In this paper, first we discuss an old problem in teaching electron configuration of transition metals and the order in which the orbitals are filled. Then we propose two simple computational experiments, in order to show that in the case of first row transition metals and the main group elements after them, the electrons occupy the 3d subshell before the 4s. It is shown that if we begin with the bare nucleus of above elements in the vacuum and then continue with adding the electrons the 19th electron firstly occupies the 3d subshell and not the 4s. Indeed, the 4s subshell in the third row of periodic table only fills first in the case of K and Ca atoms. However, the 3d subshell in transition metals and the main group elements after them is more stable than 4s and so fills first. Thus there is no scientific reason to write the electron configuration of transition elements as [Ar] 4s 3d and the correct form is [Ar] 3d 4s.
