HYDROGEN is the chemical element with atomic number 1. It is represented by the symbol H. It is the most abundant of the chemical elements constituting roughly 75% of the universe's elemental mass. At standard temperature and pressure, it is colorless, odorless, nonmetallic, tasteless, highly flammable diatomic in nature. It is the lightest element having an atomic mass of 1.00794 amu.

Natural occurrence
This element is found in great abundance in stars and gas giant planets. Molecular clouds of H2 are associated with star formation. Hydrogen plays a vital role in powering stars through proton-proton reaction and CNO cycle nuclear fusion

Chemistry
The solubility and characteristics of hydrogen with various metals are very important in metallurgy. Hydrogen is highly soluble in many compounds composed of rare earth metals and transition metals. It can be dissolved in both crystalline and amorphous metals.

Combustion
Hydrogen is highly combustible in air. It is highly flammable and will burn at concentrations as low as 4% H2 in air. It burns according to the following balanced equation:

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

When mixed with oxygen , hydrogen explodes upon ignition. Hydrogen burns violently in air. It ignites automatically at a temperature of 560 C.

Production
H2 is produced in chemistry and biology laboratories, often as a by-product of other reactions or as a means of expelling reducing equivalents in biochemical reactions. In the laboratory, H2 is usually prepared by the reaction of acids on metals such as zinc.

Zn + 2 H+ = Zn₂+ + H₂

Aluminum produces H2 upon treatment with acids but also with base
2Al + 6 H₂O = 2 Al(OH)₃ + 3 H₂

The electrolysis of water is a simple method of producing hydrogen. A low voltage current is run through the water. Gaseous oxygen forms at the anode. Gaseous hydrogen forms at the cathode. Typically the cathode is made from platinum or another inert metal when producing hydrogen for storage. The theoretical maximum efficiency (electricity used vs. energetic value of hydrogen produced) is between 80-94%.
2H₂O(aq) = 2H₂(g) + O₂(g)

Electron energy levels
The energy levels of hydrogen can be calculated using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the sun. The ground state energy level of the electron in a hydrogen atom is -13.6 eV, which is equivalent to an ultraviolet photon of roughly 92 nm.

Isotopes
Hydrogen has three naturally occurring isotopes, denoted ¹H, ²H, and ³H.

• ¹H is the most common hydrogen isotope with an abundance of more than 99.98%. Nucleus of this isotope consists of only a single proton.
  
  
• ²H, the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in its nucleus.
  
  
• ³H is known as tritium and contains one proton and two neutrons in its nucleus.

Applications

• The largest application of H2 is for the processing of fossil fuels and in the production of ammonia.
  
  
• It is used as a shielding gas in welding methods such as atomic hydrogen welding.
  
  
• H2 is used as a hydrogenating agent, particularly in increasing the level of saturation of unsaturated fats and oils and in the production of methanol.
  
  
• It is used as the rotor coolant in electrical generators at power stations, because it has the highest thermal conductivity of any gas
  
  
• It is widely used as a lifting agent in balloons and airships.
  
  
• Hydrogen is used vastly for the inflation of weather balloons.