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) [Ne]3s²3p°3d°_ (b) [Xe]6s³ (c) 1s²3s'.

Pauli's exclusion principles states that no two electrons can have the same value for all quantum numbers i.e. no more than two electrons can occupy the same orbital.The energy of an orbital can be explained on the basis of n+l value, where n is the principle quantum number and l is the azimuthal quantum number. In case if n+l value is same for two energy levels then the orbital with lower value of n will have lower energy. Ans a) [Ne]3s23p63d5 For 3p : n+l = 3+1=4 For 3d : n+l = 3+2 = 5 For 4s : n+l = 4+0 = 4 Since, 3d orbital has higher energy than 4s orbital, 4s orbital is filled first. Therefore, the electrons in the given electronic configuration are not filled in order of increasing energy. Ans b) [Xe]6s3 S is a single orbital thus it can occupy only 2 electrons according to the Pauli's exclusion principle, thus the given electronic configuration violate the Pauli's exclusion principle. Ans c) 1s23s1 For 2s : n+l = 2+0 = 2 For 2p : n+l = 2+1 = 3 For 3s : n+l = 3+0 = 3 The orbital energy level will be : 1s <2s<2p<3s, therefore, 2s and 2p orbitals should be filled first as compared to 3s and hence the electrons are not filled in order of increasing energy.