Question

After filling up of 3p orbital,electron enters into 4s orbital but not 3d orbital
Why

Answer 1

here's your answer:-

It's just because on arrangements of periodic table that you would have seen :-

(n+l) rule means according to this electrons are filed in those orbitals having less energy thats why 4s filled first ..

hope you got your answer ☺

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'