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- ← hydrogenhelium


Colorless gas with purple glow in its plasma state

Spectral lines of hydrogen
General properties
Name, symbol, number hydrogen, H, 1
Pronunciation /ˈhdrɵɨn/[1] HYE-dro-jin
Element category nonmetal
Group, period, block 11, s
Standard atomic weight 1.00794(7)g·mol−1
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)[2] g·cm−3
Liquid density at b.p. 0.07099 g·cm−3
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−1
Heat of vaporization (H2) 0.904 kJ·mol−1
Specific heat capacity (25 °C) (H2) 28.836 J·mol−1·K−1
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−1
Covalent radius 31±5 pm
Van der Waals radius 120 pm
Crystal structure hexagonal
Magnetic ordering diamagnetic[3]
Thermal conductivity (300 K) 0.1805 W·m−1·K−1
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.[4] 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.[5]

Hydrogen glows purple when it is in plasma state.[6]

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 75% 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 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:

  • It is a gas at room temperature
  • It acts like a metal when it is solid.
  • It is the lightest element in the Universe.
  • It is the most common gas element in the Universe.[4]
  • It explodes when it touches a flame.

History of Hydrogen

The name "hydrogen" comes from the Greek word for water, 'υδορ (pronounced /HEEW-dor/).

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

Uses of Hydrogen

Hydrogen as fuel (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 nuclear power stations, is that no waste is produced, and no toxic fuel like uranium is needed.

There is more than 600 million tons of hydrogen undergoing fusion every second on the Sun.[8][9]

Burning Hydrogen

1. Water can be easily broken down into hydrogen and oxygen with electricity, but it takes a lot of electricity to get a usable amount of hydrogen.

2. Burning hydrogen combines with oxygen molecules to make steam (pure water).

3. A fuel cell combines hydrogen with an oxygen molecule releasing an electron as electricity.

Hydrogen power grid

For these reasons, many people believe hydrogen power will eventually replace technologies such as diesel-electric engines and biodiesel fuel.

But it is not correct to see hydrogen as a fuel if it is used in a fuel cell. 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.


  1. Simpson, J.A.; Weiner, E.S.C. (1989). "Hydrogen". Oxford English Dictionary. 7 (2nd ed.). Clarendon Press. ISBN 0-19-861219-2 .
  2. Wiberg, Egon; Wiberg, Nils; Holleman, Arnold Frederick (2001). Inorganic chemistry. Academic Press. p. 240. ISBN 0123526515 .
  3. "Magnetic susceptibility of the elements and inorganic compounds". CRC Handbook of Chemistry and Physics (81st ed.). CRC Press.
  4. 4.0 4.1 - What is Hydrogen?
  5. Carroll, Bradley W. & Ostlie, Dale A. 2006. An introduction to modern astrophysics. 2nd ed, Addison-Wesley, San Francisco. ISBN 0-8053-0402-9
  6. "The magic of syngas". 2012 [last update]. Retrieved 7 March 2012.
  7. "Hydrogen in the Universe". 2012 [last update]. Retrieved 7 March 2012.
  8. "What is Fusion?". ITER Organization. 2012 [last update]. Retrieved 7 March 2012.
  9. "NASA's Cosmicopia". NASA. Retrieved 28 February 2013.