Question

according to which rule electron enters into 4S orbital of 3D

Answer 1

The current method of teaching students to work out electronic structures is fine as long as you realise that that is all it is - a way of working out the overall electronic structures, but not the order of filling.

You can say that for potassium and calcium, the 3d orbitals have a higher energy than the 4s, and so for these elements, the 4s levels fill before than the 3d. That, of course, is entirely true!

Then you can say that, looking at the structures of the next 10 elements of the transition series, the 3d orbitals gradually fill with electrons (with some complications like chromium and copper). That is also true.

What is not right is to imply that the 3d levels across these 10 elements have higher energies than the 4s. That is definitely not true, and causes the sort of problems we have been discussing.

Answer 2

Every atom has many subatomic particles.Electron is one of them,which determines the whole atom's electrical charge and other stuffs.

Many theories are there,which deals with the movement of the electrons in various orbital of an atom.

Similarly,the condition mentioned in the question,is controlled by the Hund's rule of the electron orbitals.