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.
[Ar] 4s² 3d¹⁰ 4p¹
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, 3
Number of electrons in each shell, from the nucleus outwards.
29.8 °C (302.91 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.
2,204 °C (2,477 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).
5.91 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
1.81 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.
5.999 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.
0.301 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.
122 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.
+3
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Gallium (Ga) is a chemical element with atomic number 31. It sits in group 13 and period 4 of the periodic table, in the p-block. It is a post-transition metal. At room temperature it is a solid. Its standard atomic weight is 69.723.
Origin of the name
Paul-Émile Lecoq de Boisbaudran found gallium in 1875 by its violet lines in the spectrum of a zinc ore and named it after Gaul, the Latin Gallia, for his native France. It was later claimed that he had hidden his own name in it too, since le coq, ‘the rooster’, is gallus in Latin; he denied it. Four years earlier Dmitri Mendeleev had predicted the element as ‘eka-aluminium’. When Lecoq measured a density of 4.7, Mendeleev wrote to tell him it should be about 5.9, and new measurements proved Mendeleev right.
Where is gallium found?
Gallium has no ores of its own: it is scattered in small traces through other minerals and is obtained only as a by-product, mostly from bauxite, the ore of aluminium, and from zinc ores.
What is gallium used for?
Most gallium goes into electronics. Gallium arsenide makes fast chips and lasers, and gallium nitride is the material of blue and violet LEDs. Because it stays liquid over a very wide range of temperatures, gallium and its alloys also replace mercury in some thermometers.
Interesting facts
Gallium melts at about 30 °C, so a piece held in the hand slowly turns into a silvery liquid.
Like water, gallium expands as it freezes, enough to crack a glass bottle.