Hydrogen is a chemical element represented
by the symbol H and an atomic number of 1. At standard
temperature and pressure it is a colorless, odorless,
nonmetallic, tasteless, highly flammable diatomic gas
(H2). With an atomic mass of 1.00794 g/mol, hydrogen is
the lightest element.
Hydrogen is the most abundant of the
chemical elements, constituting roughly 75% of the universe's
elemental mass. 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 in 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.
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.
