Select your language

Suggested languages for you:
Log In Start studying!
StudySmarter - The all-in-one study app.
4.8 • +11k Ratings
More than 3 Million Downloads
Free

All-in-one learning app

  • Flashcards
  • NotesNotes
  • ExplanationsExplanations
  • Study Planner
  • Textbook solutions
Start studying

Electron Configuration

Save Save
Print Print
Edit Edit
Sign up to use all features for free. Sign up now
X
Illustration You have already viewed an explanation Register now and access this and thousands of further explanations for free
Chemistry

Ah, electrons. Those tiny little subatomic particles, whizzing around in their orbits. They are almost 2000 times lighter than a proton and only a third of the diameter, but they are extraordinarily important. You'll remember from Fundamental Particles that whilst the number of protons tells you the element of the atom, the number of electrons and their configuration give you its reactivity and chemical properties. These are major roles for such tiny particles!

What is electron configuration?

Electron configuration is also known as electronic configuration. It is the arrangement of electrons in shells, sub-shells, and orbitals within the atom.

If you’re not familiar with the above terms, we recommend looking at Electron Shells to learn a bit more about them. For now, we’ll just provide a quick summary.

Electron shells

Electron shells are also known as energy levels. Each one has a specific principal quantum number. As shells get further from the nucleus, their principal quantum number increases and they have a higher energy level.

Electron sub-shells

Sub-shells are divisions within each shell. They also have different energy levels - the s sub-shell has the lowest energy, then p, then d, then f. Each sub-shell contains different numbers of orbitals. For example, the s sub-shell has just one orbital whilst p sub-shells have three and d sub-shells have five.

Electron Configuration, electron shells sub-shells orbitals energy levels, StudySmarterA graph showing the different energy levels of shells, subshells and orbitals.commons.wikimedia.org

Electron orbitals

Orbitals are regions of space where an electron can be found 95 percent of the time. Each orbital can contain at most two electrons. These electrons must have different spins - one has an up spin, the other a down spin. Orbitals also have different shapes depending on their subshell.

If we bring this all together, electron configuration is simply how many electrons are in each orbital, and which shell and sub-shell they are found in.

How can we find electron configuration?

There are two main rules that you should know that will help you work out an atom’s electronic configuration. These are known as Hund’s rule and the Aufbau principle. We’ll take a look at both of them in turn before putting them into practice with some examples.

The Aufbau principle

First and foremost, electrons fill the sub-shell with the lowest energy level first. Atoms like being in a lower energy state and electrons are no different. In general, that means filling the shells with lower principal quantum numbers first, and within the shell first filling the s sub-shell, then the p sub-shell, then the d subshell. But remember the sneaky exception - 3d has a lower energy level than 4s! This means that it will be filled first. The diagram below reminds you of the energy levels of the different subshells.

Electron Configuration, electron sub-shells energy levels, StudySmarterA diagram showing how energy level varies between different sub-shells. The arrow shows an increase in energy. In general, as the principal quantum number n increases, energy levels increase. Note the exception to the rule, 3d.StudySmarter Originals

Hund’s rule

Electrons don’t really get along with each other. It makes sense - they are negative particles, and so if you put two of them close together, they will repel each other quite strongly. Because of this, within sub-shells electrons prefer to occupy their own orbital if they can, and so they will fill an empty orbital first.

To work out electron configuration, it can help to draw out the different orbitals and subshells. Each box represents an orbital and we use half arrows to show the electrons and their spin states. Remember that each orbital can contain a maximum of two electrons.

Let’s have a go at working out the electronic configurations in a few examples.

Give the electron configuration of carbon in box form.

Carbon has a proton number of 6, meaning that it also contains six electrons. According to the Aufbau principle, electrons will fill the lowest energy level sub-shells first. Therefore, two electrons will first fill the single orbital in 1s. Two further electrons will then fill the single orbital in 2s, the sub-shell with the next lowest energy level. That leaves two electrons to go in 2p. However, according to Hund’s rule, the electrons will prefer to go into separate orbitals within a sub-shell. The overall electron configuration is shown below.

Electron Configuration, electron configuration carbon, StudySmarterThe electron configuration of carbon.StudySmarter Originals

Another example is magnesium.

