The Electron Configuration Review - Online Tutor, Practice Problems & Exam Prep

Understanding electron configurations is essential for grasping how electrons are distributed among orbitals in an atom. Electrons fill orbitals starting from the lowest energy level, following the Aufbau principle, which dictates the sequence from 1s to 2s, then 2p, and so forth. The periodic table can be reimagined into blocks—s, p, d, and f—that correspond to the sublevels and their maximum electron capacities: s holds 2, p holds 6, d holds 10, and f holds 14 electrons. The position of elements within these blocks helps determine their electron configuration. For a more efficient approach, condensed electron configurations use the last noble gas as a reference point, simplifying the representation of electron arrangements. This method is preferred unless a full ground state electron configuration is specifically requested. Understanding these concepts is crucial for predicting chemical properties and behaviors of elements.

Ground State Electron Configurations

When writing the electron configuration of an element, it's important that it represents the distribution of electrons from S1 to S2 to P2 and so on within orbitals using the off bow principle. Now with the off bow principle it says starting with S1 we're going to have electrons filling lower energy orbitals before moving on to higher energy orbitals. We begin with S1 when we're doing the full ground state electron configuration of an element or an ion.

To help us with this, you could take the off bound diagram approach. We start out with S1, then we move on, loop back around to S2 then we loop back around to P2, then to S3 Then we loop back around to P3 and then S4. Then continuously loop back around to D3 to P4 to S5. Now this is our off bound diagram.

In the off bound diagram we have here S1 all the way down to S8 Then we have P2 to three on to seven P7 then we have D3 to D6 and then we have F4 and F5. Another way we can look at determining the electron configuration has to do more with the periodic table. So if you click on the next video, let's reimagine what the periodic table will look like when dealing with the electron configuration of elements and ions.

Auf Bau Principle

So here, if we reimagine the periodic table, we'll see it in this depiction. Now realize here we have blue, we have yellow, we have purple, and we have red sectors. Now each of these is called a block. The S block is what's in blue. This is the P block, the D block, and the F block.

Remember, the S sublevel has one orbital and that one orbital can hold a maximum of two electrons, which is why the S block is these two columns to represent the two maximum electrons that the S sublevel can hold. The P sublevel has three orbitals, right? And each one again can hold a maximum of two electrons. So P theoretically can hold a maximum of 6 electrons. That's why the P block has 1, 2, 3, 4, 5, and 6 slots for it.

D can have up to 10 electrons because it has five orbitals and if you count you'll see in here there's 10 spots. And then for the F sub levels they can hold up to 14 electrons and if you were to count these rows in red, you'd see they come out to 14. Now realize here that in this periodic table, the first slot here which represents hydrogen, starts off our electron configuration as 1S1. Then as we move to the next one we add another electron. So Helium is 1S2.

Then when we get to the second row, since we're in the 2nd row, we now have two. This is still the block, so this is 1S22S1 and it continues onward and onward. Over here we'd go 1S22S22P1. So following this pattern this would be 3S4S5S6S7S and then here this would be 3P4P5P6P7P. When we go to the D block, realize that it's going to drop down by 1, so here this is 4S. But then when we go to D block it drops down by 1 number so now it's 3D. This would be 4D5D6D.

Now notice how this number goes from 57 to 72. That's because 58 to 71 are here. Remember this? This red line here says that this entire red row exists in between 57 and 72. And then we go between 89 and 104. Because 90 to 103 is down here. These are your F blocks. They also drop down by 1 number, so this is 4F and 5F.

So basically, if you can reimagine the periodic table in this fashion, you can use it to figure out the electron configuration of any element or ion given to you. So we're going to put this periodic table to use in order to do future electron configurations.

The Electron Configuration Example

To help us with this example question, I decided to leave this periodic table up so we can see how to best use it. Here it says write the ground state electron configuration for the following element. Here we're dealing with fluorine and they tell us that it has an atomic number of nine, which means it has nine electrons.

