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Hydroxyl (OH) is a compound with molecules each consisting of one oxygen and one hydrogen atom. The term is used when the molecule is neutral, in which case it is a radical, i.e., a compound with an unpaired electron, and also termed a hydroxyl radical, and is extremely reactive and short-lived. Negative ions of this molecule are common, one being hydroxide.
Hydroxyl has spectral lines in the cm range and shorter, including an 18 cm line, thus is commonly detected in radio astronomy. Some masers show it, and it is also visible in some molecular clouds and can serve as a tracer.