Enables the facilitated diffusion of a monoatomic ion (by an energy-independent process) by passage through a transmembrane aqueous pore or channel without evidence for a carrier-mediated mechanism. May be either selective (it enables passage of a specific ion only) or non-selective (it enables passage of two or more ions of same charge but different size). [ GOC:mtg_transport http://openisbn.com/search.php?q=0815340729&isbn=1/ GOC:pr GOC:cy ]