Hydrogen is the simplest and lightest chemical element, with the symbol H and atomic number 1. It is a colourless, odourless gas that burns easily. It is the most common element in the universe, about 75% of detectable matter, and it combines with oxygen to make water.
hydrogen quick facts
- Hydrogen’s symbol is H and its atomic number is 1.
- Hydrogen is the lightest and simplest element.
- About 75% of the detectable matter in the universe is hydrogen.
- Henry Cavendish identified hydrogen in 1766.
- A hydrogen fuel cell produces only water.
What is hydrogen?
Hydrogen is a chemical element. Britannica describes it as a colourless, odourless, tasteless gas that burns easily. It is also the simplest of all the elements.
Its symbol is H and its atomic number is 1. So it sits first in the periodic table. The Royal Society of Chemistry, or RSC, calls it the lightest element of all.
Under normal conditions, hydrogen atoms pair up to form molecules made of two atoms, written H2.
| Hydrogen at a glance | |
|---|---|
| Symbol | H |
| Atomic number | 1 |
| State at room temperature | gas |
| Boiling point | −252.879 °C |
| Discovered by | Henry Cavendish, 1766 |
| Name means | water forming |
| Share of the universe | about 75% of detectable matter |
The most common element in the universe
Hydrogen is the most common element in the whole universe. Britannica says it is three times as common as helium, the next most common element.
The RSC says hydrogen makes up about 75% of the matter we can detect in the universe. The Sun is made mostly of hydrogen, and so is the giant planet Jupiter. Discover more in our Jupiter facts.
Hydrogen was also the first element to form after the universe began. Stars shine because of hydrogen. Deep inside a star, hydrogen atoms are squeezed together so hard that they join to make helium. This releases huge amounts of energy as light and heat.
What does a hydrogen atom look like?
A hydrogen atom is the simplest atom there is. Britannica says its nucleus is a single proton, which has a positive electric charge. One electron, with a negative charge, goes with it.
Every other element has more protons and electrons than hydrogen. That is why hydrogen comes first in the periodic table.
Because it is so light, hydrogen gas rises quickly. Britannica notes that some hydrogen at the very top of the atmosphere moves so fast that it escapes into space. High above Earth there is even a layer called the protonosphere. There, hydrogen atoms and protons are the main ingredients.
Where is hydrogen on the periodic table?
Hydrogen sits at the top left of the periodic table, at the top of Group 1. Other elements in this group include lithium and sodium. They are soft metals called the alkali metals.
Hydrogen is placed there because, like them, its atom has just one outer electron. But hydrogen is not a metal. On Earth it is a gas.
Hydrogen isotopes
Atoms of the same element can have different numbers of neutrons. These different forms are called isotopes. Britannica says hydrogen has three isotopes: protium, deuterium and tritium.
Protium: the most common form, with one proton and no neutrons.
Deuterium: sometimes called heavy hydrogen, with one proton and one neutron.
Tritium: with one proton and two neutrons. It is radioactive.
Deuterium and tritium are used in experiments that try to copy the way the Sun makes energy. These experiments join atoms together.
Who discovered hydrogen?
The RSC says Henry Cavendish identified hydrogen as a separate substance in 1766.
People had noticed it earlier without knowing what it was. Hundreds of years before, alchemists such as Paracelsus mixed some metals with acid. They saw bubbles of a gas that could burn.
Where does the name come from?
The name hydrogen comes from two Greek words, hydro and genes, which together mean “water forming”.
Britannica explains why. When hydrogen burns with oxygen, it makes water, which has the chemical formula H2O: two hydrogen atoms joined to one oxygen atom.
Hydrogen on Earth
Most of Earth’s hydrogen is locked up in water. It is in the oceans, ice, rivers, lakes and the air. Britannica says hydrogen makes up only about 0.14% of Earth’s crust by weight.
Hydrogen is also found in every living thing. It is part of the carbon compounds that make up plants and animals, including you. It is in petroleum too.
Hydrogen gas on its own is rare on Earth. The RSC says there is less than one part per million of it in the air.
Hydrogen in your body
There is hydrogen inside you right now. Every molecule of water in your body contains two hydrogen atoms. The proteins, fats and sugars in your body all contain hydrogen too.
Britannica says hydrogen is found in all animal and plant tissue, as part of countless carbon compounds.
A great joiner
Hydrogen joins with almost every other element to make compounds. Britannica says it forms compounds with nearly all the elements. Only some of the noble gases are left out.
People often say carbon makes more compounds than any other element. But hydrogen is found in almost all carbon compounds, and in many others too. So Britannica says hydrogen compounds may be even more numerous.
Compounds made only of hydrogen and carbon are called hydrocarbons. Methane, the main gas in natural gas, is one carbon atom joined to four hydrogen atoms. Fuels such as petrol and natural gas are made of hydrocarbons. Find out where they come from in our fossil fuels facts.
