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.