On this periodic table, we find fluorine right here. Ground state means that we're going to start out with 1S orbital and work our way up to fluorine. So we're going to count to fluorine. So we'd say 1S2 1S2 2S2 because of 1/2 slots and then we have to count to F so that would be 2P5. We're in the 2P row. And how many slots do we have to count to get to fluorine? 1 2 3 4 5 2P5.

So here this would be the ground state electron configuration of the fluorine atom. And realize here that these are the number of electrons. So when you add them up 2 + 2 + 5, that gives me 9 electrons, which is related to the atomic number here of fluorine.

Remember, when an element is neutral, its atomic number tells us both the number of protons and the number of electrons. So again, rely on this depiction of the periodic table to help guide you to the right electron configuration of any element or ion given to you.

Which electron configuration represents a violation of the Auf Bau Principle? 

• A.
• B.
• C.
• D.

Identify the element with the given electron orbital diagram. 

• A. Silicon
• B. Fluorine
• C. Sulfur
• D. Chlorine
• E. Phosphorus

Write the electron configuration and electron orbital diagram for the following element: 

Mn (Z = 25) 

• A. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁵
• B. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 4d⁵
• C. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁴ 
• D. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁵

Write the ground-state electron configuration for the following element: 

Lead (Z = 82) 

• A. $1s^22s^22p^63s^23p^64s^23d^{10}4p^65s^24d^{10}5p^66s^24f^{10}5d^{10}6p^6$
• B. $1s^22s^22p^63s^23p^64s^23d^{10}4p^65s^24d^{10}5p^66s^24f^{14}6p^2$
• C. $1s^22s^22p^63s^23p^64s^23d^{10}4p^65s^24d^{10}5p^66s^24f^{14}5d^66p^6$
• D. $1s^22s^22p^63s^23p^64s^23d^{10}4p^65s^24d^{10}5p^66s^24f^{14}5d^{10}6p^2$

Condensed Electron Configuration

We're going to say here that the condensed electron configuration is a faster way to write out electron arrangements for elements or ions. We're going to say with condensed electron configurations, we start at the last noble gas before the desired element. And if we take a look here, remember this is our reimagining of the periodic table.

We have our S block where it begins with 1S. We have our P block here. We have our d block here, and we have our F block down here with the condensed electron configuration. It's important to know which element are we being asked to find the electron configuration of and what's the noble gas before it.

We're going to say moving forward, this will be the primary method to write electron configuration because it's the faster, easier way to do it. Unless they say full ground state electron configuration, we usually just assume that this is the method they want us to write the electron configuration.

So now that we know what the condensed electron configuration is, click on the next video and let's get to work on an example question.

The Electron Configuration: Condensed Example

Here it says to provide the condensed electron configuration for the aluminum atom. Atom means that we're dealing with the neutral form of it if you look on the periodic table. So step one, we have to find the element on the periodic table. So aluminum has an atomic number of 13, which means it has 13 electrons involved.

Step 2. We're going to locate the noble gas that comes before the element and place it inside brackets. So the noble gas before aluminum is neon, so put it in brackets. Step 3, continuing from the noble gas in brackets, complete the rest of the electron configuration. So we dealt with neon. So let's continue onward to aluminum. So next would come 3 S₂ and then 3P₁.

So this would be the condensed electron configuration of aluminum. Instead of having to write 1S₂ 2S₂ 2P₆ 3S₂ 3P₁, we have this new condensed electron configuration. Neon here is taking the spot of all of this, so it's easier and faster for us to write the electron configuration of aluminum in this regard. So just remember the condensed electron configuration saves us a lot of time in terms of writing out the electron arrangements for elements and ions.

Write the condensed electron configuration and electron orbital diagram for the following element: Zinc 

• A. [Ar] 4s² 3d⁹
• B. [Kr] 4s² 3d¹⁰
• C. [Ar] 4s² 3d¹⁰
• D. [Ar] 4s¹ 3d¹⁰