- ← hydrogenhelium


Colorless gas with purple glow in its plasma state

Spectral lines of hydrogen
General properties
Name, symbol, number hydrogen, H, 1
Pronunciation /ˈhdrn/ HYE-dro-jin
Element category nonmetal
Group, period, block 1, 1, s
Standard atomic weight 1.00794(7) g/mol
Electron configuration 1s1
Electrons per shell 1 (Image)
Physical properties
Color colorless
Phase gas
Density (0 °C, 101.325 kPa)
0.08988 g/L
Liquid density at m.p. 0.07 (0.0763 solid) g/cm3
Liquid density at b.p. 0.07099 g/cm3
Melting point 14.01 K, -259.14 °C, -434.45 °F
Boiling point 20.28 K, -252.87 °C, -423.17 °F
Triple point 13.8033 K (-259°C), 7.042 kPa
Critical point 32.97 K, 1.293 MPa
Heat of fusion (H2) 0.117 kJ/mol
Heat of vaporization (H2) 0.904 kJ/mol
Specific heat capacity (25 °C) (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
(amphoteric oxide)
Electronegativity 2.20 (Pauling scale)
Ionization energies 1st: 1312.0 kJ/mol
Covalent radius 31±5 pm
Van der Waals radius 120 pm
Crystal structure hexagonal
Magnetic ordering diamagnetic
Thermal conductivity (300 K) 0.1805 W/(m·K)
Speed of sound (gas, 27 °C) 1310 m/s
CAS registry number 1333-74-0
Most stable isotopes
Main article: Isotopes of hydrogen
iso NA half-life DM DE (MeV) DP
1H 99.985% 1H is stable with 0 neutrons
2H 0.015% 2H is stable with 1 neutron
3H trace 12.32 y β 0.01861 3He

Hydrogen is a chemical element. Its atomic number is 1, which makes it the simplest, known element in the entire universe. Hydrogen is the true primordial substance, the first atom produced after the big bang. All chemical elements were formed from hydrogen by the processes of nuclear fusion.

Hydrogen glows purple when it is in plasma state.

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. 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 two different isotopes, 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:

History of Hydrogen

Hydrogen was discovered in 1671 by Robert Boyle even though many people think it was discovered by Henry Cavendish in 1776.

The word

The name 'Hydrogen' comes 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 of hydrogen 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.

Burning Hydrogen

The electrolysis of water easily breaks water into hydrogen and oxygen, using a large amount of 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 technologies such as diesel-electric engines and biodiesel fuel.

Some people think it is not correct to see hydrogen as a fuel if it is used in a fuel cell. Because it is not found naturally on Earth, it is more of a replacement for the power grid. Such a grid and infrastructure with new vehicles might be first made in Iceland, a country that has much free geothermal energy and is quite small. Because it imports all fossil fuel, it would help Iceland to completely stop using it. The huge advantage of hydrogen is that when burnt in an engine or in a fuel cell, there is no pollution. Only a small amount of water forms.


Hydrogen for Kids. Kiddle Encyclopedia.