Hydrogen is the first element in the periodic table. The hydrogen nucleus contains one proton.
Most hydrogen nuclei have no neutrons, and are called hydrogen-1 or protium, 1H. The two other isotopes are extremely rare: 2H (hydrogen-2, also known as deuterium, symbol D) accounts for 1 in every 6500 hydrogen nuclei and 3H (hydrogen-3, also known as tritium, symbol T) exists only in trace amounts.
The electron configuration of the neutral hydrogen atom, H, is
In its standard elemental state, hydrogen exists as diatomic H2 molecules, sometimes referred to as dihydrogen.
Hydrogen forms compounds with almost every other element in the periodic table.
In compounds with very electropositive elements, as in sodium hydride, NaH, hydrogen exists as the hydride ion, H−. H− does not exist in solution because it is exceptionally reactive. For example, if you try to dissolve NaH in water, the following reaction occurs:
- H− + H2O(l) → H2(g) + OH−(aq)
In compounds with elements whose electronegativity is similar to or greater than hydrogen, such as hydrogen fluoride, HF, hydrogen is covalently bonded to the other element.
Hydrogen is present in the vast majority of organic compounds. In fact, some sources define organic compounds as those with C-H bonds, but this definition is controversial because there are some organic compounds, such as oxalic acid, which are behave exactly like other organic compounds, yet lack C-H bonds.