Question

Explain why third period has eight and not eighteen elements

Answer 1

The Period Three of the periodic table consists of the elements whose atoms has three shells occupied by electrons in its Ground State (when it is not excited or in a reaction)

According to the Aufbau's Principle (the building up principle) there can be only EIGHT elements that satisfy this. This is because the S orbital of the fourth shell sits at a lower energy level (closer to the nucleus) than the d orbital of the third shell. Therefore when the s and p orbitals of the third shell has been filled the next element (K) has its outer most electron in the S orbital of the fourth shell. Hence it is in period 4.

According to the Aufbau's principle as you list the elements in increasing order of atomic number a single electron gets added to their electronic configuration one by one to the orbital with the lowest energy level possible.