point 2233°C, 4051°F, 2506 K 
Period Boiling point 4600°C, 8312°F, 4873 K 
Block Density (g cm−3) 13.3 
Atomic number 72  Relative atomic mass 178.486  
State at 20°C Solid  Key isotopes 177Hf, 178Hf, 180Hf 
Electron configuration [Xe] 4f145d26s2  CAS number 7440-58-6 
ChemSpider ID 22422 ChemSpider is a free chemical structure database


Image explanation

Murray Robertson is the artist behind the images which make up Visual Elements. This is where the artist explains his interpretation of the element and the science behind the picture.


The description of the element in its natural form.

Biological role

The role of the element in humans, animals and plants.

Natural abundance

Where the element is most commonly found in nature, and how it is sourced commercially.

Uses and properties

Image explanation
The image is based on the civic coat of arms for the city of Copenhagen, which gives the element its name.
A shiny, silvery metal that resists corrosion and can be drawn into wires.
Hafnium is a good absorber of neutrons and is used to make control rods, such as those found in nuclear submarines. It also has a very high melting point and because of this is used in plasma welding torches.

Hafnium has been successfully alloyed with several metals including iron, titanium and niobium.

Hafnium oxide is used as an electrical insulator in microchips, while hafnium catalysts have been used in polymerisation reactions.
Biological role
Hafnium has no known biological role, and it has low toxicity.
Natural abundance
Most zirconium ores contain around 5% hafnium. The metal can be prepared by reducing hafnium tetrachloride with sodium or magnesium.
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In 1911, Georges Urbain reported the discovery of the missing element below zirconium in the periodic table, but he was wrong and the search continued. It was finally discovered by George Charles de Hevesy and Dirk Coster at the University of Copenhagen in 1923. It was found in a zirconium mineral, a Norwegian zircon, but it had proved very difficult to separate it from zirconium and this explained why hafnium remained undiscovered for so long.

Other zirconium minerals were now examined by Hevesy, and some were found to contain as much as five per cent of hafnium. (It meant the atomic weight of zirconium was wrong and hafnium-free material had to be produced in order for this to be determined.)

The first pure sample of hafnium itself was made in 1925 by decomposing hafnium tetra-iodide over a hot tungsten wire.

Atomic radius, non-bonded
Half of the distance between two unbonded atoms of the same element when the electrostatic forces are balanced. These values were determined using several different methods.

Covalent radius
Half of the distance between two atoms within a single covalent bond. Values are given for typical oxidation number and coordination.

Electron affinity
The energy released when an electron is added to the neutral atom and a negative ion is formed.

Electronegativity (Pauling scale)
The tendency of an atom to attract electrons towards itself, expressed on a relative scale.

First ionisation energy
The minimum energy required to remove an electron from a neutral atom in its ground state.

Atomic data

Atomic radius, non-bonded (Å) 2.23 Covalent radius (Å) 1.64
Electron affinity (kJ mol−1) 1.351 Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol−1)


Common oxidation states

The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Uncombined elements have an oxidation state of 0. The sum of the oxidation states within a compound or ion must equal the overall charge.


Atoms of the same element with different numbers of neutrons.

Key for isotopes

Half life
  y years
  d days
  h hours
  m minutes
  s seconds
Mode of decay
  α alpha particle emission
  β negative beta (electron) emission
  β+ positron emission
  EC orbital electron capture
  sf spontaneous fission
  ββ double beta emission
  ECEC double orbital electron capture

Oxidation states and isotopes

Common oxidation states 4
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  174Hf 173.940 0.16 2.0 x 1015
  176Hf 175.941 5.26
  177Hf 176.943 18.6
  178Hf 177.944 27.28
  179Hf 178.946 13.62
  180Hf 179.947 35.08


Data for this section been provided by the British Geological Survey.

Relative supply risk

An integrated supply risk index from 1 (very low risk) to 10 (very high risk). This is calculated by combining the scores for crustal abundance, reserve distribution, production concentration, substitutability, recycling rate and political stability scores.

Crustal abundance (ppm)

The number of atoms of the element per 1 million atoms of the Earth’s crust.

Recycling rate

The percentage of a commodity which is recycled. A higher recycling rate may reduce risk to supply.


The availability of suitable substitutes for a given commodity.
High = substitution not possible or very difficult.
Medium = substitution is possible but there may be an economic and/or performance impact
Low = substitution is possible with little or no economic and/or performance impact

Production concentration

The percentage of an element produced in the top producing country. The higher the value, the larger risk there is to supply.

Reserve distribution

The percentage of the world reserves located in the country with the largest reserves. The higher the value, the larger risk there is to supply.

Political stability of top producer

A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.

Political stability of top reserve holder

A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.

Supply risk

Relative supply risk Unknown
Crustal abundance (ppm) 3.0
Recycling rate (%) Unknown
Substitutability Unknown
Production concentration (%) Unknown
Reserve distribution (%) Unknown
Top 3 producers
  • Unknown
Top 3 reserve holders
  • Unknown
Political stability of top producer Unknown
Political stability of top reserve holder Unknown


Specific heat capacity (J kg−1 K−1)

Specific heat capacity is the amount of energy needed to change the temperature of a kilogram of a substance by 1 K.

Young's modulus

A measure of the stiffness of a substance. It provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.

Shear modulus

A measure of how difficult it is to deform a material. It is given by the ratio of the shear stress to the shear strain.

