Average mass of an atom in unified atomic mass units (1 u is 1/12 of the mass of a carbon-12 atom). Accounts for the natural mix of the element’s isotopes.
Number of the vertical column, 1 to 18 in IUPAC numbering. Elements in a group share the same outer-shell layout, so their properties are alike.
Number of the horizontal row. Usually equal to the number of electron shells in the atom; palladium is the exception, with four shells in period 5.
The subshell (s, p, d or f) that receives the last electrons in the usual filling order; it divides the table into four blocks. Real atoms sometimes differ: lanthanum starts the f-block but has no f electrons.
[Xe] 6s² 4f¹⁴ 5d²
How electrons are arranged in subshells. The bracket stands for the previous noble gas; the superscript is the number of electrons in a subshell.
2, 8, 18, 32, 10, 2
Number of electrons in each shell, from the nucleus outwards.
2,233 °C (2,506 K)
Temperature at which the solid turns liquid at standard pressure. Helium, carbon and arsenic do not melt at normal pressure: helium needs about 25 atm, carbon and arsenic turn straight into vapour.
4,603 °C (4,876 K)
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
13.3 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
1.3 Pauling
How strongly an atom pulls electrons towards itself in a chemical bond. On the Pauling scale it runs from 0.79 for caesium to 3.98 for fluorine.
6.825 eV
Energy needed to remove the most loosely bound electron from an atom. The lower it is, the more easily the element gives up an electron, which is typical of metals.
0.178 eV
Energy released when a neutral atom gains an electron. Highest for the halogens. Noble gases, nitrogen and some metals form no stable negative ion.
175 pm
Half the length of a single covalent bond between two atoms of the element. It shows the size of an atom inside a molecule or crystal: 76 pm for carbon, 132 pm for iron.
+4
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Hafnium (Hf) is a chemical element with atomic number 72. It sits in group 4 and period 6 of the periodic table, in the d-block. It is a transition metal. At room temperature it is a solid. Its standard atomic weight is 178.49.
Origin of the name
Hafnium is named after Copenhagen, in Latin Hafnia. Using his new theory of the atom, Niels Bohr predicted that element 72 would not be a rare earth, as some had claimed, but a close relative of zirconium. In 1923 Dirk Coster and George de Hevesy, working at Bohr’s institute in Copenhagen, found it by X-ray spectroscopy in zirconium ore from Norway.
Where is hafnium found?
Hafnium has no ores of its own: it is present in every zirconium ore and is so similar to zirconium that separating the two is very difficult.
What is hafnium used for?
Hafnium absorbs neutrons well and is used in the control rods of nuclear reactors. Thin layers of hafnium oxide insulate the tiny transistors in modern microchips. Hafnium carbide has one of the highest melting points of any known compound.
Interesting facts
Before hafnium was found, Georges Urbain claimed to have discovered element 72 among the rare earths and called it celtium. It was a mistake.