Give the electron configuration of magnesium in box form.

Magnesium has twelve electrons. Like carbon, its first two electrons will fill 1s and the next two will fill 2s. The next six electrons will fill 2p, leaving two electrons remaining. These will both go in 3s, the next lowest energy level, as shown.

Electron Configuration, electron configuration magnesium, StudySmarterThe electron configuration of magnesium.StudySmarter Originals

We can show how many electrons are in each sub-shell with simple notation. In this way, magnesium’s electron configuration is represented by , and carbon’s by . The superscript numbers show the number of electrons present.

You may have noticed a pattern. An element’s position on the periodic table relates to which sub-shell its outermost electron is in. A neutral atom from group 2 always has its outer electron in an s sub-shell, for example, whilst a transition metal has its outer electron in a d sub-shell. This is shown below.

Electron Configuration, periodic table, StudySmarterA diagram of the periodic table, showing how an element's position relates to which sub-shell its outer electron is in. commons.wikimedia.org

Writing out the full electronic configuration of say, strontium, could get a little boring - after all, it has thirty eight electrons! If you look at the periodic table, you’ll see that strontium has just two more electrons than krypton. Strontium therefore has the same electron configuration as krypton, but also has two extra electrons. Using the above diagram, we can work out that they fill the 5s orbital. But instead of writing out all of the electrons in all of the sub-shells up to , we can simply put [Kr] . This shows that this element has the same configuration as krypton with two additional electrons in the 5s sub-shell. It can be done with any of the noble gases.

How are ions formed?

We know how to fill in sub-shells and orbitals with electrons, but how do they empty? Remember, ions are atoms that have gained or lost electrons.

  • When gaining electrons, Hund’s rule and the Aufbau principle are followed as usual.
  • When losing electrons, electrons are lost from the highest energy level first - so in reverse order to filling up. However, there is another sneaky exception to the rule: 4s electrons are lost before 3d electrons.

Let’s look at an example.

Give the electron configuration of ions

Calcium atoms, Ca, have the electron configuration . However, ions, which have lost two electrons, have the configuration . This can also be written as .

Exceptions to the filling rules

You’ll probably have guessed by now that although chemistry is a logical subject, there are always a few cases that seem to ignore all the standard rules. Unfortunately, you just have to learn them - although taking the time to understand why they misbehave can help you to remember them.

Take chromium. Chromium, Cr, has twenty four electrons and the configuration . Hang on a second - why is there only one electron in the 4s sub-shell? We’d expect chromium's configuration to be ! Well, this is because the 4s and 3d sub-shells are very similar in energy level. The lone electron in 4s doesn’t experience any repulsion because it isn’t paired up, and this reduced electron-electron repulsion makes up for the fact that there is an extra electron in the slightly higher 3d energy level. Atoms just like to be in the lowest energy state possible.

Likewise, copper, Cu, has the configuration , not . This again is a slightly reduced energy arrangement due to the lack of electron-electron repulsion.

Electron Configuration, chromium copper observed and expected electron configuration, StudySmarterA diagram showing the expected and observed configurations of chromium and copper. Note the single electron in 4s. This is because the lack of electron-electron repulsion creates a slightly lower energy arrangement. StudySmarter Originals

Electron Configuration - Key takeaways

  • Electron configuration is also known as electronic configuration and describes the arrangement of electrons in an atom.
  • Electrons fill shells according to their energy levels, as dictated by the Aufbau principle and Hund’s rule. Electrons fill sub-shells with a lower energy level first, and within each sub-shell prefer to occupy their own orbital.
  • When forming ions, electrons are usually lost from the higher energy level sub-shell first.
  • Exceptions to the filling rules stem from the fact that the 4s and 3d sub-shells are similar in energy level. Always remember that the 4s sub-shell fills before the 3d sub-shell.

Electron Configuration

Electron configuration is the arrangement of electrons in shells, sub-shells, and orbitals within the atom.

Electron configuration is worked out by filling the shells of lowest energy level first (Aufbau’s principle), and then by filling empty orbitals within each sub-shell first (Hund’s rule).

Final Electron Configuration Quiz

Question

What is electron configuration?


