Hydrogen is the chemical element with the symbol H and atomic number 1 with a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the universe, constituting roughly 75% of all baryonic mass. Non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium has one proton and no neutrons.
The universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, non-metallic, highly combustible diatomic gas with the molecular formula H2. Since hydrogen readily forms covalent compounds with most non metallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays a particularly important role in acid-base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge ( i.e. anion ) when it is known as a hydride or as a positively charged ( i.e. cation ) species denoted by the symbol H+. The hydrogen cations is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compound are always more complex.
Hydrogen gas was first artifically produced in the early 16th century be the reaction of acids on metals. In 1766-1781, Henry cavendish was the first to recogzine that hydrogen gas was a discrete substance and that it produces water when burned, the property for which it was later named: in Greek, hydrogen means " water former ".
Appearance Colorless gas
Standard atomic weight Ar std ( H ) [ 1.00784, 1.00811 ] conventional: 1.008
Hydrogen in the periodic table
Atomic number (Z) 1
Group 1: H and alkali metals
Period Period 1
Block S- block
Electron Configuration 1s1
Electrons per shell 1
Physical properties
Phase ( at STP ) gas
Melting point (H2) 13.99K ( -259.16°C, -434.49°F)
Boiling point (H2) 20.271K ( -252.879°C, -423.182°F)
Density ( at STP ) 0.08988 g/L
When liquid ( at m.p. ) 0.07 g/cm3 ( Solid: 0.0763 g/cm3 )
