explain pauli's exclusion principle

The no. of moles of KMnO₄ that will be needed to react completely with one mole of ferrous oxalate in acidic solution is

(a) 3/5  (b)2/5

(c)4/5  (d)None of these

write lewis structure of na2so4

Why do we need to classify elements?

what is the energy level diagram of N₂ and F₂?