Show answer

Answer

The arrangement of electrons in shells, subshells, and orbitals within the atom.

Show question

Question

What is the relative mass of an electron?


Show answer

Answer

1/1840

Show question

Question

What two states can spin take?


Show answer

Answer

Up and down

Show question

Question

What is Aufbau’s principle?


Show answer

Answer

Electrons fill the lowest available energy level first.

Show question

Question

What is Hund’s rule?


Show answer

Answer

Electrons fill empty orbitals of the same energy level first before pairing up with another electron in an orbital.

Show question

Question

Sort the following sub-shells according to the order they lose electrons. Start with the sub-shell that loses electrons first:

3d, 4s, 3p, 2s



Show answer

Answer

4s, 3d, 3p, 2s

Show question

Question

An ion with a charge of 2- has the same electron configuration as argon. Name the element that forms this ion.


Show answer

Answer

Sulfur

Show question

Question

How can an atom go from the excited state to the ground state?

Show answer

Answer

Loss of energy - which means an electron will drop an energy level, meaning an emission of energy will occur. 

Show question

Question

How can an atom go from the ground state to the excited state?

Show answer

Answer

It will require an electron to gain energy and move up an energy level (shell). 

Show question

Question

How can an electron gain energy to move to a higher electronic shell?

Show answer

Answer

It can absorb a discrete frequency from the electromagnetic spectrum or a quanta of light. 

Show question

Question

The terms energy level and shell are interchangable. 

Show answer

Answer

True

Show question

Question

What three rules govern electronic configuration within an atom?

Show answer

Answer

Aufbau principle, Pauli exclusion principle, Hund's Rule

Show question

Question

Which of the following three states that electrons fill up an electron shell singly?

Show answer

Answer

Hund's rule

Show question

Question

Electrons in the same orbital can have the same spin state

Show answer

Answer

True

Show question

Question

If an electronic configuration has gaps and inconsistencies, can the specified element be in its ground state?

Show answer

Answer

No

Show question

Question

According to Aufbau, can electrons fill higher energy orbitals first?

Show answer

Answer

No, as they have to fill the lowest energy shells first. 

Show question

Question

Can electrons move from the ground state to the excited state without any external source of energy?

Show answer

Answer

No, due to the lack of presence of energy required for electrons to move to higher energy states. 

Show question

Question

If an electron has a spin state of +1/2 in an orbital, what spin state will the other electon have in the same energy level?

Show answer

Answer

-1/2 due to Pauli's exclusion principle which states that electrons must always have opposing spin states. 

Show question

Question

Is there a periodicity between the electronic configurations of atoms in the ground state?

Show answer

Answer

Yes, those with consecutive atomic numebrs can show that electonic configuration builds on top of each other with one extra electron in each case.  

Show question

Question

How are atoms in the excited state depicted? (as opposed to atoms in the ground state). 

Show answer

Answer

Atoms in the excited state are depicted with an asterisk beside the symbol of the element. 

Show question

Question

How do we call diagrams which depict electronic configuration within atoms?

Show answer

Answer

Box and arrow diagrams. 

Show question

Question

Can you determine any element by looking at its electronic configuration diagram?

Show answer

Answer

Yes, if it is in the ground state especially. 

Show question

60%

of the users don't pass the Electron Configuration quiz! Will you pass the quiz?

Start Quiz

Discover the right content for your subjects

No need to cheat if you have everything you need to succeed! Packed into one app!

Study Plan

Be perfectly prepared on time with an individual plan.

Quizzes

Test your knowledge with gamified quizzes.

Flashcards

Create and find flashcards in record time.

Notes

Create beautiful notes faster than ever before.

Study Sets

Have all your study materials in one place.

Documents

Upload unlimited documents and save them online.

Study Analytics

Identify your study strength and weaknesses.

Weekly Goals

Set individual study goals and earn points reaching them.

Smart Reminders

Stop procrastinating with our study reminders.

Rewards

Earn points, unlock badges and level up while studying.

Magic Marker

Create flashcards in notes completely automatically.

Smart Formatting

Create the most beautiful study materials using our templates.

Sign up to highlight and take notes. It’s 100% free.