Why cu2+ more stabler than cu+ though cu+ has completely filled d orbital?

cu2+ has a smaller size than Cu+ as it contains 2 electrons less than Cu+.

so, due to its small size the charge of the ion remains concentrated and is not spread widely. due to this accumulation of charge , the polar water moleculescan easily make bonds with the Cu2+ ions and release a higher amount of energy , thus making the hydration enthalpy very high.

The Stability of Cu2+ is more than Cu+ as stability dependson the hydration energy of the ions when they bond to the water molecules. The Cu2+ ion has a greater charge density than Cu+ ion and thus forms much stronger bonds releasing more energy.