Hydrogen is a gas discovered by Boyle in 1671. French chemist Antonie Lavoisier named hydrogen from the Greek words for "water former."
Hydrogen is found in the atmosphere at trace levels. It is synthetized from hydrocarbons and from water where it constitutes the lightest fraction of the H2O molecule. Hydrogen gas cannot sustain life.
Atomic number : 1
Atomic mass : 1.007825 g.mol -1
Vanderwaals radius : 0.12 nm
Ionic radius : 0.208 (-1) nm
Isotopes : 3
Electronic shell : 1s1
Hydrogen Physical Properties
- Highly flammable
- Light in weight
- Density : 0.0899*10 -3 g.cm -3 at 20 °C
- Melting point : - 259.2 °C
- Boiling point :- 252.8 °C
- Pure hydrogen is a gas under normal conditions.
- Hydrogen is diatomic and much lighter than air.
- It has such small mass that it can escape earth's gravitational
pull and fly off into space.
- The gas mixes well with air, explosive mixtures are easily
- The gas is lighter than air.
- Electronegativity according to Pauling : 2.1
- Energy of first ionisation : 1311 kJ.mol -1
- Reacts easily with other chemical substances.
- Hydrogen is slightly more soluble in organic solvents than in
- It does not usually react with other chemicals at room
- Two hydrogen molecules (H2) and one oxygen molecule (O2), combine
to form two molecules of water, or H2O. This reaction releases
- Hydrogen bonds form covalent bonds with each other and with other
- In some molecules containing hydrogen, the covalent bond between
one of the hydrogen atoms and another atom is weak and breaks
easily. Compounds made of these bonds are called acids.
- Hydrogen also forms ionic bonds with some metals, creating a
compound called a hydride.
- Hydrogen can also form a unique bond known as a hydrogen bond.
Hydrogen bonds only form between hydrogen and the elements oxygen
(O), nitrogen (N), or fluorine (F). Water is a good example of
- Many metals absorb hydrogen. Hydrogen absorption by steel can
result in brittle steel, which leads to fails in the chemical
- At normal temperature hydrogen is a not very reactive substance.
- Atomic hydrogen reacts with organic compounds to form a complex
mixture of products.
- Hydrogen reacts with oxygen to form water and this reaction is
extraordinarily slow at ambient temperature.
- Under extreme pressure hydrogen can actually act like a metal.
- Heating may cause violent combustion or explosion.
- Reacts violently with air, oxygen, halogens and strong oxidants
causing fire and explosion hazard.
- Hydrogen is widely used as a reducing agent.