Why can the electronic configuration of some transition element not be written like this;Mg=(Ne)3s2
Looks OK to me. There are two 3s electrons on top of a filled Ne electronic configuration. That is what Mg is.
The electronic configuration of transition elements cannot be written in the same way as the noble gases, like Neon (Ne), because the transition elements have partially filled d-subshells in addition to the completely filled s-subshells.
To understand why this is the case, let's take the example of magnesium (Mg), which is not a transition element but an alkaline earth metal. The electron configuration of magnesium is 1s2 2s2 2p6 3s2.
However, let's consider the transition element chromium (Cr), which has the atomic number 24. According to the order of filling of electron shells, the configuration should be 1s2 2s2 2p6 3s2 3p6 4s2 3d4. However, instead of being 3d4 4s2, it is written as 3d5 4s1.
This unusual configuration can be explained by the principle called the "Aufbau principle," which states that the electrons occupy the lowest energy orbitals available first. It is energetically favorable to have a half-filled or fully-filled d-subshell.
For chromium, in order to achieve a half-filled 3d subshell, one electron from 4s orbital moves to the 3d orbital. Thus, the electron configuration becomes 1s2 2s2 2p6 3s2 3p6 3d5 4s1. This arrangement provides additional stability to the atom.
So, in conclusion, the electronic configuration of transition elements cannot be written in the same way as noble gases because of the partially filled d-subshells, and the arrangement of electrons in these subshells is dictated by the Aufbau principle to attain greater stability.