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 4p³
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, 5
Number of electrons in each shell, from the nucleus outwards.
817 °C (1,090 K), only under pressure
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.
614 °C (887 K), sublimes
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
5.776 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
2.18 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.
9.789 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.
0.804 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.
119 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.
+5, +3, -3
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Arsenic (As) is a chemical element with atomic number 33. It sits in group 15 and period 4 of the periodic table, in the p-block. It is a metalloid. At room temperature it is a solid. Its standard atomic weight is 74.922.
Origin of the name
The name comes through Latin arsenicum from the Greek arsenikon, the yellow mineral orpiment. The Greeks took the word from Persian zarnik, ‘golden’, and reshaped it to sound like their own arsenikos, ‘male’. Arsenic minerals have been known since antiquity; the element itself is thought to have been obtained by Albertus Magnus around 1250.
Where is arsenic found?
Arsenic is found mainly in the mineral arsenopyrite and sometimes as the native element. Dissolved from rocks, it gets into groundwater, and in some regions millions of people drink well water with too much arsenic in it.
What is arsenic used for?
Small amounts of arsenic are added to semiconductors, and gallium arsenide makes fast chips and lasers. A little arsenic also hardens the lead of shot. It was once widely used in rat poison, insecticides and wood preservatives, but because of its toxicity these uses are disappearing.
Interesting facts
Arsenic was long a favourite poison, because the symptoms of poisoning looked like illness. That changed in the 1830s, when James Marsh invented a test that could detect even traces of it.
At normal pressure arsenic does not melt: when heated it turns straight from a solid into a vapour. Heated in air, it gives off fumes that smell of garlic.