Question

Why is Cr2+ unstable and easily oxidized to Cr3+?

Answer 1

Hello mate here is your answer ✌️

it's because the third ionization energy of Cr is so small that even oxygen in the air can oxidize it

Stability is all about electron configuration

Cr electron configuration is

1s2 2s2 2p6 3s2 3p6 4s1 3d5

Cr2+

1s2 2s2 2p6 3s2 3p6 3d4

Cr3+

1s2 2s2 2p6 3s2 3p6 3d3

now the reason 3d3 is more stable than 3d4

is that the d-orbital splits into t2g (low energy) and eg (high energy)

t2g has 3 orbitals while eg has 2 orbitals

so when you fill 3 electrons (3d3)

you fill t2g with 3 electrons ( each orbital filled singly )

but when you fill 4 electrons (3d4)

you fill t2g with 4 electrons ( 2 single , and 2 paired )

this paired electron is confused .. and keeps jumping from orbital to another making pairs with them

as you know that half filled orbitals are more stable ...

this paired electron can easily be taken off to reach a more stable form ( 3 single electron ) i.e Cr3+

so even the slightest oxidizing agent , or oxygen in air can do that

Hope this helps