Electron Configurations of Atoms

As we've learned, each electron in an atom occupies a unique state defined by its set of four quantum numbers: the principal quantum number (n), azimuthal quantum number (l), magnetic quantum number (ml), and spin quantum number (ms). Understanding how these numbers dictate the available orbitals is crucial for determining an atom's electron configuration.

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). This tells us the number of electrons to place for a neutral atom.
Using Aufbau's principle, you begin filling the orbitals from the bottom up (i.e., the lowest energy first). The principal quantum number (n) primarily determines the energy level. In the diagram below, you start at the top with the 1s orbital (where n=1,l=0), then 2s (where n=2,l=0), and then 2p (where n=2,l=1), then 3s, etc.
Recall that there are 3 degenerate p orbitals (corresponding to m_l=−1,0,+1 when l=1) and 5 degenerate d orbitals (corresponding to ml=−2,−1,0,+1,+2 when l=2).
Remember, only two electrons per orbital maximum is allowed, and their spins must be opposite (Pauli Exclusion Principle, directly related to the spin quantum number, m_s).
Degenerate orbitals are filled halfway until all are half-filled (Hund's rule of maximum multiplicity), before any orbital gets a second electron. This means electrons prefer to occupy separate orbitals within a subshell with parallel spins before pairing up.

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 2s²2p², which represents the electrons in the outermost 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 here to open the Periodic Table, 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

Advanced features of OpenOChem Learn require you to login. Please login or create an account.