Periodic table of the chemical elements
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57 La,Lanthanum
Lanthanides · Solid
Atomic mass 138.91
57
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.
How electrons are arranged in subshells. The bracket stands for the previous noble gas; the superscript is the number of electrons in a subshell.
Number of electrons in each shell, from the nucleus outwards.
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.
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
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.
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.
Energy released when a neutral atom gains an electron. Highest for the halogens. Noble gases, nitrogen and some metals form no stable negative ion.
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.
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Lanthanum (La) is a chemical element with atomic number 57. It sits in period 6 of the periodic table, in the f-block. It is a lanthanide. At room temperature it is a solid. Its standard atomic weight is 138.91.
The name comes from the Greek lanthanein, ‘to lie hidden’: the element hid in cerium compounds. In 1839 Carl Gustaf Mosander found its oxide in cerium nitrate. Three years later he showed that even his lanthanum was a mixture, and called the other part didymium, ‘twin’; it later turned out to be two more elements.
Lanthanum is obtained from the minerals monazite and bastnäsite, together with the other rare earths.
Lanthanum oxide makes special optical glass for camera lenses. Lanthanum is about a quarter of the mischmetal in lighter flints. Lanthanum alloys soak up hydrogen like a sponge and give it back again, which is the basis of nickel-metal hydride batteries. Lanthanum compounds were once used in the carbon arc lamps of film studios and cinema projectors.