Old English on, unstressed variant of an "in, on, into," from Proto-Germanic *ana "on" (cf. Dutch aan, German an, Gothic ana "on, upon"), from PIE root *an- "on" (cf. Avestan ana "on," Greek ana "on, upon," Latin an-, Old Church Slavonic na, Lithuanian nuo "down from"). Also used in Old English in many places where we would now use in. From 16c.-18c. (and still in northern England dialect) often reduced to o'. Phrase on to "aware" is from 1877. On time is from 1890.
A subatomic particle: neutron.
A unit: photon.
A basic hereditary unit: codon.
Inert gas: radon.
A chemical compound that is not a ketone or a compound that contains oxygen in a carbonyl group: parathion.