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.

Find the atomic or proton number from the periodic table (carbon = 6).
Using Aufbau's principle, you begin filling the orbitals from the bottom up (i.e. the lowest energy first). In the diagram below you start at the top with the 1s orbital and then 2s and then 2p then 3s etc.
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 halfway until all are half-filled (Hund's rule of maximum multiplicity).
So for a carbon atom, we have 1s²2s²2p_x¹2p_y¹ as the ground state configuration. You should be able to answer basic questions related to this.

You can simply write this as 1s²2s²2p². The valence shell configuration is 2s²2p^2, which is the electrons in the outer shell.

Take Note

Carbon has 4 valence electrons and 2 core electrons.
The core electrons are in the 1s orbital.
The valence electrons are in the 2s orbitals (i.e. the outermost 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 to know the number of valence electrons an atom has, as this determines the number of bonds it can form and influences the formal charges on the atom.

Action

Click on the Periodic Table icon () in the upper right, and then click on carbon or any other atom to see its electron configuration.
Start at H (hydrogen) and work your way up to F (fluorine), observing how the electron configuration changes.
Pay attention to how the number of valence electrons increases as you move across the period and consider how this might affect the reactivity and bonding behavior of each element.

Practice Time

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