Electrons do not exist as atoms except for the rare gas elements. Rather they form molecules or multi atomic compounds which may be molecules, crystals or glassy compounds. As atoms approach one another, the interaction between these are concerned with those electrons in the outermost orbitals. An electron in the outermost orbital is called a valence electron.
Since the valence electron possesses the highest energy, this electron is more easily removed or modified from the atomic state than inner orbital electrons. Valence electrons that have been temporarily separated from an atom are called free electrons. Only the valence electrons take part in the chemical reaction and the chemical activity of an atom is dependent only on these valence electrons. The inner electrons do not change their characteristics by bonding.
Only the valence electrons in the outermost orbit can become free electrons. Recall that the nucleus and the attraction of the protons keep electrons in the inner shells held tightly. A valence electron can be freed from an atom when energy is added to the atom. This energy permits the loosely held valence electron to escape the force of attraction between the electron and the nucleus. A free electron possesses more energy than it did as a valence electron.
The number of valence electrons determines the valency of atom. The valency of hydrogen is one. Similarly the valance of lithium, sodium and potassium is also one as all of these have only one electron in their outermost shell. Elements having the same valence electrons exhibit similar chemical properties. For example isotopes having the same number of valence electrons will have the same electronic distribution. Therefore, isotopes show similarity in their chemical properties.