ASP - Electrons in Atoms (Lesson)

Electrons in Atoms

Atomic Structure and Electrons

Electron Orbitals and Configuration

So far the course has centered around discussion of topics concerning the nucleus of atoms. The field of chemistry, however, is dominated, not by how the atomic nuclei are arranged, but by how the electrons outside of the nucleus are configured. Electrons were first discovered in 1897 before the turn of the 20th century and became a significant focus of study for many decades. At first, it was thought that electrons were just randomly associated with the nucleus (Rutherford model), but further investigation revealed a much more ordered arrangement. Current models of the atom describe electrons as occupying specific atomic orbitals or 3-dimensional regions of space outside of the nucleus.  Furthermore, the orbitals are highly ordered and arranged based on their energy levels.

Because of the highly ordered nature of electron configurations, it is possible to write an electron configuration by following the Aufbau Principle. The Aufbau Principle states that the electrons in an atom -- the electron configuration -- are filled using the orbitals closest to the nucleus and moving out to the valence electron level. In the diagram, the lower energy levels are at the bottom and as energy level increases, the distance from the nucleus increases. 

The word aufbau comes from German and literally can be interpreted as a "fill-up' diagram.  The electron orbitals are filled from lowest energy to highest energy.  A diagram depicting the order in which they are filled can be accessed HERE Links to an external site.. 

Orbital Diagrams

We draw orbital diagrams to show the distribution of electrons in a sublevel. Boxes, blanks, or circles are used to represent the individual orbital. Arrows are used to represent electrons in the orbital. If two electrons occupy the same orbital it is important to remember the Pauli Exclusion Principle which states that if two electrons occupy the same orbital then they must have opposite spin (arrows must be pointed in opposite directions).

Electrons fill orbitals that have the same energy, staying unpaired until there is no other option but to pair up. Take the 2p subshell, for example. there are 3 orbitals in the 2p subshell. These three orbitals have the exact same energy and are called 'degenerate orbitals'. Each can hold 2 electrons, for a total of 6 electrons in the entire subshell. Electrons go into each of those orbitals one at a time. This behavior is noted by what is called Hund's Rule. Hund's Rule states that every orbital in a subshell must be singly occupied with one electron before any one orbital can be doubly occupied, and all electrons in a singly occupied orbitals have the same spin. 

When 3 electrons have each occupied one of the three orbitals, the 4th electron then pairs up with another in an orbital. 

Electron 4 must pair up, but it has an opposite spin.

Electron 5 and 6 would also pair up, each with an opposite spin. 

This behavior of electrons is noted and summarized in what is known as the Pauli Exclusion Principle. The Pauli Exclusion Principle is where an orbital can contain a maximum of only two electrons, but the two electrons must have opposing spins. This means if one electron is assigned as a spin up (+1/2) electron, the other electron must a be spin-down (-1/2) electron.

Electron Configurations

Now that you know how electrons fill the orbitals within a sub-level, let's learn the order in which they fill different energy levels and sub-levels. The way that the electrons fill the orbitals is known as the electron configuration. Remember that the electrons determine much about the way that an atom behaves. Because they are so important, knowing the electron configuration will go a long way toward helping us understand atoms. An electron configuration shows the energy level, sublevel, and the number of electrons in that subshell.

Please watch the video below to learn more about electron configurations. Be sure your volume is turned on!

Each energy level, energy sublevel, and each electron is represented in an electron configuration. The orbitals are not represented. When all the superscripts for electrons are totaled, it will equal the number of electrons in the atom it represents. 

Practice Problems!

Sketch the orbital diagrams and write the electron configurations for the following elements. Check the plus sign to reveal the correct answer. 

Core and Valence Electrons

Electrons found on the energy levels closest to the nucleus feel a stronger attractive force and are known as the core electrons. Electrons in the s & p sublevels of the outermost energy level are the farthest from the nucleus and are known as the valence electrons.

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