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.
[Rn] 7s²
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, 18, 8, 2
Number of electrons in each shell, from the nucleus outwards.
700 °C (973 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.
no data
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
5.5 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
0.9 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.
5.278 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.
no data
Energy released when a neutral atom gains an electron. Highest for the halogens. Noble gases, nitrogen and some metals form no stable negative ion.
221 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.
Radium (Ra) is a chemical element with atomic number 88. It sits in group 2 and period 7 of the periodic table, in the s-block. It is an alkaline earth metal. At room temperature it is a solid. It has no standard atomic weight; the mass number of its most stable isotope is 226.
Origin of the name
The name comes from the Latin radius, ‘ray’, because radium gives off powerful radiation. In 1898 Marie and Pierre Curie found it in pitchblende, a uranium ore, a few months after polonium. Radium is so scarce that a ton of ore contains only about a seventh of a gram, and the Curies had to process tons of it. In 1911 Marie Curie and André-Louis Debierne obtained the pure metal.
Where is radium found?
Radium occurs in tiny amounts in all uranium ores, because uranium slowly decays into it. It was obtained as a by-product of refining uranium.
What is radium used for?
In the early 20th century radium was mixed into paint that glowed in the dark, used on watch dials and aircraft instruments, until its danger became clear. It was also used to treat cancer and, with beryllium, as a source of neutrons. Today it has been replaced by safer substances almost everywhere.
Interesting facts
The women who painted luminous dials with radium paint, moistening their brushes with their lips, fell seriously ill. The fate of these ‘Radium Girls’ in the 1920s made the danger of radium clear to everyone.
The curie, an old unit of radioactivity, was defined by the activity of one gram of radium-226.