April 2019 Limerick Project

Electron Configurations

“Use noble gas configurations
As a shorthand for core information.
How the atom arrays
Its electrons: display
This arrangement in compact notation.”

The periodic table arranges elements according to their physical and chemical behaviors… but why are those behaviors different from one another in the first place? The electronic structure of an atom (that is, how the electrons in that atom are arranged) of each element is unique and governs each element’s behaviors. The 12 April 2019 limerick introduces the skill of representing these arrangements in a compact form: writing “noble gas” configurations.

“Use noble gas configurations/ As a shorthand for core information.
Electron configurations represent the number of electrons in an element and their probable locations, which are called orbitals. (Why are orbitals “probable” instead of “definite” locations? This is due to the unusual rules of quantum mechanics.)

Sodium (Na), for instance, has eleven electrons. These are distributed into orbitals of increasingly higher energy, as represented by the following full electron configuration: 1s22s22p63s1
The 1s orbital is lowest in energy, followed by 2s, followed by the three orbitals in the 2p subshell, followed by the 3s orbital. (The s and p labels come from abbreviations for “sharp” and “principal,” which are historical descriptions related to these concepts.) The superscripts denote how many electrons populate a given orbital or subshell. By adding up the superscripts, we confirm that we’ve accounted for all eleven electrons.

Commonly, chemists use “noble gas” notation, where the symbol of the nearest noble gas to the element in question is placed in brackets to replace part of the electron configuration. The nearest noble gas for sodium is neon (Ne), which contains ten electrons, so we can also write: [Ne]3s1
Here, the ten innermost electrons (1s22s22p6) are accounted for by comparison to neon’s configuration; the eleventh, outermost electron is notated separately.  These inner electrons are called core electrons (we often refer to “the noble gas core”); the outer electron is called a valence electron. 

“How the atom arrays/ Its electrons: display/
This arrangement in compact notation.”
Compared to a full electron configuration, a noble gas configuration is a more efficient notation for arranging electron configuration information, as seen above. This compactness becomes quite dramatic with atoms containing large numbers of electrons.