Contents
Hydrogen
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H
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Li
General
Name, symbol, number hydrogen, H, 1
Element category nonmetal
Group, period, block 1, 1, s
Appearance colorless gas
Standard atomic weight 1.00794(7) g·mol−1
Electron configuration 1s1
Electrons per shell 1
Physical properties
Phase gas
Density (0 °C, 101.325 kPa)
0.08988 g/L
Melting point 14.01 K
(−259.14 °C, −434.45 °F)
Boiling point 20.28 K
(−252.87 °C, −423.17 °F)
Triple point 13.8033 K (-259°C), 7.042 kPa
Critical point 32.97 K, 1.293 MPa
Heat of fusion (H2) 0.117 kJ·mol−1
Heat of vaporization (H2) 0.904 kJ·mol−1
Specific heat capacity (25 °C) (H2)
28.836 J·mol−1·K−1
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k
at T/K 15 20
Atomic properties
Crystal structure hexagonal
Oxidation states 1, −1
(amphoteric oxide)
Electronegativity 2.20 (Pauling scale)
Ionization energies 1st: 1312.0 kJ·mol−1
Atomic radius 25 pm
Atomic radius (calc.) 53 pm
Covalent radius 37 pm
Van der Waals radius 120 pm
Miscellaneous
Thermal conductivity (300 K) 180.5 m W·m−1·K−1
Speed of sound (gas, 27 °C) 1310 m/s
CAS registry number 1333-74-0
Selected isotopes
Main article: Isotopes of hydrogen iso NA half-life DM DE (MeV) DP
1H 99.985% 1H is stable with 0 neutrons
2H 0.015% 2H is stable with 1 neutron
3H trace 12.32 y β− 0.01861 3He
References
Hydrogen (pronounced /ˈhaɪdrədʒən/[1]) is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H2. With an atomic weight of 1.00794, hydrogen is the lightest element.
Hydrogen is the most abundant chemical element, constituting roughly 75% of the universe's elemental mass.[2] Stars in the main sequence are mainly composed of hydrogen in its plasma state. Elemental hydrogen is relatively rare on Earth. Industrial production is from hydrocarbons such as methane with most being used "captively" at the production site. The two largest uses are in fossil fuel processing (e.g., hydrocracking) and ammonia production mostly for the fertilizer market. Hydrogen may be produced from water by electrolysis at substantially greater cost than production from natural gas.[3]
The most common isotope of hydrogen is protium with a single proton and no neutrons. In ionic compounds it can take a positive charge (a cation composed of a bare proton) or a negative charge (an anion known as a hydride). Hydrogen forms compounds with most elements and is present in water and most organic compounds. It plays a particularly important role in acid-base chemistry with many reactions exchanging protons between soluble molecules. As the only neutral atom with an analytic solution to the Schrödinger equation, the study of the energetics and bonding of the hydrogen atom played a key role in the development of quantum mechanics.
Hydrogen is important in metallurgy as it can embrittle many metals[4], complicating the design of pipelines and storage tanks.[5] Hydrogen is highly soluble in many rare earth and transition metals[6] and is soluble in both crystalline and amorphous metals.[7] Hydrogen solubility in metals is influenced by local distortions or impurities in the crystal lattice.[8]
Combustion
Hydrogen gas (dihydrogen[9]) is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume.[10] The enthalpy of combustion for hydrogen is −286 kJ/mol:[11]
2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ (286 kJ/mol)[12]Hydrogen/oxygen mixtures are explosive across a wide range of proportions. It ignites spontaneously in air at 560 °C.[13] Pure hydrogen-oxygen flames emit ultraviolet light and are nearly invisible to the naked eye as illustrated by the faint plume of the Space Shuttle main engine compared to the highly visible plume of a Space Shuttle Solid Rocket Booster). The detection of a burning hydrogen leak may require a flame detector; such leaks can be very dangerous. The explosion of the Hindenburg airship was an infamous example of hydrogen combustion; the cause is debated, but the visible flames were the result of combustible materials in the ship's skin.[14] Because hydrogen is buoyant in air, hydrogen flames tend to ascend rapidly and cause less damage than hydrocarbon fires. Two-thirds of the Hindenburg passengers survived the fire, and many deaths were instead the result of falls or burning diesel fuel.[15]
H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding halides: hydrogen chloride and hydrogen fluoride.[16]
Electron energy levels
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.[17]
The energy levels of hydrogen can be calculated fairly accurately 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. However, the electromagnetic force attracts electrons and protons to one another, while planets and celestial objects are attracted to each other by gravity. Because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.[18]
A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation or the equivalent Feynman path integral formulation to calculate the probability density of the electron around the proton.[19]
Elemental molecular forms
See also: Spin isomers of hydrogen