Periodic table of the chemical elements
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2 He,Helium
Noble gases · Gas
Atomic mass 4.0026
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
2 He,Helium
Noble gases · Gas
Atomic mass 4.0026
2
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.
Helium (He) is a chemical element with atomic number 2. It sits in group 18 and period 1 of the periodic table, in the s-block. It is a noble gas. At room temperature it is a gas. Its standard atomic weight is 4.0026.
Helium is named after the Sun, Greek hḗlios. It was first seen in 1868 as a yellow line in the spectrum of sunlight during a solar eclipse, and Norman Lockyer took it for a new element. He gave it the ending -ium of metals, since he thought it was one. On Earth helium was found only in 1895, when William Ramsay released it from the uranium mineral cleveite.
Helium is the second most abundant element in the universe, about a quarter of ordinary matter; stars make it from hydrogen. On Earth it is rare: about 5 parts per million of the air, and it keeps escaping into space. Earth’s helium comes from the radioactive decay of uranium and thorium deep underground and collects in natural gas, from which it is extracted.
The largest use of helium is cooling: liquid helium keeps the superconducting magnets of MRI scanners and particle accelerators cold. It fills balloons and airships, safely, since unlike hydrogen it does not burn. Divers breathe helium mixed with oxygen, welders shield hot metal with it, and rockets use it to pressurise their fuel tanks.