Draw the Electron Configuration for a Neutral Atom of Fluorine

4.6 Electronic configuration (ESABE)

The energy of electrons (ESABF)

The electrons of an atom all have the same charge and the same mass, but each electron has a different amount of energy. Electrons that have the lowest energy are found closest to the nucleus (where the attractive force of the positively charged nucleus is the greatest) and the electrons that have higher energy (and are able to overcome the attractive force of the nucleus) are found further away.

Electron arrangement (ESABG)

We will start with a very simple view of the arrangement or configuration of electrons around an atom. This view simply states that electrons are arranged in energy levels (or shells) around the nucleus of an atom. These energy levels are numbered 1, 2, 3, etc. Electrons that are in the first energy level (energy level 1) are closest to the nucleus and will have the lowest energy. Electrons further away from the nucleus will have a higher energy.

In the following examples, the energy levels are shown as concentric circles around the central nucleus. The important thing to know for these diagrams is that the first energy level can hold 2 electrons, the second energy level can hold 8 electrons and the third energy level can hold 8 electrons.

Lithium

Lithium (\(\text{Li}\)) has an atomic number of 3, meaning that in a neutral atom, the number of electrons will also be 3. The first two electrons are found in the first energy level, while the third electron is found in the second energy level (Figure 4.7).

Figure 4.7: Electron arrangement of a lithium atom.
Fluorine

Fluorine (\(\text{F}\)) has an atomic number of 9, meaning that a neutral atom also has 9 electrons. The first 2 electrons are found in the first energy level, while the other 7 are found in the second energy level (Figure 4.8).

Figure 4.8: Electron arrangement of a fluorine atom.
Neon

Neon (\(\text{Ne}\)) has an atomic number of 10, meaning that a neutral atom also has 10 electrons. The first 2 electrons are found in the first energy level and the last 8 are found in the second energy level. (Figure 4.9).

Figure 4.9: Electron arrangement of a neon atom.

But the situation is slightly more complicated than this. Within each energy level, the electrons move in orbitals. An orbital defines the spaces or regions where electrons move.

Atomic orbital: An atomic orbital is the region in which an electron may be found around a single atom.

The first energy level contains only one s orbital, the second energy level contains one s orbital and three p orbitals and the third energy level contains one s orbital and three p orbitals (as well as five d orbitals). Within each energy level, the s orbital is at a lower energy than the p orbitals. This arrangement is shown in Figure 4.10.

Figure 4.10: The positions of the first ten orbitals of an atom on an energy diagram.

Each block in Figure 4.10 is able to hold two electrons. This means that the s orbital can hold two electrons, while the p orbital can hold a total of six electrons, two in each of the three blocks.

This diagram also helps us when we are working out the electron configuration of an element. The electron configuration of an element is the arrangement of the electrons in the shells and subshells. There are a few guidelines for working out the electron configuration. These are:

Each orbital can only hold two electrons. Electrons that occur together in an orbital are called an electron pair.
An electron will always try to enter an orbital with the lowest possible energy.
An electron will occupy an orbital on its own, rather than share an orbital with another electron. An electron would also rather occupy a lower energy orbital with another electron, before occupying a higher energy orbital. In other words, within one energy level, electrons will fill an s orbital before starting to fill p orbitals.
The s subshell can hold 2 electrons
The p subshell can hold 6 electrons

The way that the electrons are arranged in an atom is called its electron configuration.

When there are two electrons in an orbital, the electrons are called an electron pair. If the orbital only has one electron, this electron is said to be an unpaired electron. Electron pairs are shown with arrows pointing in opposite directions.

Electron configuration: Electron configuration is the arrangement of electrons in an atom, molecule or other physical structure.

Aufbau diagrams (ESABH)

An element's electron configuration can be represented using Aufbau diagrams or energy level diagrams. An Aufbau diagram uses arrows to represent electrons. You can use the following steps to help you to draw an Aufbau diagram:

Determine the number of electrons that the atom has.
Fill the s orbital in the first energy level (the 1s orbital) with the first two electrons.
Fill the s orbital in the second energy level (the 2s orbital) with the second two electrons.
Put one electron in each of the three p orbitals in the second energy level (the 2p orbitals) and then if there are still electrons remaining, go back and place a second electron in each of the 2p orbitals to complete the electron pairs.
Carry on in this way through each of the successive energy levels until all the electrons have been drawn.

You can think of Aufbau diagrams as being similar to people getting on a bus or a train. People will first sit in empty seats with empty seats between them and the other people (unless they know the people and then they will sit next to them). This is the lowest energy. When all the seats are filled like this, any more people that get on will be forced to sit next to someone. This is higher in energy. As the bus or train fills even more the people have to stand to fit on. This is the highest energy.

Aufbau is the German word for "building up". Scientists used this term since this is exactly what we are doing when we work out electron configuration, we are building up the atoms structure.

Hund's rule and Pauli's principle

Sometimes people refer to Hund's rule for electron configuration. This rule simply says that electrons would rather be in a subshell on their own than share a subshell. This is why when you are filling the subshells you put one electron in each subshell and then go back and fill the subshell, before moving onto the next energy level.

Pauli's exclusion principle simply states that electrons have a property known as spin and that two electrons in a subshell will not spin the same way. This is why we draw electrons as one arrow pointing up and one arrow pointing down.

Spectroscopic electron configuration notation (ESABI)

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 4.11 and Figure 4.12 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 \(1\text{s}^{2}2\text{s}^{2}2\text{p}^{6}3\text{s}^{2}3\text{p}^{6}\).

Figure 4.11: An Aufbau diagram showing the electron configuration of fluorine

Figure 4.12: An Aufbau diagram showing the electron configuration of argon

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.

Worked example 3: Aufbau diagrams and spectroscopic electron configuration

Give the electron configuration for nitrogen (N) and draw an Aufbau diagram.

Give the number of electrons

Nitrogen has seven electrons.

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.

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.

Place three electrons in the 2p orbital

We place three electrons in the 2p orbital: \(2\text{p}^{3}\).

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.