What is hydrogen used for?
Fertilisers: Britannica says the biggest use of hydrogen is making ammonia. This uses about two-thirds of all the hydrogen produced. Ammonia is used to make fertilisers that help crops grow.
Food: hydrogen is used to turn vegetable oils into solid fats for making margarine.
Fuel refining: the RSC says hydrogen is used to remove sulfur from fuels.
Rockets: NASA used hydrogen to power rockets such as the Saturn V and the Space Shuttle.
Hydrogen as a clean fuel
Hydrogen can power cars, buses and buildings using a fuel cell. The US Department of Energy says that when hydrogen is used in a fuel cell, it produces only water.
That makes hydrogen an exciting clean fuel. Fuel cells do not give off harmful fumes. They are quiet, too.
But there is a catch. The Department of Energy says about 95% of hydrogen today is made from natural gas. Hydrogen can also be made by using electricity to split water. This is cleaner if the electricity comes from the sun or wind.
How hydrogen is made
Because hydrogen gas is so rare in the air, it has to be made. The Department of Energy says about 95% of hydrogen today comes from natural gas. It is made using hot steam, in a process called steam reforming.
Hydrogen can also be made from water. A machine called an electrolyser uses electricity to split water into hydrogen and oxygen. The Department of Energy says it works much like a fuel cell in reverse.
Scientists are also testing other ways to make hydrogen. One uses sunlight directly, and another uses tiny living things called microbes.
How a fuel cell works
A fuel cell turns the chemical energy in hydrogen straight into electricity. Inside, hydrogen and oxygen react. The only waste is water.
Britannica says fuel cells can power vehicles. They can also give main or backup power to power plants, hospitals and office buildings.
Hydrogen in space travel
Hydrogen is a powerful rocket fuel when it burns with oxygen. Britannica says it is also favoured as a fuel for nuclear-powered rockets and space vehicles.
Hydrogen is used in metal making too. Britannica says it helps turn iron ore into iron. It is also used to make metals such as tungsten and molybdenum.
Is hydrogen dangerous?
Hydrogen burns very easily, so it must be handled carefully.
Long ago, hydrogen was used to fill balloons and huge airships. It was chosen because it is so light. The RSC says the Hindenburg airship disaster in 1937 was filmed and seen by shocked audiences around the world. It brought an end to hydrogen airships.
Today, party balloons are filled with helium, which does not burn.
Super-cold liquid hydrogen
Hydrogen only turns into a liquid when it is incredibly cold. Its boiling point is −252.879 °C, just a little above the coldest temperature possible.
Rockets carry hydrogen as a super-cold liquid, because a liquid takes up far less space than a gas. It is kept in specially insulated tanks, because at room temperature it would boil away very quickly.
Three things people get wrong
Hydrogen is not a metal. It sits above the alkali metals in the periodic table, but on Earth it is a gas.
Water is not hydrogen gas. Water is a compound of hydrogen and oxygen joined together.
Hydrogen is not rare. It is the most common element in the universe, even though there is very little hydrogen gas in Earth’s air.
Did you know? The name hydrogen means “water forming”, because hydrogen burns with oxygen to make water.
Try it
Look at a glass of water and remember that two out of every three atoms in it are hydrogen atoms.
Knowledge check
What is the atomic number of hydrogen?
Show answer
1.
What does a hydrogen fuel cell produce?
Show answer
Only water.
Frequently asked questions
What is hydrogen?
Hydrogen is the simplest chemical element, with the symbol H and atomic number 1. It is a colourless, odourless, tasteless gas that burns easily.
Who discovered hydrogen?
Henry Cavendish identified hydrogen as a separate substance in 1766. Earlier alchemists such as Paracelsus had seen it bubble from metals and acid.
What is hydrogen used for?
Hydrogen is mostly used to make ammonia for fertilisers. It is also used in fuel refining, margarine, rocket fuel and fuel cells.
Why is hydrogen called hydrogen?
The name comes from Greek words meaning “water forming”, because hydrogen burns with oxygen to make water.
Is hydrogen the most common element?
Yes. Hydrogen is the most common element in the universe, making up about 75% of detectable matter, though it is rare as a gas on Earth.
What are the isotopes of hydrogen?
Hydrogen has three isotopes: protium, deuterium and tritium. They have zero, one and two neutrons, and tritium is radioactive.
Sources
- Encyclopaedia Britannica: hydrogen — properties, the atom, abundance, isotopes, name, where it is found and main uses
- Royal Society of Chemistry: hydrogen — discovery, name, uses, abundance, boiling point and the Hindenburg
- US Department of Energy: hydrogen fuel basics — fuel cells, how hydrogen is produced and its uses
