# A-level Chemistry/OCR (Salters)/Hydrogen

Hydrogen is the first element in the periodic table. The hydrogen nucleus contains one proton.

## Isotopes

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.

## Electron configuration

The electron configuration of the neutral hydrogen atom, H, is

${\displaystyle 1{\mbox{s}}^{1}\,\!}$

## Dihydrogen

In its standard elemental state, hydrogen exists as diatomic H2 molecules, sometimes referred to as dihydrogen.

## Compounds

Hydrogen forms compounds with almost every other element in the periodic table.

### Hydrides

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.

### Organic compounds

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.