1 H Hydrogen 1.008
Nonmetal s-block Period 1 Group 1

Hydrogen

H · Element 1 · Nonmetal

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The simplest atom there is — one proton, one electron — and about 90% of every atom in the universe.

State at 20°C Gas
Atomic mass 1.008 u
Electron configuration1s¹
All 118 2He

Structure

The hydrogen atom

Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.

Orbital cloud

Measured values

Property sheet

Every bar shows where hydrogen sits among all 118 elements for that property.

Physical

Density 8.99e-5 g/cm³ 0%
Melting point 14 K 1%
Boiling point 20.3 K 1%
Specific heat 14.304 J/g·K
Thermal conductivity 0.18 W/m·K 11%

Atomic

Atomic radius 53 pm 3%
Covalent radius 31 pm
Van der Waals radius 120 pm

Electronic

Electronegativity 2.2 78%
Ionisation energy 1312.2 kJ/mol 94%
Electron affinity 72.4 kJ/mol 57%

Occurrence

Abundance in crust 1400 mg/kg 91%

Identity

SymbolH
Atomic number1
Atomic mass1.008 u
CategoryNonmetal
Blocks
Crystal structurehexagonal
Oxidation states-1, +1
Discovered1766
Discovered byHenry Cavendish

Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.

Same element, different neutrons

Isotopes of hydrogen

All hydrogen atoms have the same number of protons, 1, and differ only in their number of neutrons: those variants are its isotopes. The most common, H-1, has 0 neutrons and makes up 99.9855% of natural hydrogen.

Isotopes of hydrogen found in nature, with their share of the natural element, and well-known radioactive ones
ISOTOPE Neutrons Natural Abundance Half-life Decay Mode
H-1 most common 0 99.9855% Stable —
H-2 1 0.0145% Stable —
H-3 2 — 12.32 yrs β⁻

Here the classroom shortcut, rounded atomic mass (1.008) minus atomic number, gives the same answer: 0.

Decay modes

β⁻
beta-minus decay

Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.

Size, to scale

How big is a hydrogen atom?

Radius 53 pm — that is 0.053 nm, so about 9.43 million of them side by side would span a millimetre.

Thermal range

Solid, liquid, gas — and when

Hydrogen is liquid over a 6 K window, from 14 K to 20 K.

Where it sits

Position in the table

Hydrogen sits in period 1, group 1. Everything in group 1 shares the same outer-electron count, which is why they behave so similarly.

Other nonmetals

6C 7N 8O 15P 16S 34Se
All nonmetals

The story

What hydrogen is, and how we found it

Hydrogen (H) is element 1, the lightest and simplest element: a typical hydrogen atom is one proton and one electron. It is the most abundant element in the universe, making up about three quarters of ordinary matter by mass and roughly nine out of every ten atoms. It sits at the top of group 1 of the periodic table, but it is a nonmetal, not an alkali metal.

At room temperature hydrogen is a colorless, odorless gas made of H₂ molecules. Its density is 0.08988 g/L at 0 °C and normal pressure, about 14 times lower than that of air. It condenses to a liquid at −252.87 °C (20.28 K) and freezes at −259.14 °C (14.01 K).

Henry Cavendish identified hydrogen as a distinct substance in 1766, calling it "inflammable air"; earlier experimenters such as Robert Boyle had made the gas from metals and acids without recognizing it as an element. In 1783 Antoine Lavoisier named it hydrogène, from the Greek for "water-former", because it burns in oxygen to form water.

Atomic structure and isotopes

Hydrogen's single electron occupies the first and only shell (configuration 1s¹). The atom can lose that electron to form H⁺, share it in covalent bonds as in water and methane, or gain a second electron to form the hydride ion, H⁻.

Hydrogen has three isotopes. Protium (hydrogen-1) has no neutron and makes up about 99.985% of natural hydrogen. Deuterium (hydrogen-2) has one neutron and makes up about 0.015%. Tritium (hydrogen-3) has two neutrons, is radioactive with a half-life of 12.3 years, and occurs only in traces.

Applications

What hydrogen is used for

Ammonia, refining and methanol

Most hydrogen is used where it is made, in chemical plants and oil refineries. The largest uses are making ammonia for fertilizers by the Haber-Bosch process (N₂ + 3H₂ → 2NH₃), refining crude oil by hydrocracking and by removing sulfur from fuels, and making methanol. According to the International Energy Agency, world demand is close to 100 million tonnes a year, and almost all of it is still produced from fossil fuels.

