[moderate]

Hello,

I have read in several textbooks that alkali metals react with O₂ to form oxides, peroxides and superoxides.

Lithium reacts to form just the oxide (Li₂O), sodium forms predominantly the peroxide (Na₂O₂), and potassium (and those lower in the group) form primarily superoxides (MO₂).

I've been trying to reason this out for myself in terms of the reaction mechanism and the trend in ionization energy of the alkali metals, but haven't been able to come up with anything satisfactory.

I realize that the more reactive alkali metals end up "surrounded" by a greater number of oxygens, but I can't really say that they are oxidized more, since each always forms a +1 ion.

Also, I noticed that the oxygens have progressively lower negative charge per oxygen atom (going down the group). This seems strange, this I assumed that as the ionization energy falls, more electrons should be transferred.

All in all, I am quite lost on this.