Question

the electron enters into 4s orbital after filling 3p orbital but not into 3d . explain the reason.

Answer 1

Because according to Aufbau's principle,electrons get filled in the increasing level of energy of the orbitals,after 3p, among 4s(n+l=4) and 3d(n+l=5) energy of 4s is lower due to lower (n+l) value, therefore electrons tend to fill in the 4s orbital first.

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'