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
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, 18, 8, 2
Number of electrons in each shell, from the nucleus outwards.
727 °C (1,000 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.
1,845 °C (2,118 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).
3.62 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
0.89 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.
5.212 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.
0.145 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.
215 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.
+2
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Barium (Ba) is a chemical element with atomic number 56. It sits in group 2 and period 6 of the periodic table, in the s-block. It is an alkaline earth metal. At room temperature it is a solid. Its standard atomic weight is 137.33.
Origin of the name
The name comes from the Greek barys, ‘heavy’, because the barium mineral baryte is unusually heavy for a stone. Carl Wilhelm Scheele recognised that baryte contained a new ‘earth’, and in 1808 Humphry Davy isolated the metal by electrolysis.
Where is barium found?
Barium is too reactive to occur free. It is mined mainly as baryte, barium sulfate, and less often as witherite, barium carbonate.
What is barium used for?
Most baryte is ground up and added to the mud used in drilling oil and gas wells, to make it heavier. Barium sulfate stops X-rays, so patients swallow it to make the stomach and gut visible on X-ray images. Barium nitrate gives fireworks their green colour, and barium compounds are used in white paints and glass.
Interesting facts
Barium compounds that dissolve in water are poisonous. Barium sulfate can be swallowed safely only because it does not dissolve.
Alchemists were fascinated by ‘Bologna stones’: pebbles of baryte from near Bologna that, once prepared, glowed in the dark after being exposed to light.