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Hydrogen,  1H
General properties
Name, symbol Hydrogen, H
Appearance Colorless gas
Purple glow in an electric field
Hydrogen in the periodic table


- ← HydrogenHelium
Atomic number 1
Standard atomic weight (Ar) 1.008
Group, block , s-block
Period period 1
Electron configuration 1s1
per shell
Physical properties
Colorless gas
Phase Gas
Melting point 13.99 K K ​(−259.16 °C, ​​−434.49 °F)
Boiling point 20.271 K ​(−252.879 °C, ​−423.182 °F)
Density at  (0 °C and 101.325 kPa) 0.08988 g/L
when liquid, at  0.07 g/cm3 (solid: 0.0763 g/cm3)
when liquid, at  0.07099 g/cm3
Triple point 13.8033 K, ​7.041 kPa
Critical point 32.938 K, 1.2858 MPa
Heat of fusion 0.117 kJ/mol
Heat of 0.904 kJ/mol
Molar heat capacity 28.836 J/(mol·K)
Atomic properties
Oxidation states −1, +1 ​(an amphoteric oxide)
Electronegativity Pauling scale: 2.20
energies 1st: 1312.0 kJ/mol
Covalent radius 31±5 pm
Van der Waals radius 120 pm
Crystal structure ​Hexagonal
Speed of sound 1310 m/s (gas, 27 °C)
Thermal conductivity 0.1805 W/(m·K)
Magnetic ordering Diamagnetic
CAS Registry Number 12385-13-6 1333-74-0 (H2)
Discovery Henry Cavendish (1766)
Named by Antoine Lavoisier (1783)
· references

Hydrogen is an element with the symbol H and the atomic number 1. It is a colorless, light gas at room temperature, and with most hydrogen atoms consisting of a single proton and electron, they are the simplest possible atoms in the universe. Some atoms may have one or two neutrons in their nucleus, forming the isotopes deuterium and tritium.


Physical properties

Hydrogen is a very light gas, with an atomic mass of 1.00797 and a density of 0.08988 g/L. Balloons filled with hydrogen will readily rise. Producing liquid hydrogen is completely infeasible to the amateur, but it has one of the highest energy densities of all fuels per mass. Hydrogen normally exists as a diatomic gas, which has two spin isomers: orthohydrogen and parahydrogen.

Pure hydrogen has no smell, but if the gas is freshly prepared from acids and zinc, or alkali and aluminium, it may have a pungent smell because of corrosive droplets or acid vapors carried by it.

Chemical properties

Hydrogen will form compounds with many different elements. Combustion with hydrogen produces water and a large amount of heat, light and sound.

H2 + ½ O2 → H2O

With fluorine, the corresponding redox reaction occurs explosively when the gases contact each other, forming hydrogen fluoride. Hydrogen and chlorine will react explosively on exposure to ultraviolet light to form hydrogen chloride, which can be dissolved in water to form hydrochloric acid. Bromine and iodine need sufficient activation energy to form their respective hydrogen halides. Some metals will react with hydrogen to form hydrides. Others alloy with the gas - palladium is notable for being able to absorb 900 times its weight in hydrogen.

With the proper catalyst (palladium, platinum, or Raney nickel) alkene and alkyne functionality can be converted to alkane functionality. With Lindlar's catalyst, a poisoned variant, alkynes can be reduced only to alkenes.

In air, hydrogen burns with a flame that is very dimly bluish in darkness and invisible under any kind of light. If hydrogen is freshly prepared, gasiform or aerosol contaminants may color the flame and make it visible; for example, hydrogen made by reacting aluminium with sodium hydroxide produces a sodium yellow flame.


Hydrogen is available as compressed gas in cylinders, though it's availability varies.

An important thing to remember is that hydrogen must NEVER be stored in other common gas cylinders, such as propane tanks. The presence of oxygen in the cylinder poses a risk when liquifying the gas inside. Hydrogen liquifies at 800 atm, a pressure difficult to achieve by an amateur chemist. Lastly, hydrogen will also cause embrittlement in many types of steels, such as high-strength and low-alloy steels, as well as titanium and nickel alloys.


Hydrogen gas can be liberated by dissolving any sufficiently electropositive metal in a non-oxidizing acid, such as hydrochloric acid or dilute sulfuric acid. Normally, aluminium, magnesium or zinc are used for this process. Iron is sometimes used. The addition of excess metal to acid or vice-versa will also reduce some of the acid to chlorine or hydrogen sulfide, which may contaminate your hydrogen.

Zn + 2 HCl → ZnCl2 + H2

The acid can also be replaced with a strong base, such as sodium hydroxide, but this will only work with amphoteric metals that can form hydroxometalate salts with sodium, such as aluminium and zinc; purely basic metals such as magnesium will not work. All these reactions produce large amounts of heat and may pose a fire or explosion hazard.

2 Al + 2 NaOH + 2 H2O → 2 NaAl(OH)4 + 3 H2

Another way to produce hydrogen is to electrolyze water with a small amount of electrolyte. Hydrogen gas is produced at the anode and oxygen is produced at the cathode.

H2O → H2 + ½ O2


  • Hydrogen balloons
  • Metal hydrides
  • Reduction of organic compounds
  • Ammonia synthesis



Hydrogen gas poses an asphyxiant hazard at high concentrations, in closed environments. When mixed with air, it poses a great explosive hazard in concentrations from 5% to 95% by volume, meaning any significant build-up in an enclosed room is a serious risk of explosion.

Liquid hydrogen is a very powerful cryogenic liquid, and contact with naked skin will cause frostbites.


Compressed and cryogenic hydrogen should only be stored in cylinders made of metal that is not susceptible to embrittlement. The cylinders should be checked from time to time for signs of corrosion and to make sure the valves work properly. They must also be stored in dark cold places, away from any heat source, and if possible in a semi-open space to prevent a possible build-up.

Due to the very small size of diatomic hydrogen, hydrogen can easily diffuse out of many materials such as latex. Thus, filling a balloon with hydrogen should be done only when you plan to use the balloon.


Hydrogen can be safely released in open air. Ignition of the hydrogen may be preferable to prevent an unexpected explosion from occurring. This however, should never be done in a closed environment, as even a small amount of hydrogen in the air can cause a fire or explosion. Continuous venting is highly recommended.


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