Hydrogen
Hydrogen is the simplest element in the universe, and also the most common. Every atom of hydrogen contains just one proton and one electron, and nothing else. It is so light that on Earth most of it has floated away into space, but out among the stars it is everywhere.
- Atomic Number11 proton, 1 electron
- Atomic Mass1.008 uLightest of all elements
- State at Room TempGasInvisible, no smell
- Density0.00009 g/cm³14× lighter than air
- Melting / Boiling-259 °C / -253 °CLiquid only when very cold
- Discovered1766by Henry Cavendish
How light is hydrogen?
Hydrogen is the lightest of all elements. A single atom weighs less than any other atom in the universe.
A hydrogen atom weighs approx. 1 atomic mass unit. An iron atom weighs 56 times more, which is why a balloon full of hydrogen floats up and a lump of iron drops to the floor.
What is hydrogen?
Hydrogen is a colourless, odourless gas that is lighter than air. It is the first element on the periodic table because its atom has just one proton in its nucleus. When two hydrogen atoms join together they form a molecule written as H₂, which is the form hydrogen normally takes as a gas.
Hydrogen got its name from two Greek words: "hydro" meaning water and "genes" meaning maker. It was called the "water maker" because when it burns in oxygen the only thing it produces is water.
Where you find hydrogen
In space
Stars are giant balls of hydrogen. The Sun is roughly 73% hydrogen by mass, and deep inside its core, hydrogen atoms are squeezed together so hard that they fuse into helium. That fusion is what makes the Sun shine.
The two biggest planets in our solar system, Jupiter and Saturn, are also mostly hydrogen. They are sometimes called gas giants because they have no solid surface to land on. If you tried to fly down into Jupiter you would just keep falling through thicker and thicker hydrogen until the pressure crushed your ship.
On Earth
On Earth, almost no hydrogen exists as a pure gas. It is too light, so any pure hydrogen escapes into space. Instead, hydrogen is locked up inside other molecules:
- Water (H₂O) is two hydrogen atoms joined to one oxygen atom. Around 11% of every drop of water is hydrogen by mass.
- Living things are full of hydrogen. Sugar, fat, protein, and DNA all contain it.
- Fuel like petrol and natural gas is made of hydrogen joined to carbon. We call these hydrocarbons.
How we use hydrogen
People use hydrogen for several important jobs:
- Rocket fuel. Liquid hydrogen burned with liquid oxygen gives one of the most powerful rocket fuels known. The Space Shuttle's huge orange tank held hydrogen and oxygen to lift it into orbit.
- Fertiliser. Hydrogen is combined with nitrogen from the air to make ammonia, which is then used to make fertilisers. Around half the food the world eats depends on this process.
- Clean energy. Hydrogen fuel cells can power cars and even buildings. The only thing that comes out of the exhaust pipe is water, which is why some people call hydrogen the fuel of the future.
How it was discovered
The first scientist to study hydrogen on its own was an English chemist called Henry Cavendish, in 1766. He noticed that a strange "inflammable air" came out when he poured acid on metal. When he set fire to it, the air burned with a pale flame and produced water. He had no idea he was looking at the most common element in the universe.
Deeper dive: atoms, isotopes and chemistry
A hydrogen atom is the simplest possible atom: one proton in the nucleus and one electron going round it. But there are actually three different versions of hydrogen, called isotopes, depending on how many neutrons sit in the nucleus alongside the proton.
- Protium (no neutrons) is the normal one. Over 99.9% of all hydrogen is protium.
- Deuterium (one neutron) is sometimes called heavy hydrogen. Water made with deuterium is called heavy water and is used in some nuclear reactors.
- Tritium (two neutrons) is radioactive and very rare. It is made in reactors and used in special glow-in-the-dark signs and in fusion research.
Hydrogen sits in Group 1 of the periodic table along with the alkali metals like sodium and potassium, because all of them have just one electron in their outer shell. But hydrogen is not really a metal at all; at normal temperatures it is a gas, not a shiny solid. Scientists put it where they did because of its electron count, and you will sometimes see it floated above the table by itself to show it is a bit of an oddball.
Hydrogen has two main oxidation states. When it gives away its single electron it becomes H⁺ (an acid), and when it accepts an extra one it becomes H⁻ (a hydride). That flexibility is part of why hydrogen joins up with almost every other element.
Hydrogen is the start of the periodic table because of its single proton, but its story reaches all the way to the heart of every star. The next element along, with two protons instead of one, is helium, which is what hydrogen turns into when it fuses inside the Sun.