Chemical element · reference page

Hydrogen

Hydrogen is element 1 (H), located in period 1, group 1 and the s block. At room temperature its reference phase is gas. Its electron configuration is 1s1.

Atomic number1

Standard atomic weight[1.00784, 1.00811]

Group1

Period1

Blocks

Electron configuration1s1

Electronic structure

What controls its chemistry

Hydrogen has one electron and no inner electron shell. It can share that electron in covalent bonds, lose it to form H⁺ in strongly ionising conditions, or gain one electron to form hydride H⁻ in suitable compounds. This flexibility explains why hydrogen appears in water, acids, hydrocarbons and many biological molecules.

The configuration 1s¹ also makes hydrogen a special case in the periodic table. It is placed above group 1 because it has one valence electron, but its chemistry is not simply that of an alkali metal.

Chemical forms

Compounds, ions and materials

Elemental hydrogen normally occurs as H₂ molecules. In compounds it appears in water, hydrocarbons, acids, metal hydrides and countless organic molecules. The oxidation state can be +1, 0 or −1 depending on the bonding partner.

Physical behaviour

State and phase changes

The reference phase at room temperature is gas. The listed density is 0.0899 g/L. The melting point is 13.84 K (-259.3 °C). The boiling point is 20.27 K (-252.9 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where hydrogen is found and used

In practice, hydrogen is encountered in water and the chemistry of life, fertiliser production and oil refining.

In practice

Examples of real uses

Water and the chemistry of life

Hydrogen is part of water and of the carbon–hydrogen framework found throughout biological chemistry.

Fertiliser production

Hydrogen is part of fertiliser chemistry or the industrial processes used to supply essential plant nutrients.

Oil refining

Hydrogen is used in catalytic or hydrogen-processing steps that remove impurities and upgrade fuels.

Periodic position

What its neighbours tell us

Hydrogen is in period 1 and group 1. In the neighbourhood shown here, lithium is directly below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationdiatomic nonmetal

Phase at room temperatureGas

Electron shells1

Density0.0899 g/L

Melting point13.84 K (-259.3 °C)

Boiling point20.27 K (-252.9 °C)

Pauling electronegativity2.2

First ionisation energy1 312 kJ/mol

Electron affinity72.77 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of hydrogen is associated with Henry Cavendish. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is given as the interval [1.00784, 1.00811]. Natural samples can differ slightly in isotope proportions, so a single universal decimal value would hide real variation between materials.

Hydrogen has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The hazards associated with hydrogen depend on chemical form, dose and route of exposure. Pure hydrogen, its ions and its compounds can have very different biological and environmental effects, so safety data should be checked for the actual substance being handled.