Term coined by Max Born for treating electron as standing wave that takes on following configurations:
        

     Electrons are organized about the atom by energy levels or shells (n = 1, 2, 3, 4, ...).

     When l=0 (zero nodal planes), 1 region of probability and 1 orbital exist:

 
 

     When l=1 (1 nodal plane), 2 regions of probability and 3 orbitals exist:
:
 

     When l=2 (2 nodal planes), 4 regions of probability and 5 orbitals exist:
       
 

     When l=3 (3 nodal planes), 8 regions of probability and 7 orbitals exist:
     
 
 

     The first 5 shells are organized as: