hydroxyl

The terms hydroxyl and hydroxide refer in a general way to OH, i.e., a oxygen atom and a hydrogen atom bound together. The term hydroxyl, according to context, can refer to:

• the hydroxyl group aka hydroxy group - OH as a functional group, which means a specific configuration of atoms that can be a section of a larger molecule.
• hydroxyl radical - neutral OH, which is reactive because its outer shell has an unpaired electron, one being paired with a hydrogen electron.
• hydroxyl ion - ionic OH, typically a negative ion, i.e., anion. It has a negative charge but its stability is enhanced from what it would be, for having no unpaired electrons.

The term hydroxide, according to context, can refer to:

• hydroxide ion - another name for a negative hydroxyl ion.
• any compound that contains a hydroxyl group, such as sodium hydroxide (NaOH) or ammonium hydroxide (NH₄OH).

The hydroxyl radical produces spectral lines in the cm range and shorter, within the radio atmospheric window, including an 18-cm line, and is commonly detected in radio astronomy. Some masers (hydroxyl masers) show it, and it also shows in some molecular clouds and can serve as a tracer, including for shock.