When going through list of oxidation states on Wikipedia I encounterd there that sodium, potassium, rubidium and caesium exhibit oxidation states of -1, but not lithium, even though its electronegativity is more than that of the others. How's that possible? Can some one provide an example.
Answer
You are talking about alkalides, salts where the anion is an alkali metal. There is a very brief overview on Wikipedia which also provides a couple of examples, including $\ce{[Na(\text{cryptand[2.2.2]})]+Na-}$. A good university level inorganic text book such as Greenwood and Earnshaw or Housecroft and Sharpe will provide more detail.
If you are interested in unusual oxidation states of alkali metals you might also like to know that in electrochemical experiments there is some evidence for $\ce{Cs^3+}$, which is isoelectronic with $\ce{Xe^2+}$. Again I think Greenwood and Earnshaw discusses this, but I don't have it to hand at the moment to confirm.
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