The following do not represent valid ground-state electron configurations for an atom either because they violate the Pauli exclusion principle or because orbitals are not filled in order of increasing energy. Indicate which of these two principles is violated in each example. 

(a) 1s²2s²3s¹, 

(b) [Xe]6s²5d⁴ 

(c) [Ne]3s²3d⁵.