• # question_answer Which of the following electronic configuration is not possible    [MHCET 2003] A)                 $1{{s}^{2}}2{{s}^{2}}$  B)                 $1{{s}^{2}},2{{s}^{2}}2{{p}^{6}}$ C)                 $[Ar]\,3{{d}^{10}},4{{s}^{2}}4{{p}^{2}}$              D)                 $1{{s}^{2}},2{{s}^{2}}2{{p}^{2}},3{{s}^{1}}$

We know from the Aufbau principle, that 2p orbital will be filled before 3s orbital. Therefore, the electronic configuration $1{{s}^{2}},\,2{{s}^{2}},\,2{{p}^{2}},\,3{{s}^{1}}$ is not possible.