Question

the electron enters into 4S orbital after filling 3p orbital but not in 3D explain reason

Answer 1

THE ORDER OF FILLING 3d AND 4s ORBITALS

This page looks at some of the problems with the usual way of explaining the electronic structures of the d-block elements based on the order of filling of the d and s orbitals. I am grateful to Dr Eric Scerri from UCLA, who pointed these problems out to me and provided me with some useful academic papers I wouldn't otherwise have been able to get hold of.

The way that the order of filling of orbitals is normally taught gives you an easy way of working out the electronic structures of elements. However, it does throw up problems when you come to explain various properties of the transition elements.

This page takes a closer look at this, and offers a more accurate explanation which avoids the problems.

Answer 2

$\large\tt\red{Answer\::)}$

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'