Hydrogen

Hydrogen is a chemical element; it has symbol H and atomic number 1. It is the lightest element and, at standard conditions, is a gas of diatomic molecules with the formula H₂, sometimes called dihydrogen, but more commonly called hydrogen gas, molecular hydrogen or simply hydrogen. It is colorless, odorless, tasteless, non-toxic, and highly combustible. Constituting approximately 75% of all normal matter, hydrogen is the most abundant chemical substance in the universe. Stars, including the Sun, primarily consist of hydrogen in a plasma state, while on Earth, hydrogen is found in water, organic compounds, and other molecular forms. The most common isotope of hydrogen (symbol ¹H) consists of one proton, one electron, and no neutrons.

In the early universe, the formation of hydrogen's protons occurred during the first second following the Big Bang, with neutral hydrogen atoms only forming approximately 370,000 years later during the recombination epoch as the universe cooled and plasma had cooled enough for electrons to remain bound to protons. Hydrogen, typically nonmetallic except under extreme pressures, readily forms covalent bonds with most nonmetals, contributing to the formation of compounds like water and various organic substances. Its role is crucial in acid-base reactions, which predominantly involve proton exchange among soluble molecules. In ionic compounds, hydrogen can take the form of either a negatively charged anion, where it is known as hydride, or as a positively charged cation denoted by the symbol H⁺. The cation, simply a proton (symbol p), exhibits specific behavior in aqueous solutions and in ionic compounds involves screening of its electric charge by surrounding polar molecules or anions. Hydrogen's unique position as the only neutral atom for which the Schrödinger equation can be directly solved has significantly contributed to the foundational principles of quantum mechanics through the exploration of its energetics and chemical bonding.

Historically, hydrogen gas was first produced artificially in the early 16th century through the reaction of acids with metals. Henry Cavendish, between 1766 and 1781, identified hydrogen gas as a distinct substance and discovered its property of producing water when burned—hence its name derived from the Greek "water-former".

Today, the majority of hydrogen production occurs through steam reforming of natural gas, with a smaller portion derived from energy-intensive methods such as the electrolysis of water. Its primary industrial uses include fossil fuel processing, such as hydrocracking, and ammonia production, with emerging applications in fuel cells for electricity generation and as a heat source. When used in fuel cells, hydrogen's only emission at the point of use is water vapor, though combustion can produce nitrogen oxides. Hydrogen's interaction with metals may cause embrittlement.