Hydrogen is a chemical element. It has the symbol H and atomic number 1.  It has a standard atomic weight of 1.008, meaning it is the lightest element in the periodic table.

Hydrogen is the most common chemical element in the Universe, making up 75% of all normal (baryonic) matter (by mass). Most stars are mostly hydrogen. Hydrogen's most common isotope has one proton with one electron orbiting around it.

Hydrogen is classed as a reactive nonmetal, unlike the other elements appearing in the first column of the periodic table, which are classed alkali metals. The solid form of hydrogen is expected to behave like a metal, however.

When alone, hydrogen usually binds with itself to make dihydrogen (H2). At standard temperature and pressure, this hydrogen gas (H2) has no colour, smell or taste. It is not toxic. It is a nonmetal and burns very easily.

In its pure form on Earth, hydrogen is usually a gas. Hydrogen is also one of the parts that make up a water molecule. Hydrogen is important because it is the fuel that powers the Sun and other stars.
Hydrogen makes up about 74% of the entire universe.[7] Hydrogen's symbol on the Periodic Table of Elements is H.

Pure hydrogen is normally made of two hydrogen atoms connected together. Scientists call these diatomic molecules. Hydrogen will have a chemical reaction when mixed with most other elements. It has no color or smell.

Pure hydrogen is very uncommon in the Earth's atmosphere. In nature, it is usually in water.  Hydrogen is also in all living things, as a part of the organic compounds that living things are made of.  In addition, hydrogen atoms can combine with carbon atoms to form hydrocarbons.  Petroleum and other fossil fuels are made of these hydrocarbons and commonly used to create energy for human use.

Hydrogen has three isotopes; the others are called deuterium and tritium. Like regular hydrogen, they both have only one proton and one electron, but deuterium also has one neutron and tritium has two. These other types of hydrogen are important in nuclear energy and organic chemistry reactions.

Some other facts about hydrogen:

Hydrogen was first separated in 1671 by Robert Boyle. Henry Cavendish in 1776 identified it as a distinct element and discovered that burning it made water.

Antoine Lavoisier give Hydrogen its name, from the Greek word for water, 'υδορ (pronounced /HEEW-dor/) and gennen meaning to "generate" as it forms water in a chemical reaction with oxygen.

The main uses are in the petroleum industry and in making ammonia by the Haber process. Some is used elsewhere in the chemical industry. A little of it is used as fuel, for example in rockets for spacecraft. Most of the hydrogen that people use comes from a chemical reaction between natural gas and steam.

Nuclear fusion is a very powerful source of energy. It relies on forcing atoms together to make helium and energy, exactly as happens in a star like the Sun, or in a hydrogen bomb. This needs a large amount of energy to get started, and is not easy to do yet. A big advantage over nuclear fission, which is used in today's nuclear power stations, is that it makes less nuclear waste and does not use a toxic and rare fuel like uranium.  More than 600 million tons of hydrogen undergo fusion every second on the Sun.[10][11]

Hydrogen is mostly used in the petroleum industry, to change heavy petroleum fractions into lighter, more useful ones. It is also used to make ammonia. Smaller amounts are burned as fuel. Most hydrogen is made by a reaction between natural gas and steam.

The electrolysis of water breaks water into hydrogen and oxygen, using electricity. Burning hydrogen combines with oxygen molecules to make steam (pure water vapor). A fuel cell combines hydrogen with an oxygen molecule, releasing an electron as electricity. For these reasons, many people believe hydrogen power will eventually replace other synthetic fuels.

Hydrogen can also be burned to make heat for steam turbines or internal combustion engines. Like other synthetic fuels, hydrogen can be created from natural fuels such as coal or natural gas, or from electricity, and therefore represents a valuable addition to the power grid; in the same role as natural gas. Such a grid and infrastructure with fuel cell vehicles is now planned by a number of countries including Japan, Korea and many European countries. This allows these countries to buy less petroleum, which is an economic advantage. The other advantage is that used in a fuel cell or burned in a combustion engine as in a hydrogen car, the motor does not make pollution. Only water, and a small amount of  nitrogen oxides, forms.