Bulk modulus

A measure of how difficult it is to compress a substance. It is given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour pressure

A measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

Specific heat capacity
(J kg−1 K−1)
144 Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) Unknown
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- - - - - 1.35
x 10-11
x 10-9
x 10-6
x 10-5
0.00272 0.0437
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Listen to Hafnium Podcast
Transcript :

Chemistry in its element: hafnium


You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

(End promo)

Meera Senthilingam

This week, super alloys, nuclear reactors and space rockets. Just some of the reasons that this week's uncommon and unknown element Hafnium is cherished by scientists worldwide. Here's Eric Scerri.

Eric Scerri

Today I am going to talk about an uncommon element that is also not very well known. However it has a rather interesting history and some important commercial applications including its use in the nuclear power industry and in the making of super-alloys.

The element is number 72 in the periodic table, and is called hafnium. It takes its name from hafnium, the old Latin name for Copenhagen which is the city in which it was first isolated in 1922. But first let me back-track a little. In 1913, the physicist Henry Moseley, working in Manchester and later Oxford, discovered an experimental method for ordering the elements according to their atomic numbers. Prior to this work the elements in the periodic table had been ordered by using their atomic weights, which gave rise to a series with uneven gaps between each element. As a result, nobody could be sure how many elements remained to be discovered. All this changed following Moseley's discovery because atomic number increases in whole number steps as one moves through the periodic table.

One of the gaps that opened up, was between element 71, lutetium, and element 73, tantalum. Moreover this particular case was complicated by the fact that it was not clear if element 72 would turn out to be a transition metal, or perhaps a rare earth element, since element 72 falls at the boundary between these two types of elements. Some chemists thought the element would be a rare earth element and carried out many fruitless searches for the element among minerals containing rare earths. But some other chemists suggested that the new element would be a transition metal. The chemical argument for this was quite simple. According to some versions of the periodic table, element 72 fell underneath titanium and zirconium in the periodic table, and both of these elements were known transition elements. Then an argument from physics was proposed by Niels Bohr, one of the founders of quantum theory. According to the electronic configuration that Bohr predicted for element 72 he also agreed that it had be a transition metal.

In 1923 Coster and Hevesy a couple of young researchers in Bohr's institute decided to try to isolate the element as a test of Bohr's theory. In order to do this they followed the chemists' suggestion and decided to look among the ores of zirconium. Within just a few weeks they succeeded by examining some Norwegian zircon and by detecting the X-ray spectral line frequencies expected for this element. It was the discovery of one of the only six then remaining gaps in the periodic table. It also turned out to be the one but last discovery of any naturally occurring element, the last one being rhenium a few years later.

Hafnium is not all that uncommon compared to many other exotic elements. It occurs to the extent of 5.8 ppm of the Earth's upper crust by weight. The reason why it took a long time to isolate is that its atoms have almost the identical size to those of zirconium, along with which it typically occurs in minerals. This makes it difficult to separate from zirconium. But these days a number of methods of extraction have been developed and hafnium has found many of applications because of its rather specific properties. It is a shiny, silvery metal that is corrosion resistant to a remarkable degree. More important perhaps, it has a very high ability to capture neutrons which renders it ideal for making control rods in nuclear reactors, especially those that need to operate under harsh conditions such as today's pressurized water reactors.

Hafnium is also very good at forming super-alloys, which can withstand very high temperatures and has found applications in making a variety of parts for space vehicles. In terms of regular compounds rather than alloys, hafnium carbide has the highest melting point, of any compound consisting of just two elements, at just under

3,9900C. Moving up to compounds of three elements, the mixed carbide of tungsten and hafnium has the single highest melting point of any known compound at 41250C.

Hafnium is not cheap given how difficult it is to extract and because of its relative scarcity. But there are some cases where one just has to pay the price! In the case of nuclear reactors for example, it costs in excess of a million dollars just for the neutron absorbing hafnium rods.

On my recent trip to Copenhagen I spent a long time looking for the famous little mermaid that is symbolic of the city. When I found it I was surprised to see that it is rather insignificant but this did not seem to lessen the special attention that it held from tourists from all over the world. I think it's a little bit like the metal hafnium, first discovered in the mermaid's city of Copenhagen. It too seems somewhat insignificant at first sight and yet it holds the attention of a variety of scientists because of its rather special properties.

Meera Senthilingam

The ability to capture neutrons and the highest melting point of any compound, you can see why scientists consider this element as special as the little mermaid. That was Eric Scerri revealing the powers of Hafnium. Now next week, we meet the King of the elements.

Brian Clegg

Forget 10 Downing Street or 1600 Pennsylvania Avenue, the most prestigious address in the universe is number one in the periodic table, hydrogen. In science, simplicity and beauty are often equated - and that makes hydrogen as beautiful as they come, a single proton and a lone electron making the most compact element in existence.

Meera Senthilingam

And Brian Clegg will be revealing the beauty of hydrogen in next week's Chemistry in its element. Until then, thanks for listening, I'm Meera Senthilingam from the and I'll see you next week.


Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by There's more information and other episodes of Chemistry in its element on our website at

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Visual Elements images and videos
© Murray Robertson 1998-2017.



W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.
Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.
J. S. Coursey, D. J. Schwab, J. J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions (version 4.1), 2015, National Institute of Standards and Technology, Gaithersburg, MD, accessed November 2016.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.


Uses and properties

John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.
Thomas Jefferson National Accelerator Facility - Office of Science Education, It’s Elemental - The Periodic Table of Elements, accessed December 2014.
Periodic Table of Videos, accessed December 2014.


Supply risk data

Derived in part from material provided by the British Geological Survey © NERC.


History text

Elements 1-112, 114, 116 and 117 © John Emsley 2012. Elements 113, 115, 117 and 118 © Royal Society of Chemistry 2017.



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