All practice pages/Interatomic theory/Elements
Element 31, a post-transition metal
A metal that melts in your hand, predicted by Mendeleev before it was found, and a key ingredient of the blue LED.
Period 4, group 13. The other post-transition metals are tinted, and every cell opens its own article.
Gallium has 31 electrons, one for every proton, so the atom carries no charge. Below are the same 31 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p¹
Short form: [Ar] 3d¹⁰ 4s² 4p¹, where [Ar] stands for the 18 electrons argon already has.
That list runs by shell. In the order the subshells fill it reads [Ar] 4s² 3d¹⁰ 4p¹, 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 3 here is 3s² 3p⁶ 3d¹⁰, and 2 + 6 + 10 is 18.
Gallium sits in period 4 and has 4 shells. The period number counts the shells.
The outer shell holds 3 electrons. Those are the ones gallium can lose, gain or share, so they are the ones that decide what it bonds with. For a main-group element the group number gives that count: group 13 means 3.
Three valence electrons, which gallium gives up to form Ga³⁺. With arsenic and nitrogen it forms the covalent semiconductors gallium arsenide and gallium nitride.
Never mined for itself. It is recovered in small amounts while bauxite is processed into aluminum and while zinc ores are refined, and China produces nearly all of it.
Paul-Émile Lecoq de Boisbaudran spotted two new violet lines in the spectrum of a zinc ore in 1875 and isolated the metal the same year. It matched Mendeleev's predicted eka-aluminium so closely, down to its density once it was remeasured, that the periodic table won wide acceptance. He named it after Gallia, Latin for France; some suspected a pun on his own name, since gallus means rooster, le coq in French.
Almost all goes into semiconductors: gallium arsenide chips in phones and in solar cells for satellites, and gallium nitride in blue and white LEDs, laser diodes and compact phone chargers. Galinstan, a gallium-indium-tin alloy that is liquid at room temperature, replaces mercury in some thermometers.
Not very toxic, but liquid gallium attacks aluminum and makes it brittle, so it is kept away from aluminum parts.