為什麼Cu2+ 較 Cu+ 穩定?

Electron in box can only partially explanation the stability of several ions. It is not the full story to tell you whether it is stable or not even the 3d orbital of Cu+ is fully filled.
To understand this question, we can't miss the explanation of enthalpy of formation of Cu2+ and Cu+ in aqueous state. The calculations are as follows.
The enthalpy formation of Cu+ is
ΔHfCu+(aq) =ΔHatmCu(s) +ΔHI.E1Cu +(g) +ΔHhydCu +(aq)
ΔHfCu+(aq) = 339+744+(-481)
ΔHfCu+(aq) = +602 Kjmol-1
The enthalpy formation of Cu2+ is
ΔHfCu2+(aq) =ΔHatmCu(s) +ΔHI.E.1Cu +(g) +ΔHI.E2.Cu 2+ (g) +ΔHhydCu 2+(aq)
ΔHfCu2+(aq) = 339+744+1956(-2244)
ΔHfCu2+(aq) = +795 Kjmol-1

From the above calculation, we know that the ΔHfCu2+(aq) is much greater than that of ΔHfCu+(aq)due to more ionization energy required to remove the second electron from Cu+ ion. This energy is much more greater and can't be called off evenΔHhydCu 2+(aq) is very negative (-2244Kjmol-1) comparing that of ΔHhydCu +(aq) is just slightly negative (- 481)

By drawing the haber cycle, we have:


2Cu+ (aq) →→→ΔH2→→→→.Cu 2+ (aq) + Cu (s)
↑********************************↗
↑ΔH1*****************************↗ΔH3
↑*************************↗
2Cu (s) ****************↗

ΔH3=2(ΔH1)+ΔH2
ΔH2=ΔH3 - 2 (ΔH1)
ΔH2= +795 – 2(+602)
ΔH2= - 409 Kjmol-1

Comparing the result obtained from the haber cycle, we know that the conversion of Cu(s) to Cu+(aq) is energy unfavourable due to

2(ΔH1) = 2(+602) = +1204Kjmol-1 (an endothermic reaction). However, the formation of Cu+ from Cu2+ is more energy favourable due to ΔH2 = -409Kjmol-1 (an exothermic reaction).As a result, from energy point of view, the formation of Cu2+(aq)much more favourable than Cu+(aq). Hence Cu2+(aq) is much more stable than Cu+(aq).

I hope your will find my answer helpful.

因為還有其他原因影響穩定性。
Cu(I) 會進行 self-proportionation。
Cu+ + Cu+ -----------> Cu2+ + Cu

因為Cu 2+容易跟其它離子反應,因為易得電子,變得很穩定,但Cu+即可能失電子,又可以得電子,所以Cu+并不穩定