Hydrogen · element 1
The lightest atom there is, and the raw material for almost everything else in the universe
Key facts
Hydrogen is atomic number 1 for a simple reason: its atom has a single proton and a single electron, and nothing else. It is a colourless, odourless gas at room temperature, and it is so light that any hydrogen released into the open air drifts upward and eventually leaks away into space.
Although it is scarce as a free gas on Earth, hydrogen is the most common element in the universe by a wide margin. Stars are mostly hydrogen, and the energy that reaches us from the Sun comes from hydrogen nuclei fusing into helium. On our own planet almost all hydrogen is locked up in compounds, above all in water, where two hydrogen atoms are bonded to one oxygen atom.
Where it is found
Free hydrogen gas makes up only a tiny fraction of the air we breathe, far less than a thousandth of one per cent. Earth's gravity is too weak to hold on to such a light molecule for long. Yet hydrogen is everywhere in a chemically bound form. Every water molecule contains it, and so does every living thing, because hydrogen is part of carbohydrates, fats, proteins and DNA.
Look beyond Earth and the picture flips. Roughly three quarters of the ordinary matter in the universe is hydrogen. Gas giants like Jupiter and Saturn are mostly hydrogen, and deep inside them the pressure is so high that hydrogen is thought to behave like a liquid metal.
How it was discovered
The English scientist Henry Cavendish is credited with the discovery. In 1766 he collected the gas, measured its properties carefully and called it 'inflammable air'. He later showed that burning it produced water, a result that surprised everyone, because water was thought to be a basic element itself.
The French chemist Antoine Lavoisier gave the gas its modern name in the 1780s. He combined the Greek words for water and forming, so hydrogen literally means water-maker. The name records the experiment that changed chemistry: water is not elemental, it is a compound of two gases.
What we use it for
The biggest single use of hydrogen today is making ammonia for fertiliser through the Haber-Bosch process, which feeds a large share of the world's population. Oil refineries use huge amounts to strip sulfur from fuels and to break heavy oils into lighter ones.
Liquid hydrogen burned with liquid oxygen is a powerful rocket fuel, used in the upper stages of many launch vehicles. Fuel cells combine hydrogen with oxygen from the air to generate electricity, with water as the only exhaust. Whether hydrogen becomes a widespread clean fuel depends largely on how it is made. Most hydrogen today comes from natural gas and releases carbon dioxide, while so-called green hydrogen is produced by splitting water with renewable electricity.
Reading the numbers
Hydrogen boils at about minus 253 degrees Celsius and freezes at about minus 259, only a few degrees above absolute zero. At room temperature its density is roughly 0.09 grams per litre, about one fourteenth that of air, which is why hydrogen balloons rise so eagerly and why leaks disperse upwards.
Its electron configuration is 1s1: a single electron in the first shell. That shell can hold two electrons, so hydrogen is one short of a full shell and readily shares its electron in a covalent bond. This is why hydrogen exists as H2 molecules rather than lone atoms. It sits at the top of group 1 because it has one outer electron like the alkali metals, but it is a non-metal and behaves very differently from them. Its first ionisation energy of 1312 kilojoules per mole is far higher than that of lithium or sodium, so hydrogen does not give up its electron easily.
Safety and the environment
Hydrogen is highly flammable and burns with a flame that is almost invisible in daylight. Mixed with air it can explode across a wide range of concentrations, so any leak in an enclosed space is dangerous. The gas itself is not poisonous, but it can displace oxygen.
Burning hydrogen produces only water, so it causes no local air pollution. Its environmental footprint depends on production. Making hydrogen from natural gas emits carbon dioxide unless that gas is captured. Leaked hydrogen also has an indirect warming effect, because it slows the breakdown of methane in the atmosphere, which is one reason engineers work hard to keep hydrogen systems leak-free.
Did you know?
- Around three quarters of the ordinary matter in the universe, by mass, is hydrogen.
- Hydrogen has three natural isotopes. Ordinary hydrogen has no neutron, deuterium has one, and tritium has two and is radioactive.
- Water made with deuterium instead of ordinary hydrogen is called heavy water. Ice made from it sinks in normal water.
- A hydrogen flame is nearly invisible in daylight, so hydrogen fires are often detected by their heat before they are seen.
- The Sun converts hundreds of millions of tonnes of hydrogen into helium every second.
- The Hindenburg airship, filled with hydrogen, was destroyed by fire in 1937, and hydrogen was abandoned for passenger airships after that.
More in the non-metals
The nearest neighbours of hydrogen in the family, by atomic number.
See all 7 non-metals on one page, or browse all 118 elements.
Compare Hydrogen with another element
Each comparison has a side-by-side data table and a plain-English summary of the differences.
Frequently asked questions
Is hydrogen flammable?
Yes, extremely. Hydrogen burns in air across a very wide range of mixtures and can explode in a confined space. Its flame is pale and hard to see in daylight, which adds to the danger.
Why is hydrogen the first element?
Elements are ordered by the number of protons in the nucleus. Hydrogen has exactly one proton, the smallest number possible, so it is atomic number 1. Nothing simpler can count as an element.
Is hydrogen a metal?
No. At ordinary temperatures and pressures it is a non-metallic gas. It is placed above the alkali metals in group 1 only because it has a single outer electron. Under the crushing pressures inside Jupiter, hydrogen is thought to become metallic.
Where does hydrogen fuel come from?
Most industrial hydrogen is made by reacting natural gas with steam, which releases carbon dioxide. A cleaner route is electrolysis, splitting water into hydrogen and oxygen using electricity. When that electricity comes from wind or solar power the result is called green hydrogen.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Hydrogen reference article. Spotted an error? Tell us.