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Hydrogen, 1H
Hydrogen discharge tube.jpg
Purple glow in its plasma state
Appearance colorless gas
Standard atomic weight Ar, std(H) [1.007841.00811] conventional: 1.008
Hydrogen in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson


– ← hydrogenhelium
Atomic number (Z) 1
Group group 1: hydrogen and alkali metals
Period period 1
Block   s
Electron configuration 1s1
Electrons per shell 1
Physical properties
Phase at STP gas
Melting point 13.99 K ​(−259.16 °C, ​−434.49 °F)
Boiling point 20.271 K ​(−252.879 °C, ​−423.182 °F)
Density (at STP) 0.08988 g/L
when liquid (at m.p.) 0.07 g/cm3 (solid: 0.0763 g/cm3)
when liquid (at b.p.) 0.07099 g/cm3
Triple point 13.8033 K, ​7.041 kPa
Critical point 32.938 K, 1.2858 MPa
Heat of fusion (H2) 0.117 kJ/mol
Heat of vaporization (H2) 0.904 kJ/mol
Molar heat capacity (H2) 28.836 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 15 20
Atomic properties
Oxidation states −1, +1 (an amphoteric oxide)
Electronegativity Pauling scale: 2.20
Ionization energies
  • 1st: 1312.0 kJ/mol
Covalent radius 31±5 pm
Van der Waals radius 120 pm
Color lines in a spectral range
Spectral lines of hydrogen
Other properties
Natural occurrence primordial
Crystal structure ​hexagonal
Hexagonal crystal structure for hydrogen
Speed of sound 1310 m/s (gas, 27 °C)
Thermal conductivity 0.1805 W/(m⋅K)
Magnetic ordering diamagnetic
Molar magnetic susceptibility −3.98·10−6 cm3/mol (298 K)
CAS Number 12385-13-6
1333-74-0 (H2)
Discovery Henry Cavendish (1766)
Named by Antoine Lavoisier (1783)
Main isotopes of hydrogen
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
1H 99.98% stable
2H 0.02% stable
3H trace 12.32 y β 3He

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) which is very stable, due to its high bond dissociation energy of 435.7 kJ/mol. 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.


Molecular hydrogen is flammable and reacts with oxygen:

2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ (286 kJ/mol)

At temperatures above 500 degrees Celsius, hydrogen spontaneously ignites in air.


While hydrogen gas in its pure form is not reactive, it does form compounds with many elements, particularly halogens, which are very electronegative. Hydrogen also forms vast arrays with carbon atoms, forming hydrocarbons. The study of the properties of hydrocarbons are known as organic chemistry.

The H- anion (negatively charged atom) is called a hydride, although the term is not widely used. An example of a hydride is lithium hydride (LiH), which is used as a "spark plug" in nuclear weapons.


Acids dissolved in water typically contain high levels of hydrogen ions, in other words, free protons. The level of them is usually used to determine its pH, which basically means the content of hydrogen ions in a particular volume. For example, hydrochloric acid, found in people's stomachs, can dissociate into a chloride anion and a free proton, and the property of the free proton is how it can digest food by corroding it.

Although rare on Earth, the H3+ cation is one of the most common ions in the universe.


Main article: isotopes of hydrogen

Hydrogen has 7 known isotopes, two of which are stable (1H and 2H), which are commonly referred to protium and deuterium. The isotope 3H is known as tritium and has a half life of 12.33 years, and is produced in small amounts by cosmic rays. The remaining 4 isotopes have half lives on the scale of yoctoseconds.

Hydrogen in nature

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. 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, because nearly all primordial hydrogen would have escaped into space due to its weight. 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.

Some other facts about hydrogen:

History of 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 gave 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.

Uses of Hydrogen

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

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.

Using hydrogen

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.

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