what is differentiating electron

The term differentiating is used for something which can distinguish between two entities, measurements, quantities, properties etc. Differentiating electron is an electron which would distinguish one atom from the other. In the periodic table, the atomic number increases by a factor of one as we move across the period. So, the differentiating electron would be the last electron to be filled in an atom and therefore would be present in the valence shell of the atom. It distinguishes a particular element present in a given period of the periodic table from the element present just before it (and has atomic number less than it by a factor of one).