The Electron Configuration

An element’s electron configuration represents the location of its electrons within various shells and orbitals. From this distribution of electrons we can relate the chemical and physical properties of different elements. 

Setting up an electron configuration

Before you begin the electron configuration of an element it is important to remember the basic principles of quantum mechanics

Basics of Quantum Theory 

The main atomic sub-levels or subshells are s, p, d and f. Each atomic sub-level has a set number of atomic or electron orbitals with each being able to hold up to 2 electrons. 

Sublevels-Subshells-Orbitals-Arrows-ElectronsQuantum Mechanical Model of the Atom

Electron Configurations

Using this redesigned Periodic Table is an easy way to determine the electron configurations of all elements.  

Periodic-Table-Electron-ConfigurationPeriodic Table (Electron Configuration)

For example, what is the ground state electron configuration of Aluminum? 

STEP 1: Locate the element on the periodic table. 

Periodic-Table-Electron-ConfigurationPeriodic Table (Aluminum)

STEP 2: Starting at the 1s orbital write the orbitals and the number of electrons involved with aluminum. This follows the aufbau principle, which in german means, “to build up”. 

Periodic-Table-AluminumPeriodic Table (Aluminum)

Looking at the periodic table we find that aluminum is in Group 3A and with an atomic number of 13 has 13 electrons total. Now we simply count to aluminum while filling in our various orbitals. 

Al (13 Electrons)         1s22s22p63s23p1

If you wished to include the orbital diagram for the electron configuration for aluminum you would write the following: 

Electron-Orbital-Diagram-AluminumElectron Orbital Diagram (Aluminum)

Full vs. Condensed Electron Configurations

The full or longhand electron configuration involves going from the 1s orbital and to the last orbital of a specific element. 

Al (13 Electrons)            1s22s22p63s23p1

The condensed electron configuration begins at the noble gas just before we reach our specific element. For example, the noble gas before we reach aluminum is neon. 

Al (13 Electrons)            [Ne] 3s23p1

Substituting in the [Ne] is equivalent to writing 1s22s22p6 because both describe how 10 electrons are organized into orbitals. 

Exceptions to the Electron Configuration 

Some elements will break the order of electrons and orbitals. When an element is in its neutral form its electron configuration cannot possess a d4 or d9 orbital because of energetic reasons. 

For example, Copper (Cu) has an atomic number of 29 and so possesses 29 electrons. 

Periodic-Table-CopperPeriodic Table (Copper)

Following the steps we have done we would obtain the following electron configuration for Copper:

Electron-Configuration-CopperElectron Configuration of Copper

Remember that a neutral element is not allowed to possess a d4 or d9 orbital and so an electron from the s-orbital is promoted to the d-orbital. 

Electron-Configuration-Exception-Copper-CuElectron Configuration Exception of Copper

Connecting the electron dots

As stated previously, the electron configurations of the elements are essential in understanding the correlations between chemical and physical properties. With electron configurations we relate together Slater's rules and the Madelung rules when constructing our orbital diagrams. 

From what we've learned we can make connections with the quantum numbers, the bohr diagram when discussing the excited state vs. ground state of electrons, the periodic trends such as ionization energy, and using valence electrons when drawing Lewis Dot Structures