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] 7s¹
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, 1
Number of electrons in each shell, from the nucleus outwards.
no data
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).
no data
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
no data
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.
4.073 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.
no data
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.
+1
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Francium (Fr) is a chemical element with atomic number 87. It sits in group 1 and period 7 of the periodic table, in the s-block. It is an alkali metal. At room temperature it is predicted to be a solid. It has no standard atomic weight; the mass number of its most stable isotope is 223.
Origin of the name
Francium is named after France, the homeland of its discoverer, Marguerite Perey of the Curie Institute in Paris. Chemists had long expected an alkali metal below caesium, ‘eka-caesium’. In 1939 Perey found it among the products of the radioactive decay of actinium. It was the last element to be discovered in nature rather than made in a laboratory.
Where is francium found?
Francium forms in uranium minerals from the decay of actinium, but its longest-lived form lasts only about 22 minutes, so at any moment the whole Earth’s crust contains less than about 30 grams of it.
What is francium used for?
Francium has no uses outside research. No one has ever made enough of it to weigh; what is known of its chemistry comes from experiments with a few atoms, which show that it behaves like caesium.