-on 1
a suffix used in the names of subatomic particles ( gluon; meson; neutron ), quanta ( graviton ), and other minimal entities or components ( cistron; codon; magneton; photon ).
[ prob. extracted from ION; cf. PROTON ]
-on 2
a suffix used in the naming of inert gaseous elements: neon.
[ -on, neut. of -os adj. ending ]