oxides of group 1 elements are Li2O , Na2O2 , KO2 , RbO2 Oxygen’s oxidation state in each compound is -2 , -1 , -1/2 ,-1/2 respectively I.e oxide-> peroxide-> superoxide

my question The oxidation state of the oxygen Is increasing down the group why is that ?

My attempt

the tendency to donate electron increases down the group , by this statement the oxidation state of oxygen in the oxides down the group should decrease as oxygen gets easily reduced.

What am I missing here?

  • 2
    $\begingroup$ What about the size of the cations? $\endgroup$ – Karsten Theis Apr 3 '19 at 12:58

Firstly, the oxides you have listed are not the only ones that exist. It's that on reacting with excess oxygen, Li forms monoxide, Na forms peroxide, and the elements further down form superoxide. These are not necessarily the strongest oxide form too, infact, according to this link:- K2O2; the most stable oxide of K is peroxide despite being able to form a superoxide. (Also, Li, Na can also make superoxides (though not very stable) and K, Rb can also make monoxides.)

So I think the question changes to why are the elements down the group more able to form oxygen compounds with decreasing oxidation state of oxygen, and that is due to the size of the metal atoms; the larger the metal atom, the higher number of oxygen atom containing species it can (stably) accommodate around it.


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