Spectroscopic Electron Configuration Notation
A special type of notation is used to show an atom’s electron configuration. The notation describes the energy levels, orbitals and the number of electrons in each. For example, the electron configuration of lithium is \(1\text{s}^{2}2\text{s}^{1}\). The number and letter describe the energy level and orbital and the number above the orbital shows how many electrons are in that orbital.
Aufbau diagrams for the elements fluorine and argon are shown in figure 1 and figure 2 below respectively. Using spectroscopic notation, the electron configuration of fluorine is \(1\text{s}^{2}2\text{s}^{2}2\text{p}^{5}\) and the electron configuration of argon is \(1\text{s}^{2}2\text{s}^{2}2\text{p}^{5}3\text{s}^{2}3\text{p}^{6}\).
Figure 1: An Aufbau diagram showing the electron configuration of fluorine
Figure 2: An Aufbau diagram showing the electron configuration of argon
Tip:
The spectroscopic electron configuration can be written in shorter form. This form is written as [noble gas]electrons, where the noble gas is the nearest one that occurs before the element. For example, magnesium can be represented as \([\text{Ne}]3\text{s}^{2}\) and carbon as \([\text{H}]2\text{s}^{2}2\text{p}^{2}\). This is known as the condensed electron configuration.
Example: Aufbau Diagrams and Spectroscopic Electron Configuration
Question
Give the electron configuration for nitrogen (N) and draw an Aufbau diagram.
Step 1: Give the number of electrons
Nitrogen has seven electrons.
Step 2: Place two electrons in the 1s orbital
We start by placing two electrons in the 1s orbital: \(1\text{s}^{2}\).
Now we have five electrons left to place in orbitals.
Step 3: Place two electrons in the 2s orbital
We put two electrons in the 2s orbital: \(2\text{s}^{2}\).
There are now three electrons to place in orbitals.
Step 4: Place three electrons in the 2p orbital
We place three electrons in the 2p orbital: \(2\text{p}^{3}\).
Step 5: Write the final answer
The electron configuration is: \(1\text{s}^{2}2\text{s}^{2}2\text{p}^{3}\). The Aufbau diagram is given in the step above.
Aufbau Diagrams for Ions
When a neutral atom loses an electron it becomes positively charged and we call it a cation. For example, sodium will lose one electron and become \(\text{Na}^{+}\) or calcium will lose two electrons and become \(\text{Ca}^{2+}\). In each of these cases, the outermost electron(s) will be lost.
When a neutral atom gains an electron it becomes negatively charged and we call it an anion. For example chlorine will gain one electron and become \(\text{Cl}^{-}\) or oxygen will gain two electrons and become \(\text{O}^{2-}\).
Aufbau diagrams and electron configurations can be done for cations and anions as well. The following worked example will show you how.
Example: Aufbau Diagram for an Ion
Question
Give the electron configuration for (\(\text{O}^{2-}\)) and draw an Aufbau diagram.
Step 1: Give the number of electrons
Oxygen has eight electrons. The oxygen anion has gained two electrons and so the total number of electrons is ten.
Step 2: Place two electrons in the 1s orbital
We start by placing two electrons in the 1s orbital: \(1\text{s}^{2}\).
Now we have eight electrons left to place in orbitals.
Step 3: Place two electrons in the 2s orbital
We put two electrons in the 2s orbital: \(2\text{s}^{2}\).
There are now six electrons to place in orbitals.
Step 4: Place six electrons in the 2p orbital
We place six electrons in the 2p orbital: \(2\text{p}^{6}\).
Step 5: Write the final answer
The electron configuration is: \(1\text{s}^{2}2\text{s}^{2}2\text{p}^{6}\). The Aufbau diagram is:
Did You Know?
When we draw the orbitals we draw a shape that has a boundary (i.e. a closed shape). This represents the distance from the nucleus in which we are \(\text{95}\%\) sure that we will find the electrons. In reality the electrons of an atom could be found any distance away from the nucleus.