Abstract: Particulate metal oxide compositions having reactive atoms stabilized on particulate surfaces and methods for reacting the compositions with saturated and unsaturated species are provided. The preferred particulate metal oxides of the compositions are nanocrystalline MgO and CaO with an average crystallite size of up to about 20 nm. The preferred reactive atoms of the compositions are atoms of the halogens and Group IA metals. In one embodiment, chlorine atoms are stabilized on the surface of nanocrystalline MgO thus forming a composition which is capable of halogenating compounds, both saturated and unsaturated, in the absence of UV light and elevated reaction temperatures.