Periodic Classification of the Elements

The electrons occupying the outermost shell of an atom are valence electrons, while electrons occupying the inner principal energy levels are core electrons.

A sodium atom with a [Ne]3s¹ electron configuration has three principal energy levels. The complete inner principal energy levels with [He]2s²2p⁶ indicate that there are ten core electrons followed by the third outermost level containing the remaining one electron. Therefore sodium has one valence electron. Similarly, chlorine has ten core electrons and seven valence electrons.

Valence electrons are the farthest from the nucleus and are held most loosely. Hence, they are the easiest to lose or share and play an important role in chemical bonding.

Elements that have the same number of valence electrons exhibit similar chemical properties, as can be seen from the arrangement in the modern periodic table.

For main-group elements, the lettered group number equals the number of valence electrons and the row number equals the highest principal quantum number of that element.

Each element in a group has the same number of electrons available for bonding. Down the group, the principal quantum number increases by one, whereas the numbe