Question

Between cuprous and cupric which is more stable and why

Answer 1

Cupric which is copper(II) is more stable than Cuprous, copper(I).
The standard reduction potentials of copper at 25C:

Cu2+ + 2e- -> Cu, E = +0.34V
Cu+ + e- -> Cu, E = +0.15V

This means that, subtracting one equation from the other,

Cu2 + e- -> Cu+, E = +0.19V.

It takes only 0.19V of energy to oxidize Cu+ to Cu2+, so in the earth's atmosphere that is 1/5 oxygen, there is plenty of oxidizing power around to scavenge Cu+ ions. Compare this to the theoretical minimum potential difference needed to split water to make oxygen and hydrogen, ~1.2V

On the other hand, you can't oxidize copper any further than Cu2+ (Cupric). You can't strip any more electrons away, so it is stable in the earth's atmosphere.