Hydrogen (H)
Hydrogen (H) is a reactive nonmetal with atomic number 1 and a standard atomic weight of 1.0080. At room temperature it is a gas that melts at -259 °C and boils at -253 °C.
Hydrogen key facts
Hydrogen at a glance
Identity
| Symbol | H |
|---|---|
| Atomic number (Z) | 1 |
| Atomic weight | 1.0080 ± 0.0002 (CIAAW 2024) |
| Category | Reactive nonmetal |
| Group | 1 — alkali metals (with hydrogen) |
| Period | 1 |
| Block | s-block |
| CAS number | 1333-74-0 |
Atomic properties
| Electron configuration | 1s1 |
|---|---|
| Electrons per shell | 1 |
| Oxidation states | -1, +1 |
| Electronegativity (Pauling) | 2.2 |
| First ionisation energy | 13.598 eV (1,312 kJ/mol) |
| Electron affinity | 0.754 eV |
| Atomic radius (empirical) | 25 pm |
| Covalent radius | 32 pm |
| Van der Waals radius | 110 pm |
Physical properties
| State at 25 °C | Gas |
|---|---|
| Melting point | -259.2 °C · -434.5 °F · 13.99 K |
| Boiling point | -252.9 °C · -423.2 °F · 20.27 K |
| Density | 0.0899 g/L (gas at 0 °C, 1 atm) |
| Specific heat capacity | 14.304 J/(g·K) |
| Thermal conductivity | 0.181 W/(m·K) |
| Crystal structure | Hexagonal |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 1H 99.9855%, 2H 0.0145% |
| Stable isotopes | 2 |
| Known isotopes | 7 |
| Abundance in Earth's crust | 1,400 mg/kg |
| Abundance in seawater | 108,000 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Hydrogen electron configuration
Hydrogen has 1 electron. Written in short form, its ground-state configuration is 1s1.
As a member of group 1 it has 1 valence electron in its outer shell (shell 1), which largely decides its chemistry.
Discovery and name
1766 — Henry Cavendish · England.
Greek hydro (water) + genes (forming) — it forms water when burned.
Known for
The most abundant element in the universe — about three-quarters of all normal matter by mass. Used to make ammonia for fertiliser, to refine petroleum, and increasingly as a clean fuel for fuel cells.
Isotopes of hydrogen
Naturally occurring hydrogen is a mix of 2 isotopes. Each has 1 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 1H | 1 | 0 | 99.9855% | stable |
| 2H | 1 | 1 | 0.0145% | stable |
7 isotopes of hydrogen are known in our data set (half-lives as published in nuclear data tables).
Hydrogen compared with group 1
The other elements in group 1 (alkali metals (with hydrogen)), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Hydrogen | 1 | 1.0080 | -259 °C | -253 °C | 0.0899 g/L | 2.2 |
| Lithium | 3 | 6.94 | 180 °C | 1,342 °C | 0.534 g/cm³ | 0.98 |
| Sodium | 11 | 22.990 | 98 °C | 883 °C | 0.97 g/cm³ | 0.93 |
| Potassium | 19 | 39.098 | 64 °C | 759 °C | 0.89 g/cm³ | 0.82 |
| Rubidium | 37 | 85.468 | 39 °C | 688 °C | 1.53 g/cm³ | 0.82 |
| Cesium | 55 | 132.91 | 28 °C | 671 °C | 1.873 g/cm³ | 0.79 |
| Francium | 87 | [223] | 21 °C (est.) | — | 1.87 g/cm³ (pred.) | 0.7 |