Question

explain the principle in your own words which supports the statement that an electron enters into 4s orbital after filling up of 3p orbital

Answer 1

We say that the 4s orbitals have a lower energy than the 3d, and so the 4s orbitals are filled first. We know that the 4s electrons are lost first during ionization. The electrons lost first will come from the highest energy level, furthest from the influence of the nucleus.