Question

explain why electron enters 4s orbital after filling 3p orbital without entering 3d orbital

Answer 1

n+l value of 4s is 4 whereas of
3d is 5
so the electron enters into 4s other than entering into 3d (or) according to the aufbau principle an electron enters into lower energy shell and the order of energies of 4s and 3d is 4s<3d

Answer 2

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As per Aufbau principle, electron enters into the orbital whose (n + l) value is less. The (n + l) values for 4a and 3d are

4s = 4 + 0 = 4

3d = 3 + 2 = 5

Here the (n + l) value of 4s < 3d. Hence electron enters into 4s orbital after filling '3p' instead of '3d'