Other industrial uses

  • Food: hydrogenation with a nickel catalyst turns liquid vegetable oils into solid fats.
  • Metals and glass: hydrogen atmospheres prevent oxidation in metal heat treatment and float glass production, and hydrogen can reduce metal ores to metal; pilot plants use it to make iron without coke.
  • Electronics: very pure hydrogen is a process gas in semiconductor manufacturing.

Energy and spaceflight

  • Rocket fuel: Liquid hydrogen fuels NASA's Space Launch System; SpaceX Starship uses liquid methane and liquid oxygen.
  • Fuel cells: fuel cells combine hydrogen with oxygen from the air to make electricity, with water as the only exhaust. They power a small number of cars, buses, trucks and forklifts.
  • Energy storage: hydrogen made by splitting water with renewable electricity (electrolysis) can store energy for industry or power generation, though this is still a small share of supply.

Where it comes from

Natural occurrence

1400 mg/kg of Earth's crust · more abundant than 91% of elements

In the universe

Hydrogen formed in the first minutes after the Big Bang and is still the main ingredient of stars and interstellar gas. Stars, including the Sun, shine by fusing hydrogen into helium, and the giant planets Jupiter and Saturn are mostly hydrogen.

On Earth

Free hydrogen gas is rare on Earth. It is so light that it escapes from the upper atmosphere, and air contains only about 0.5 parts per million. Almost all of Earth's hydrogen is bound in compounds: water (11% hydrogen by mass), natural gas, oil and coal, clay minerals, and every living thing. It makes up about 0.14% of Earth's crust by mass. A few natural underground accumulations of hydrogen gas are known, such as one tapped by a well in Mali, and geologists are exploring for more.

How it is produced

Industry makes hydrogen mainly by steam reforming of natural gas (CH₄ + H₂O → CO + 3H₂, followed by the water-gas shift reaction) and by gasifying coal; some is a byproduct of oil refining. Electrolysis of water, which can run on renewable electricity, supplies well under 1% of production (IEA).

Handling

Safety

Fire and explosion

Hydrogen is extremely flammable. It burns in air at any concentration from about 4% to 75% by volume, one of the widest ranges of any gas, and very little energy is needed to ignite it. Its flame is pale blue and almost invisible in daylight, so a hydrogen fire can be hard to see.

Health effects

Hydrogen is not toxic, and no occupational exposure limit is set for it. In a closed space it can displace oxygen and cause suffocation. Liquid hydrogen, at about −253 °C, causes severe cold burns on contact.

Handling and storage

  • Store cylinders upright and secured, in well-ventilated areas away from heat, sparks and oxidizers.
  • Hydrogen leaks easily through small gaps and rises; install hydrogen detectors near ceilings and ventilate from the top.
  • Hydrogen can make some steels brittle over time (hydrogen embrittlement), so pipes and tanks must be made of compatible materials.
  • If a hydrogen flame cannot be stopped at its source, let it burn and cool nearby equipment, because extinguishing it can let an explosive cloud build up.

Keep exploring

Use hydrogen in the tools

Quick answers

Hydrogen: common questions

What is hydrogen?

Hydrogen (symbol H) is element 1 on the periodic table, a nonmetal in period 1, group 1. The simplest atom there is — one proton, one electron — and about 90% of every atom in the universe. At room temperature it is a gas.

What is the electron configuration of hydrogen?

Hydrogen's ground-state electron configuration is 1s¹, giving 1 occupied shell with the distribution 1.

What are the melting and boiling points of hydrogen?

Hydrogen melts at 14 K (-259.1 °C) and boils at 20.3 K (-252.9 °C).

What is the atomic mass of hydrogen?

The standard atomic weight of hydrogen is 1.008 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.

How many neutrons does hydrogen have?

The most common hydrogen atom, H-1, has 0 neutrons: its mass number, 1, minus its atomic number, 1. It makes up 99.9855% of natural hydrogen. The classroom shortcut, rounded atomic mass (1.008) minus atomic number, gives the same 0.

How dense is hydrogen?

Hydrogen is a gas at room temperature; at 0 °C and 1 atm its density is about 0.09 g/L, so it is lighter than air.

What is the electronegativity of hydrogen?

Hydrogen has a Pauling electronegativity of 2.2. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.

Who discovered hydrogen, and when?

Hydrogen was discovered in 1766 by Henry Cavendish. Origin of the name: Greek hydro + genes, "water-former".

How common is hydrogen on Earth?

Hydrogen makes up about 0.14% of the Earth's crust — common enough to be mined at scale.

Next → He Helium Element 2 · Noble gas