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Electron Configuration

3 min readdecember 1, 2021

Rupi Adhikary

Rupi Adhikary

Rupi Adhikary

Rupi Adhikary

Writing Electron Configurations Based on the number of Electrons in an Atom

An element’s electron configuration is how we depict the arrangement of electrons around the atom’s nucleus. First, let’s go over some key terms and concepts! 

Sublevels: denoted by s, p, d, and f, sublevels are general energy levels. 

Orbital: Based on its energy level, the general location of an electron is within an orbital. Each orbital can hold up to 2 electrons. 

  • S sublevel = 1️⃣orbital = holds a maximum of 2 electrons 

  • P sublevel = 3️⃣orbitals = holds a maximum of 6 electrons 

  • D sublevel = 5️⃣orbitals = holds a maximum of 10 electrons 

  • F sublevel = 7️⃣ orbitals = holds a maximum of 14 electrons 

How to Find Electron Configurations

To find a neutral atom’s electron configuration, you’ll need two things: the name of the atom and a periodic table such as this:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-EAPeMl8QRKfi.png?alt=media&token=ff1236de-8fd3-46a8-9ee4-9fc793cfe317

Image Courtesy of ChemFiesta

A diagram such as this can help getting used to the order of orbitals as well! You can find some more helpful tips here

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F2880px-Klechkowski_rule_2-zsgwp7wFKDFg.png?alt=media&token=16e0e7ed-2fb0-4d2f-8e48-87d708ab9582

Image Courtesy of Wikipedia

First, you’ll have to find the element on the table (for example, aluminum/Al). We can see that Al is in the green 🟩 or ‘p’ section. Then, all we have to do is go through the table until we get to aluminum! 

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-AY19KNPbBvMJ.png?alt=media&token=3c118832-3928-4db6-85bc-ddfaf1e5d9b3

We can see that aluminum is past the 1s2, 2s2, 2p6, and 3s2 orbitals. However, the 3p orbital isn’t completely full. For that last orbital, the ‘exponent’ will be how far the element in question is. In this case, aluminum is the first element in the 3p row. Therefore, the exponent will be 1!

So, the electron configuration for Aluminum is as follows:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-X4YdnuHFFVMe.PNG?alt=media&token=b2cea66b-2a90-44af-a203-2afaf7b6d4fd

Noble Gas Configurations

But what if you had to find the configuration for something further down the periodic table and didn’t want to write out all the orbitals that come before it? That’s where noble gas configurations come in!

Noble gas configurations are a way to shorthand electron configurations. They’re done by placing the most recent noble gas in brackets and placing the rest of the appropriate configuration after it.

For example, let’s take silver🥈! A typical electron configuration for Ag is:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-3vYECfGgttWk.PNG?alt=media&token=2325828b-aabd-4a8a-a0f9-01e38263fb76

The most recent noble gas to silver is Krypton (Kr). Therefore, we can shorthand this configuration by placing [Kr] at the beginning and picking up the rest of the configuration where Kr left off:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-ggspsYhX85jU.PNG?alt=media&token=091f2be0-e08a-4cfb-a8ff-1e7d1a6ee409

Exceptions and Special Cases 

Exceptions

There are two main exceptions to the electron configuration rule: Copper (Cu) 🔑 and Chromium (Cr) 🦾. These exceptions are due to the fact that the ‘d’ sublevel is unstable unless it is half (has 5 electrons) or completely full (has 10 electrons)

Typically, we would say that Cu has the electron configuration:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-VoXBg2A9VrGD.PNG?alt=media&token=df9408a5-b0a5-490b-8373-a6ec0acd2ef5

However, we can see that the 3d orbital has 4 electrons, making it very unstable. To fix this, we take 1 electron from the 4s orbital and add it to the 3d orbital to give us:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-Q9ZCGcS6QSge.PNG?alt=media&token=fe6ee780-43af-4c3a-be81-9dee1fa37747

Through this same process, Cr has the electron configuration:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-J6llk36R0HOi.PNG?alt=media&token=9333f720-510c-4daa-81f9-de1b6ceb1d06

Anions

In the case of having to write out the electron configuration of an anion, you’ll first need to determine the original atom’s configuration. From there, depending on how negative the anion is, you’ll have to add that many electrons and continue with the same pattern as above. 

For example, Chlorine (Cl) has the electron configuration:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-cOdPnwjAnj4V.PNG?alt=media&token=47bdc5ed-2182-4542-bb96-2210fe5a4fa7

To find the electron configuration of Cl⁻, we’ll add one electron to get:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-19lIz9eJY9AC.PNG?alt=media&token=c53da01a-d2cb-4378-bae2-a8a14f7e489f

Cations

Cations are a bit different from anions. We’ll start off with the original atom’s electron configuration. However, when determining what orbitals to take electrons out from, we start from the outer most/highest level. 

For example, Iron has the electron configuration:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-p8rau8QfRrqe.PNG?alt=media&token=5ca41c1d-70b3-4ffb-99d5-27bf070b7570

To find the electron configuration of Fe³⁺- we have to take electrons out starting from the 4th level and working our way down. Therefore, 2 electrons would be taken out from the 4s orbital and 1 from the 3d orbital to give us: 

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-SB2RWrEByEyP.PNG?alt=media&token=084fe751-895b-469f-ad3e-d67a2ad644ab

🎥Check out this replay for some more practice!

