Shells (Energy levels) of electrons

The chemistry of a substance to a large measure depends on the arrangement of electrons in the atoms present in that substance.

Electrons are grouped in shells (energy levels). 
The electrons in incomplete or outer shells are available for bond formation. These are known as valence electrons.

The number and type of valence electrons for any atom can be correlated with its position in the Periodic Table.

As valence electrons are used for bond formation, being able to correlate the number of valence electrons to the periodic table position is of great help in the learning of Chemistry.