Oxidation state of Cu

If copper has 1 valence electron in the 4s subshell, how is it able to form Cu2+ with oxidation state of 2? I know the energy level difference between the 3d and 4s subshells has a part to play here, but I can't seem to put my finger on it. 




lets just put it like this.....you wouldn't be too excited to be without your parents.....and wouldn't be really stabilised in their absence.....but put forward a condition..like you get 6-7 of your best friends to stay with you all the while they're away.....now that is good enough compensation for your parents being away for a period of time....similarly....water....surrounds the $\ce{Cu^2+}$ ion...and stabilises it through hydration...as $\ce{Cu+}$ has a much higher hydration enthalpy that $\ce{Cu+}$....it gets stabilised..and is ready to lose that last electron..though left configurationally unstable


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