Question

Why is Cl- more stable than ClO4- even though the latter has 4 equivalent resonance structures

Answer 1

Number of electrons in Cl^- = 17 + 1 = 18 electrons
Electronic configuration of Cl^- = 2,8,8 = Electronic configuration of Argon
So, Cl has to gain only one electron to get into stable noble gas configuration.
In the case of ClO₄^-, oxidation state of Cl ⇒ x + (4*-2) = -1
⇒ +7
So, Cl has to lose 7 electrons to attain stable noble gas configuration which needs a lot of energy.
Hence, Cl^- more stable than ClO₄^- .