Correlating shell (energy level) occupancy with periodic table row

In the periodic table elements are arranged in order of increasing atomic number across the rows.
Each row is referred to as a period.
Each successive element has one more proton in its nucleus.
Because atoms have no overall charge, atoms of successive elements also have one more electron.

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

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.

Electrons occupy the lowest energy shells available.  

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.