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Why Cu+2 is more stable than Cu+1 in spite of the inert gas configuration [Ar]3d104s0 for Cu+1
Copper has an electronic configuration of [Ar] 3d104s1 which means that 29 electrons are distributed as:
1)18 with an inert gas configuration of argon
2)10 electrons are present in the five 3d orbitals; each with 2 electrons
3)1 electron in 4s orbital
Variable oxidation state is a very important phenomenon of transition elements. Copper is also a transition element. It belongs to the 3d transition series. Copper exhibits an oxidation state of +2 which is more common than +1. To achieve +1 oxidation state copper loses lone, unpaired or single electron present in 4s orbital leading to a stable inert gas configuration [Ar] 3d104s0. Half-filled, completely-filled or un-filled configurations are the most stable configurations according to the rules of chemical bonding.
So what is reason for this anomaly?
We can answer this question by looking at the ionization potential values. Ionization potential is the energy required to remove an electron from a configuration.
1st ionization potential i.e energy required to remove first electron from copper configuration is equal to 745 Kj/mol.
2nd ionization potential i.e energy required to remove second electron which is present in 3d orbital is equal to 1958 Kj/mol
If we total these two energies we see that quite a good amount of energy is required to achieve an oxidation state of +2. But there are two more energies which play part in chemical bonding. One of them is hydration energy and the other lattice energy. Hydration energy is related to formation of chemical bonds with water and lattice energy is the one involved in arrangement in solid state. To make it extremely simple I don’t want to go into the values of these two energies but they OFFSET the high energy requirement of copper +2. In aqueous (water) solutions or in solid state, energy gained through formation of bonds with water molecules and in solid state energy gained through lattice formation offsets the energy required to remove an electron from 3d orbital making Cu+2 more stable than Cu+1. CuSO4.5H2O-copper sulfate pentahydrate is a common example of Cu+2 and CuCl-cuprous chloride is a Cu+1 compound.
THIS IS THE SIMPLEST ANSWER THAT CAN BE GIVEN FOR THE QUESTION: