The order in which electrons fill orbitals about a nucleus is very systematic. It is not however in direct order from s orbital to f orbital at a level. In many cases orbitals at a higher energy level fill before all orbitals at a lower energy level have been completely filled.
The graphic below illustrates the order in which orbitals fill about the nucleus. Each orbital is described with a number for the energy level (each blue column represents an energy level) and a letter for the shape of the orbital. The number beside the arrow indicates the order that the orbitals fill in. Notice how the orbitals fill on the diagonal going from left to right.The example below illustrates how to use the table.
Calcium ( 12 protons and electrons)
  • First two electrons in the 1s orbital (s orbitals hold a maximum of 2 electrons )
  • Next two electrons in the 2s orbital
  • Next six electrons in the 2p orbital ( p orbitals hold a maximum of 6 electrons )
  • Last two electrons in the 3s orbital
    Diagram #4
 
n =1
  
n = 2
n = 3
n = 4
n = 5
n = 6
n = 7
1
1s
2
2s
4
3s
6
4s
9
5s
12
6s
16
7s
 
3
2p
5
3p
8
4p
11
5p
15
6p
19
7p
 
7
3d
10
4d
14
5d
18
6d
 
13
4f
17
5f

Assignment #3
Filling of Orbitals