Answer :

o The filling up of electrons in corresponding orbitals decides the electronic stable ground state configuration of an atom and usually is determined by *Aufbau* principle (in the figure).

According to the principal “In the ground state of the atoms, the orbitals are filled according to the order of their increasing energies.” Which actually means that, lower energy orbitals get occupied by electrons before the higher ones; hence according to the effective nuclear charge and other several factors ( n , lvalue etc.) the order of energy goes like : 1s<2s<2p<3s<3p<4s<3d....so on... and for the first 3 options :**(i) 1s ^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{8} 4s^{2} (ii) 1s^{2} 2s^{2} 2p6 3s^{2} 3p^{6} 3d^{9} 4s^{2}**

**(iii) 1s ^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10} 4s^{1}**this order is maintained as we can see the 4s orbital is filled first (maximum capacity is 2 electrons ) and then the 3d (maximum capacity is 10 electrons)

o According to this, the electron should enter 4s orbital first and then to 3d orbital but in case of **(iv) 1s ^{2} 2s^{2} 2p6 3s^{2} 3p^{6} 3d^{5} 4s^{1,}** this is a disparity to the rule hence cannot be considered as ground state electronic configuration of an atom. But it will be stable as 4s and 3d orbitals are almost the same in terms of inherent energy and d

^{5}i.e. a half-filled orbital has generally attained a state of stability.

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