Hydrogen
Hydrogen (H) is a reactive nonmetal with atomic number 1 and an atomic mass of 1.008. Its electron configuration is 1s1, which gives it 1 valence electron.
Hydrogen key data
| Symbol | H |
|---|---|
| Atomic number | 1 |
| Atomic mass (standard atomic weight) | 1.008 |
| Molar mass | 1.008 g/mol |
| Protons | 1 |
| Neutrons (most abundant isotope, hydrogen-1) | 0 |
| Electrons (neutral atom) | 1 |
| Valence electrons | 1 |
| Category | Reactive nonmetal |
| Period | 1 |
| Group | 1 |
| Block | s-block |
| Noble gas notation | 1s1 |
| Full electron configuration | 1s1 |
| Electrons per shell | 1 |
Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.
The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.
Measured properties of hydrogen
From PubChem, the US National Institutes of Health. Public domain.
| Melting point | 13.81 K (−259.34 °C), 102nd of 103 by melting point |
|---|---|
| Boiling point | 20.28 K (−252.87 °C), 92nd of 93 by boiling point |
| Density | 8.988 × 10⁻⁵ g/cm³, 96th of 96 by density |
| State at room temperature | Gas |
| Electronegativity | 2.2 (Pauling), joint 17th of 95 by electronegativity |
| Van der Waals radius | 120 pm, 99th of 99 by atomic radius |
| First ionisation energy | 13.598 eV (1312 kJ/mol), 8th of 102 by first ionisation energy |
| Electron affinity | 0.754 eV (72.7 kJ/mol), 29th of 57 by electron affinity |
| Common oxidation states | +1, −1 |
| Discovered | 1766 |
Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.
Hydrogen electron configuration
Hydrogen has no noble-gas core, so its electron configuration is simply 1s1.
Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.
Mass of one hydrogen atom
A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.
Mass of one atom
- one atom = 1.008 / (6.02214076 × 10²³)
- = 1.6738 × 10⁻²⁴ g
Atoms in one gram
- atoms per gram = 6.02214076 × 10²³ / 1.008
- = 5.9743 × 10²³ atoms
Hydrogen on the periodic table
Hydrogen is in period 1, group 1, in the s-block.
The other elements of group 1 are lithium, sodium, potassium, rubidium, caesium and francium.