Hydrogen

01 / A closer look

Meet Hydrogen

Hydrogen has atomic number 1: every atom of this element contains 1 protons. It belongs to period 1, group 1 of the periodic table.

In a neutral atom, the number of electrons equals the number of protons. Isotopes of this element share the same atomic number but have different neutron counts.

02 / Physical properties

Melting point

13.99 K

Boiling point

20.271 K

Density

0.000082 g/cm³

Atomic weight

1.0080 ± 0.0002

Melting points, boiling points and densities follow RSC fact boxes. Density is referenced to room temperature; consult the source for pressure and allotrope details. These are reference values, not a claim that every value was experimentally measured. Atomic weights follow CIAAW.

Physical-property source: Royal Society of Chemistry ↗ · Reviewed 24 September 2026

Standard atomic weights follow CIAAW 2024 (abridged). Brackets show an isotope mass number from IUPAC 2022, not a standard atomic weight.

03 / Atomic structure

Atomic structure

Electron configuration

1s1

Oxidation states

+1, -1

Electronegativity · Pauling

2.2

First ionisation energy

13.598 eV

Atomic radius

120 pm

04 / Where it is found

Where it is found

Water and organic compounds; abundant in stars.

Production and export figures depend on the commodity and reporting year. No comparable country quantities have been included for this element in this edition.