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

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

Dihydrogen

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

• 
  length of the H−H single bond in H₂
• 
  space-filling model of H₂

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.