Periodic table of the chemical elements
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18 Ar,Argon
Noble gases · Gas
Atomic mass 39.95
Periodic table of the chemical elements
Hover an element, or Tab into the table and move with the arrow keys. Press an element to open its card.
Tap an element or slide your finger across the table
18 Ar,Argon
Noble gases · Gas
Atomic mass 39.95
18
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.
Argon (Ar) is a chemical element with atomic number 18. It sits in group 18 and period 3 of the periodic table, in the p-block. It is a noble gas. At room temperature it is a gas. Its standard atomic weight is 39.95.
Argon is Greek for ‘idle’ or ‘lazy’: the gas refused to react with anything. As early as 1785 Henry Cavendish noticed that about 1% of air would not combine with oxygen however long he sparked it. In 1894 Lord Rayleigh, puzzled that nitrogen taken from air was denser than nitrogen made from ammonia, found the cause with William Ramsay: a new gas, the first noble gas to be discovered. Until 1957 its symbol was simply A.
Argon makes up 0.93% of the air, which makes it the third most common gas in the atmosphere after nitrogen and oxygen, and far more common than all the other noble gases. It is obtained as a by-product when liquefied air is separated into oxygen and nitrogen.
Because there is so much of it in the air, argon is the cheapest noble gas. Welders use it to shield hot metal from oxygen, it fills incandescent bulbs and fluorescent tubes, and it insulates the gap in double-glazed windows. Silicon crystals for electronics are grown under argon.