▁am mon ium , ▁N H , ▁the ▁pro ton ated ▁derivative ▁of ▁am mon ia , ▁N H 3 . ▁ ▁The ▁name ▁on ium ▁is ▁also ▁used ▁for ▁c ations ▁that ▁would ▁result ▁from ▁the ▁substitution ▁of ▁hydro gen ▁atoms ▁in ▁those ▁ ions ▁by ▁other ▁groups , ▁such ▁as ▁organ ic ▁radical s , ▁or ▁hal og ens ; ▁such ▁as ▁t et raph en yl ph os phon ium , ▁( C 6 H 5 ) 4 P + . ▁The ▁subst itu ent ▁groups ▁may ▁be ▁div al ent ▁or ▁tri val ent , ▁yield ing ▁ ions ▁such ▁as ▁im in ium ▁and ▁nit ril ium . ▁ ▁A ▁simple ▁on ium ▁ion ▁has ▁a ▁charge ▁of ▁+ 1 . ▁A ▁larger ▁ion ▁that ▁has ▁two ▁on ium ▁ion ▁sub groups ▁is ▁called ▁a ▁double ▁on ium ▁ion , ▁and ▁has ▁a ▁charge ▁of ▁+ 2 . ▁A ▁triple ▁on ium ▁ion ▁has ▁a ▁charge ▁of ▁+ 3 , ▁and ▁so ▁on . ▁ ▁Comp ounds ▁of ▁an ▁on ium ▁c ation ▁and ▁some ▁other ▁an ion ▁are ▁known ▁as ▁on ium ▁comp ounds ▁or ▁on ium ▁sal ts . ▁ ▁O ni um ▁ ions ▁and ▁on ium ▁comp ounds ▁are ▁in vers ely ▁analog ous ▁to ▁- ate ▁ ions ▁and ▁a te ▁complex es : ▁Lewis ▁bases ▁form ▁on ium ▁ ions ▁when ▁the ▁central ▁atom ▁g ains ▁one ▁more ▁bond ▁and ▁becomes ▁a ▁positive ▁c ation . ▁Lewis ▁ac ids ▁form ▁- ate