All practice pages/Interatomic theory/Elements
Element 72, a transition metal
A near-identical twin of zirconium that hid inside its ores for over a century, now inside the transistors of computer chips.
Period 6, group 4. The other transition metals are tinted, and every cell opens its own article.
Hafnium has 72 electrons, one for every proton, so the atom carries no charge. Below are the same 72 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d² 6s²
Short form: [Xe] 4f¹⁴ 5d² 6s², where [Xe] stands for the 54 electrons xenon already has.
That list runs by shell. In the order the subshells fill it reads [Xe] 6s² 4f¹⁴ 5d², because a subshell fills by its energy, not by its shell number.
The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 4 here is 4s² 4p⁶ 4d¹⁰ 4f¹⁴, and 2 + 6 + 10 + 14 is 32.
Hafnium sits in period 6 and has 6 shells. The period number counts the shells.
The outer shell holds 2 electrons. Those are the ones hafnium can lose, gain or share, so they are the ones that decide what it bonds with.
Like other transition metals, hafnium loses its outer s electrons, and often some d electrons as well, to form positive ions, and it can form more than one kind.
Always found with zirconium, about 2% as much, in the mineral zircon. It is separated out when zirconium is refined for nuclear reactors.
Its chemistry is so close to zirconium's that no one noticed it until 1923, when Dirk Coster and George de Hevesy found it with X-ray spectroscopy in Copenhagen, guided by Niels Bohr's theory of the atom. They named it after Hafnia, the Latin name of Copenhagen.
Hafnium oxide replaced silicon dioxide as the insulating layer in computer chip transistors in 2007, letting them shrink further. Hafnium absorbs neutrons strongly, the opposite of zirconium, so it makes control rods for nuclear reactors, including those in submarines. It also strengthens superalloys and tips plasma cutting torches.