follow exactly the same pattern as the elements from lithium to neon with the difference that the s and p orbitals are getting filled now. This process can be simplified if we represent the total number of electrons in the first two shells by the name of element neon (Ne). The electronic configuration of the elements from sodium to argon can be written as (Na, [Ne] s ) to (Ar, [Ne] s p ). The electrons in the completely filled shells are known as core electrons and the electrons that are added to the electronic shell with the highest principal quantum number are called valence electrons.
For example, the electrons in Ne are the core electrons and the electrons from Na to Ar are the valence electrons. In potassium (K) and calcium (Ca), the s orbital, being lower in energy than the d orbitals, is occupied by one and two electrons respectively. A new pattern is followed beginning with scandium (Sc). The d orbital, being lower in energy than the p orbital, is filled first.
Consequently, in the next ten elements, scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) and zinc (Zn), the five d orbitals are progressively occupied. We may be puzzled by the fact that chromium and copper have five and ten electrons in d orbitals rather than four and nine as their position would have indicated with two-electrons in the 4s orbital. The reason is that fully filled orbitals and half- filled orbitals have extra stability (that is, lower energy). Thus p , p , d , d , f , f etc.
configurations, which are either half-filled or fully filled, are more stable. Chromium and copper therefore adopt the d and d configuration (Section . . )[ caution: exceptions do exist ] With the saturation of the d orbitals, the filling of the p orbital starts at gallium (Ga) and is complete at krypton