Hydrogen
Hydrogen is a chemical element with atomic number Z = 1 and chemical symbol H. It is the most abundant element in the universe, (about 90% in number of atoms, about 70% in mass. It exists primarily in interstellar gas clouds and stars). On earth hydrogen occurs mainly bound to other elements, to carbon in carbohydrates, to oxygen in water, and to nitrogen in ammonia. Measured in number of atoms, hydrogen is the third abundant element (15%) on earth. Atomic hydrogen is very reactive and does not naturally appear in free form. The hydrogen molecule H2 is a stable gas at ambient temperature and pressure.
Chemical properties
The neutral atom has nuclear charge e (the elementary charge) and one electron (also with charge e). The electronic wave functions of the ground and excited states of the H-atom are known exactly, because the Schrödinger equation can be solved analytically for one-electron atoms, see hydrogen-like atoms. In the ground state the electron is in a 1s orbital (a nodeless spherically symmetric function with energy -13.6 eV).
Hydrogen resembles alkali metals, like Li and Na, in that it can lose its (outer) electron and become the cation H+. Strong acids, dissolved in water, dissociate and release H+, for instance HCl → Cl− + H+. Note, however, that the free hydrogen nucleus, being a bare charge, will not remain free in solution, but will immediately associate with solvent molecules, e.g., with water it will associate to H3O+.
Hydrogen also resembles halogens, like Cl, in that it can form an anion. It does so in saline hydrides (LiH, NaH, CaH2, etc.), in which to a large extent the binding is ionic, i.e., it is of the type M+–H−.
Hydrogen also forms bonds that are mainly covalent, as in carbohydrates and ammonia.
Hydrogen gas is highly flammable and will burn at low concentrations in air. It combusts according to the following exothermic equation:
- 2 H2 + O2 → 2 H2O