Energy levels and rows

Experimental evidence has shown that the electrons in an atom may have only certain energies.  The allowed energies are referred to as energy levels (or shells).  

The electrons on an atom occupy the lowest energy shells available.  The energy of the electrons in a shell depends on the distance from the nucleus.

Shell 1 < Shell 2 < Shell 3...

The maximum number of electrons in a shell also depends on the distance from the nucleus.

 

The shells occupied by electrons on a particular atom is related to the period in which that atom appears in the periodic table.

• first period elements (2 electrons maximum) have electrons only in shell 1.
• second period elements (10 electrons maximum) have electrons in shells 1 and 2.
• third period elements (18 electrons maximum) have electrons in shells 1, 2 and 3.
• fourth period elements (36 electrons maximum) have electrons in shells 1, 2, 3 and 4.