Hydrogen is a chemical element with chemical symbol H and atomic number 1. With an atomic weight of 1.00794 u, hydrogen is the lightest element on the periodic table. Its monatomic form (H) is the most abundant chemical substance in the universe, constituting roughly 75% of all baryonic mass.[note 1] Non-remnant stars are mainly composed of hydrogen in its plasma state. The most common isotope of hydrogen, termed protium (name rarely used, symbol 1H), has a single proton and zero 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, nonmetallic, 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 in the form of water or organic compounds. Hydrogen plays a particularly important role in acid–base reactions. In ionic compounds, hydrogen can take the form of a negative charge (i.e., anion) known as a hydride, or as a positively charged (i.e., cation) species denoted by the symbol H+. The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex species than that would suggest.
As the simplest atom known, the hydrogen atom has had considerable theoretical application. For example, the hydrogen atom is the only neutral atom with an analytic solution to the Schrödinger equation.
Hydrogen gas was first artificially produced in the early 16th century, via the mixing of metals with acids. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, and that it produces water when burned, a property which later gave it its name: in Greek, hydrogen means "water-former".
Industrial production is mainly from the steam reforming of natural gas, and less often from more energy-intensive hydrogen production methods like the electrolysis of water. Most hydrogen is employed near its production site, with the two largest uses being fossil fuel processing (e.g., hydrocracking) and ammonia production, mostly for the fertilizer market.
Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks.
Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table and is a highlyreactive nonmetallic element and oxidizing agent that readily forms compounds (notably oxides) with most elements.By mass, oxygen is thethird-most abundant element in the universe, after hydrogen and helium. At STP, two atoms of the element bind to form dioxygen, a diatomicgas that is colorless, odorless, and tasteless; with the formula O
2.
Many major classes of organic molecules in living organisms, such as proteins, nucleic acids, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that are constituents of animal shells, teeth, and bone. Most of the mass of living organisms is oxygen as it is a part of water, the major constituent of lifeforms (for example, about two-thirds of human body mass). Elemental oxygen is produced bycyanobacteria, algae and plants, and is used in cellular respiration for all complex life. Oxygen is toxic to obligately anaerobic organisms, which were the dominant form of early life on Earth until O
2 began to accumulate in the atmosphere. Free elemental O
2 only began to accumulate in the atmosphere about 2.5 billion years ago (see Great oxygenation event) about a billion years after the first appearance of these organisms.Diatomic oxygen gas constitutes 20.8% of the volume of air. Oxygen is the most abundant element by mass in the Earth's crust as part of oxide compounds such as silicon dioxide, making up almost half of the crust's mass.
Oxygen is an important part of the
atmosphere, and is necessary to sustain most terrestrial life as it is used in respiration. However, it is too chemically reactive to remain a free element in
Earth's atmosphere without being continuously replenished by the
photosynthetic action of living organisms, which use the energy of sunlight to produce elemental oxygen from water. Another form (
allotrope) of oxygen,
ozone (
O
3), strongly absorbs
UVB radiation and consequently the high-altitude
ozone layer helps protect the biosphere from
ultraviolet radiation, but is a pollutant near the surface where it is a by-product of
smog. At even higher
low earth orbit altitudes, atomic oxygen is a significant presence and a cause of
erosion for spacecraft.
Oxygen is produced industrially by
fractional distillation of liquefied air, use of
zeolites with
pressure-cycling to concentrate oxygen from air,
electrolysis of water and other means. Uses of elemental oxygen include the production of
steel,
plastics and
textiles,
brazing, welding and cutting of steels and other
metals,
rocket propellant,
oxygen therapy and
life support systems in
aircraft,
submarines,
spaceflight and
diving.
