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.
1s¹
How electrons are arranged in subshells. The bracket stands for the previous noble gas; the superscript is the number of electrons in a subshell.
1
Number of electrons in each shell, from the nucleus outwards.
-259.2 °C (13.99 K)
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.
-252.9 °C (20.28 K)
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
0.00009 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
2.20 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.
13.598 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.
0.754 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.
31 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.
+1, -1
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Hydrogen (H) is a chemical element with atomic number 1. It sits in group 1 and period 1 of the periodic table, in the s-block. It is a nonmetal. At room temperature it is a gas. Its standard atomic weight is 1.0080.
Origin of the name
The name hydrogen was coined in 1783 by the French chemists Antoine Lavoisier and Louis-Bernard Guyton de Morveau, from Greek hydro- ‘water’ and -genes ‘forming’: when hydrogen burns, it forms water. The symbol H is the first letter of the name.
Where is hydrogen found?
Hydrogen is the most abundant element in the universe: about three quarters of ordinary matter by mass and more than 90% of all atoms. Stars, the Sun among them, shine by fusing hydrogen into helium. On Earth almost all hydrogen is bound in compounds, above all in water, and in organic matter: living things, oil, natural gas and coal.
What is hydrogen used for?
About half of all hydrogen goes into making ammonia by the Haber process, mostly for fertilisers, and much of the rest into oil refining. Hydrogen also turns vegetable oils into solid fats (hydrogenation), and liquid hydrogen burned with liquid oxygen is a rocket fuel. Fuel cells turn hydrogen into electricity, with water as the only product. Today more than three quarters of hydrogen is made from natural gas.
Interesting facts
A hydrogen atom has just one proton and one electron: it is the simplest and lightest of all atoms.
Hydrogen is the only element whose isotopes have names of their own: protium, deuterium and tritium.
Air with as little as 4% hydrogen can catch fire.
Radioactive tritium makes exit signs and watch dials glow without electricity.