The electron by which the electron configuration of the given element differs from that of its preceding element is called differentiating electron is the last entering electron of its atom .
Periods in the long form of the periodic table : In each period, the differentiating electron enters the s-orbital in the element p-orbital in the last element . When s and p orbitals are completely filled with electrons as ns2np6 , the element is the noble gas with stable configuration of octet .