Electron Configuration

3 min readdecember 1, 2021

Rupi Adhikary

Rupi Adhikary

Rupi Adhikary

Rupi Adhikary

Writing Electron Configurations Based on the number of Electrons in an Atom

An element’s electron configuration is how we depict the arrangement of electrons around the atom’s nucleus. First, let’s go over some key terms and concepts! 

Sublevels: denoted by s, p, d, and f, sublevels are general energy levels. 

Orbital: Based on its energy level, the general location of an electron is within an orbital. Each orbital can hold up to 2 electrons. 

  • S sublevel = 1️⃣orbital = holds a maximum of 2 electrons 

  • P sublevel = 3️⃣orbitals = holds a maximum of 6 electrons 

  • D sublevel = 5️⃣orbitals = holds a maximum of 10 electrons 

  • F sublevel = 7️⃣ orbitals = holds a maximum of 14 electrons 

How to Find Electron Configurations

To find a neutral atom’s electron configuration, you’ll need two things: the name of the atom and a periodic table such as this:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-EAPeMl8QRKfi.png?alt=media&token=ff1236de-8fd3-46a8-9ee4-9fc793cfe317

Image Courtesy of ChemFiesta

A diagram such as this can help getting used to the order of orbitals as well! You can find some more helpful tips here

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F2880px-Klechkowski_rule_2-zsgwp7wFKDFg.png?alt=media&token=16e0e7ed-2fb0-4d2f-8e48-87d708ab9582

Image Courtesy of Wikipedia

First, you’ll have to find the element on the table (for example, aluminum/Al). We can see that Al is in the green 🟩 or ‘p’ section. Then, all we have to do is go through the table until we get to aluminum! 

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-AY19KNPbBvMJ.png?alt=media&token=3c118832-3928-4db6-85bc-ddfaf1e5d9b3

We can see that aluminum is past the 1s2, 2s2, 2p6, and 3s2 orbitals. However, the 3p orbital isn’t completely full. For that last orbital, the ‘exponent’ will be how far the element in question is. In this case, aluminum is the first element in the 3p row. Therefore, the exponent will be 1!

So, the electron configuration for Aluminum is as follows:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-X4YdnuHFFVMe.PNG?alt=media&token=b2cea66b-2a90-44af-a203-2afaf7b6d4fd

Noble Gas Configurations

But what if you had to find the configuration for something further down the periodic table and didn’t want to write out all the orbitals that come before it? That’s where noble gas configurations come in!

Noble gas configurations are a way to shorthand electron configurations. They’re done by placing the most recent noble gas in brackets and placing the rest of the appropriate configuration after it.

For example, let’s take silver🥈! A typical electron configuration for Ag is:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-3vYECfGgttWk.PNG?alt=media&token=2325828b-aabd-4a8a-a0f9-01e38263fb76

The most recent noble gas to silver is Krypton (Kr). Therefore, we can shorthand this configuration by placing [Kr] at the beginning and picking up the rest of the configuration where Kr left off:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-ggspsYhX85jU.PNG?alt=media&token=091f2be0-e08a-4cfb-a8ff-1e7d1a6ee409

Exceptions and Special Cases 

Exceptions

There are two main exceptions to the electron configuration rule: Copper (Cu) 🔑 and Chromium (Cr) 🦾. These exceptions are due to the fact that the ‘d’ sublevel is unstable unless it is half (has 5 electrons) or completely full (has 10 electrons)

Typically, we would say that Cu has the electron configuration:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-VoXBg2A9VrGD.PNG?alt=media&token=df9408a5-b0a5-490b-8373-a6ec0acd2ef5

However, we can see that the 3d orbital has 4 electrons, making it very unstable. To fix this, we take 1 electron from the 4s orbital and add it to the 3d orbital to give us:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-Q9ZCGcS6QSge.PNG?alt=media&token=fe6ee780-43af-4c3a-be81-9dee1fa37747

Through this same process, Cr has the electron configuration:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-J6llk36R0HOi.PNG?alt=media&token=9333f720-510c-4daa-81f9-de1b6ceb1d06

Anions

In the case of having to write out the electron configuration of an anion, you’ll first need to determine the original atom’s configuration. From there, depending on how negative the anion is, you’ll have to add that many electrons and continue with the same pattern as above. 

For example, Chlorine (Cl) has the electron configuration:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-cOdPnwjAnj4V.PNG?alt=media&token=47bdc5ed-2182-4542-bb96-2210fe5a4fa7

To find the electron configuration of Cl⁻, we’ll add one electron to get:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-19lIz9eJY9AC.PNG?alt=media&token=c53da01a-d2cb-4378-bae2-a8a14f7e489f

Cations

Cations are a bit different from anions. We’ll start off with the original atom’s electron configuration. However, when determining what orbitals to take electrons out from, we start from the outer most/highest level. 

For example, Iron has the electron configuration:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-p8rau8QfRrqe.PNG?alt=media&token=5ca41c1d-70b3-4ffb-99d5-27bf070b7570

To find the electron configuration of Fe³⁺- we have to take electrons out starting from the 4th level and working our way down. Therefore, 2 electrons would be taken out from the 4s orbital and 1 from the 3d orbital to give us: 

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-SB2RWrEByEyP.PNG?alt=media&token=084fe751-895b-469f-ad3e-d67a2ad644ab

🎥Check out this replay for some more practice!



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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.


© 2024 Fiveable Inc. All rights reserved.

AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.