Electron Configurations of Atoms

Ground State and Valence Shell Electron Configuration.

In general chemistry, you learned to write the ground state and valence shell electron configurations.  Here is the procedure you used.  In this example, we will determine the ground-state electron configuration for a Carbon atom.

  1. Find the atomic or proton number from the periodic table (carbon = 6).
  2. Using Aufbau's principle you begin filling the orbitals from the bottom up (i.e. lowest energy first).  In the diagram below you actually start at the top with the 1s orbital and then 2s and then 2p then 3s etc. 


  3. Recall that there are 3 degenerate p orbitals and 5 d orbitals.  Remember only two electrons per orbital maximum is allowed and their spins must be opposite (Pauli Exclusion Principle).  Degenerate orbitals are filled half way until all are half filled (Hund's rule of maximum multiplicity).
    So for carbon atom we have 1s22s22px12py1 as the ground state configuration.  You should be able to draw answer basic question related to this.

Aufbaus Principle

You can simply write this as 1s22s22p2.  The valence shell configuration is 2s22p2, which is the electrons in the outer shell.

Take NoteThus carbon has 4 valence electrons and 2 core electrons.  The core electrons are in the 1S orbital.  The valence electrons are the glue that binds atoms together in bonds.  You should be able to do this for any atom in the 2nd row.  It's important early on to determine the number of valence electrons an atom has since this is related to the number of bonds and formal charges on atoms. 

Click on the Periodic Table icon () in the upper right and then click on the carbon or any other atom to see its electron configuration.  Start at H (hydrogen) and work you way up to F (fluorine) observing the change in the electron configuration.