Question

why do the electron entersinto 4s orbital but not 3d orbital after filling up 3p orbital?

Answer 1

(n+l) rule....the shell with Lower value of (n+l ) is filled prior to the one with greater value

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'