All alkali metal peroxides contain the `[-O-O]^(2-)` ion. They are

All alkali metal superoxides contain the ion [O_(2)] . They are

Alkali metals contain

Lithium forms monoxide only when heated in oxygen. Sodium forms monoxide and peroxide in excess of oxygen. Other alkali metals form superoxide with oxygen i.e MO_(2) . The abnormal behaviour of lithium is due to small size. The larger size of nearer alkali metals also decides the role in formation of superoxides. The three ions are related to each other as follows O^(2-) overset(1/2O_(2))rarrunderset(O_(2^(2-)))overset(O_(2))rarrunderset("superoxide ion")2O_(2) Lithium does not form stable peroxide because

Assertion : Superoxides of alkali metals are paramagnetic. Reason : Superoxides contain the ion O_(2)^(-) which has one unpaired electron.

Assertion : Superoxides of alkali metals are paramagnetic. Reason : Superoxides contain the ion O_2^- which has one unpaired electron.

Assertion : Super-oxides of alkali metals are para-magnetic. Reason : Super-oxides contain the ion O_(2)^(-) which has one unpaired electron.

Assertion: Superoxide ion is paramagnetic whereas peroxide ion is diamagnetic. Reason: Superoxide ion [O=O]^(-) has one unpaired electron whereas peroxide ion [O=O]^(-) has no unpaired electron.