Electron Configurations 
When creating electron configuration diagrams it is best to remember the following:
- Shells expand outwards with the K shell, n=1, closest to the nucleus.
- 's' subshells hold only 2 electrons in 1 orbital
- 'p' subshells hold 6 electrons in 3 orbitals
- 'd' subshells hold 10 electrons in 5 orbitals
- 'f' subshells hold 14 electrons in 7 orbitals
- each orbital can hold 0, 1 or 2 electrons but never more than 2.  (Pauli's Exclusion Principle)
- each electron is to be placed into the lowest possible energy orbital first. As lower levels fill then higher levels can be filled. (Aufbrau (Boiling) Principle)
- in the 'p' orbitals of which there are three of equal energy, each orbital gets one electron first.  Only when all three are full can you go back and fill in the orbitals with the second electron. 

(Hund's law - When electrons are placed in a set of orbitals of equal energy, they are spread out as much as possible to give as many unpaired electrons as possible.)    Hund's law also applies to the 'd' orbitals of which there are five and the 'f' orbitals of which there are seven the same rules apply.


Using the above rules we can predict which orbitals in an atom will have electrons and the number of electrons found in each orbital. This arrangement is called the atom's electronic structure or electron configuration.  Knowing how to predict an atom's electronic configuration is important because it is the arrangement of electrons that controls an atom's chemical properties. Specific examples follow:

Hydrogen (Z = 1)   A neutral atom has 1 electron. In its ground state the electron will occupy the lowest energy level and the lowest orbital in that energy level.  We use two methods to illustrate this. One is an orbital diagram the other is a form of chemical shorthand.
                            1s
              H          O        or     1s1

Let's take a look at helium. (Z=2)
                             1s
              He         O          or      1s2

Now let's take a look at Li (Z = 3)
                             1s        2s
              Li           O    O          or       1s2 2s1

Now take a look at boron (Z = 5)
                             1s        2s         2p
              B           O    O  OOO        or      1s2 2s2 2p1

One final example before you get turned loose to do some yourself:   Ne (Z = 8)
                             1s        2s          2p
              Ne         O    O   OOO        or      1s2 2s2 2p6

Both the orbital diagram and shorthand have uses. The orbital diagram is best for showing how bonding takes place. While the shorthand method is used on the periodic table to show the electronic configuration of the outermost orbitals.

One last thing. The orbitals would seem to be filled in the following order:

                     1s  2s  2p  3s  3p  3d   4s   4p   4d   4f    5s etc.

BUT THIS IS NOT SO.


The electron order of filling is in fact:  1s  2s  2p  3s  3p  4s  3d   4p  5a   4d   5p and so on.
The order can be predicted using a periodic table as a reference or this simple chart:
 

                                7s   7p   7d   7f

                                6s   6p   6d   6f

                                5s   5p   5d   5f

                                4s   4p   4d   4f

                                3s   3p   3d

                                2s   2p

                                1s 
 

It can also be determined if you know how to read the periodic table.
For a visual diagram click here.
               Go to the Electron Configuration Worksheet