Among ferrous and ferric ions, which one is more stable? Why? 

Explanation

Electronic configuration of `Fe(26): [Ar]4s^(2) 3d^(6)`
Ferrous ion `Fe^(2+) : [Ar] 4s^(0) 3d^(5)`
Ferric ion `Fe^(3+) : [Ar] 4s^(0) 3d^(5)`
Ferric ion is more stable than ferrous ion. This is because ferric ion has half filled d-subshell stable electronic configuration.