Standard conditions in the context of "Graphite"

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

Hydrogen gas was first produced artificially in the 17th century by the reaction of acids with metals. Henry Cavendish, in 1766–1781, identified hydrogen gas as a distinct substance and discovered its property of producing water when burned: this is the origin of hydrogen's name, which means 'water-former' (from Ancient Greek: ὕδωρ, romanized: húdōr, lit. 'water', and γεννάω, gennáō, 'I bring forth'). Understanding the colors of light absorbed and emitted by hydrogen was a crucial part of the development of quantum mechanics.

The bond dissociation energy (BDE, D₀, or DH°) is one measure of the strength of a chemical bond A−B. It can be defined as the standard enthalpy change when A−B is cleaved by homolysis to give fragments A and B, which are usually radical species. The enthalpy change is temperature-dependent, and the bond dissociation energy is often defined to be the enthalpy change of the homolysis at 0 K (absolute zero), although the enthalpy change at 298 K (standard conditions) is also a frequently encountered parameter.

As a typical example, the bond dissociation energy for one of the C−H bonds in ethane (C₂H₆) is defined as the standard enthalpy change of the process