a) `1s^(2) 2s^(2) 2p^(3) implies `

(b) `1s^(2) 2s^(2) 2p^(6) 3s^(2) 3p^(6) 3d^(2) implies Z = 20`
c) Electronic configuration of He is `1 s^(2) implies `

Which one contradicts the Hund's rule ?

Explanation

1) 'c' contradicts Pauli's principle.
2) According to Pauli's principle. no two electrons of the same atom can have all four quantum numbers the same.
3) But here , in both electron have same values of four quantum numbers that is n = 1 , l = 0 , `m_(l) = 0 , m_(s) = + (1)/(2)`
4) lt contradicts Pauli's principle.