http://www.ck12.org Chapter 5. Electrons in Atoms
5.3 Electron Arrangement in Atoms
Lesson Objectives
- Understand how to apply the Aufbau principle, the Pauli exclusion principle, and Hund’s rule to determine
ground state electron configurations. - Be able to write correct orbital filling diagrams and electron configurations for all elements.
- Know how to use the noble gas notation shorthand method.
- Be able to determine the number of valence electrons and the number of unpaired electrons in any atom.
- Understand that some electron configurations are exceptions to the normal Aufbau process.
Lesson Vocabulary
- Aufbau principle
- electron configuration
- Hund’s rule
- noble gas notation
- Pauli exclusion principle
- valence electron
Electron Configurations
The quantum mechanical model provides what is now recognized as the modern and accepted model of the atom.
An atom’selectron configurationdescribes the arrangement of all electrons in that atom. Although we do not
know the exact locations of any electrons, each electron will exist primarily in a region of most probable locations
that is defined by one of the orbitals described in the previous section. Writing the electron configuration of an atom
essentially amounts to listing which orbitals contain electrons and how many electrons are in each type of orbital.
Since every element has a different number of electrons, each has a unique electron configuration. Recall that the
natural tendency for all systems is to be in the lowest possible energy state, which is also known as the ground state.
Thus, the ground state electron configuration for an element is the lowest-energy arrangement of electrons possible
for that element. Most ground state electron configurations can be determined from the quantum number guidelines
learned in the previous lesson, “The Quantum Mechanical Model,” and a few basic rules.
Aufbau Principle
To determine the ground state electron configuration for a given atom, it is first necessary to organize the atomic
sublevels in order of increasing energy. The figure below (Figure5.18) shows the relative energies of various
sublevels.
The lowest energy sublevel is always the 1ssublevel, which consists of one orbital. The single electron of the
hydrogen atom will occupy the 1sorbital when the atom is in its ground state. As we move on to atoms with more