Periodic table - Extended periodic table General Name, symbol, number hydrogen, H, 1 Chemical series nonmetals Group, period, block 1, 1, s Appearance colorless Standard atomic weight 1.00794(7) g·mol1 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·mol1 Heat of vaporization (H2) 0.904 kJ·mol1 Specific heat capacity (25 °C) (H2) 28.836 J·mol1·K1 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·mol1 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·m1·K1 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 ² 0.019 3He References This box: view " talk " edit Hydrogen (pronounced /ÈhajdrYd’Yn/[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 mass of 1.00794 amu, hydrogen is the lightest element. Hydrogen is the most abundant of the chemical elements, 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, and is industrially produced from hydrocarbons such as methane, after which most elemental hydrogen is used "captively" (meaning locally at the production site), with the largest markets about equally divided between fossil fuel upgrading (e.g., hydrocracking) and ammonia production (mostly for the fertilizer market). Hydrogen may be produced from water using the process of electrolysis, but this process is presently significantly more expensive commercially than hydrogen production from natural gas.[3] The most common naturally occurring isotope of hydrogen, known as protium, has a single proton and no neutrons. In ionic compounds it can take on either a positive charge (becoming a cation composed of a bare proton) or a negative charge (becoming an anion known as a hydride). Hydrogen can form compounds with most elements and is present in water and most organic compounds. It plays a particularly important role in acid-base chemistry, in which many reactions involve the exchange of protons between soluble molecules. As the only neutral atom for which the Schrödinger equation can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics. The solubility and characteristics of hydrogen with various metals are very important in metallurgy (as many metals can suffer hydrogen embrittlement[4]) and in developing safe ways to store it for use as a fuel.[5] Hydrogen is highly soluble in many compounds composed of rare earth metals and transition metals[6] and can be dissolved in both crystalline and amorphous metals.[7] Hydrogen solubility in metals is influenced by local distortions or impurities in the metal crystal lattice.[8]