The Electron Configuration: Exceptions (Simplified): Study with Video Lessons, Practice Problems & Examples

Electron orbital stability is enhanced when d subshells are half-filled or fully filled, following Hund's rule. Notable exceptions in electron configurations begin with chromium (Cr, Z=24) and include copper (Cu). For chromium, the configuration is adjusted from argon 4s²3d⁴ to 4s¹3d⁵ to achieve half-filled stability. Similarly, copper's configuration shifts from argon 4s²3d⁹ to 4s¹3d¹⁰ for full stability, illustrating the drive for optimal electron arrangements.

The Electron Configuration: Exceptions (Simplified) Concept 1

So when it comes to electron orbital stability, you just need to remember that d subshell orbitals are most stable when they are half-filled or totally filled with electrons because of symmetry. So what we mean by half-filled, remember, we're going to follow Hund's rule, which says that n orbitals that have the same energy that are degenerate are first half-filled. So we're dealing with up, up, up, up, up. Here we have a set of d orbitals electrons that are all facing up. So this set of orbitals are half-filled. Now, totally filled, we go up, up, up, up, up, then come back around, down, down, down, down, down. So here we have an example of a half-filled set of d orbitals, and here a set of totally filled in d orbitals.

The Electron Configuration: Exceptions (Simplified) Concept 2

Now when looking at exceptions to the electron configurations, we're going to say starting from chromium, which is Cr, as the atomic number z increases, exceptions to electron configurations can be observed. A memory tool here we can have is that chromium has an atomic number of 24. So think about that, 24. We're going to say that the exceptions happen with these 2 elements and with these 4 elements, so 24. We're going to start out with chromium, we know that's where it starts, and we're going to skip the next 4 columns. Right? So we start out with chromium and you skip the next 4. So skip Manganese, skip Iron, skip Cobalt, skip Nickel, and then you land on copper where the next group of exceptions can exist. So just remember, these are the 6 major types of elements where we're going to see exceptions to the electron configuration. So keep this in mind when we're looking at their electron configurations. Now that we know that these are the 6 to that we have to deal with, let's see how these exceptions arise. So click on the next video and let's see what happened.

The Electron Configuration: Exceptions (Simplified) Concept 3

So remember, exceptions start with chromium. So let's look at chromium. If we were to determine its electron configuration initially, we would see that it would look like argon 4s23d4. Now here, what do we have here? We have 3d4 with four electrons within it. But remember, earlier we said that s and d subshells or sublevels have this urge to try to be half-filled or totally filled. Now we're going to say, an s orbital electron can be promoted to create half-filled orbitals with d4 electrons. So what we're saying here is if you're doing the electron configuration of chromium, you're going to end with a d4. That's a key to tell you that, oh, d4. We have only four electrons within these d orbitals. But if I could somehow get one more electron in there, those d orbitals will be half-filled. So what's going to happen here is we're going to take a d5, and our 4s2 just gave up an electron so it becomes 4s1. So it now looks like this. This would be the correct electron configuration of chromium. So again remember, chromium has this type of exception, and the driving force is trying to get a half-filled set of d orbitals. Okay. So here, we're not going to land we're not going to stay as d4, when it's neutral it's going to become a d5. Now that we've seen this with the first column, let's see what happens with the second column. So click on the next video and let's see what happens with them.

The Electron Configuration: Exceptions (Simplified) Concept 4

So if we look at the 2nd column, let's look at copper. Now copper, if we were to look at the periodic table, we'd initially think it's argon s2d9. But if we look at the 3d9 orbitals, what we should notice is we just need one more electron here and it will be completely filled. Remember there's this need, this drive by your p and d sublevels or subshells to be either half-filled or totally filled. So we're going to say here, an electron, an s orbital electron, can be promoted to create completely filled orbitals with d 9 elements. Here copper is a d nine element. It ends with d 9. If you just get one more electron it can become d 10, and that's what's going to happen. We take one from the s orbital, before s orbital, and we donate it over to the d. Doing this now, we only have one electron here within 4s, and this now becomes totally filled in and therefore more stable. So now this is s1d10. So just remember, we have these 6 elements we discussed previously in the videos, they have this drive where we're going to take 1 electron from an s orbital and promote it to 1 of the d orbitals, so that we can create either a half-filled d orbitals or totally filled, completely filled d orbitals. Remember this driving force that causes the exceptions within these neutral elements.

The Electron Configuration: Exceptions (Simplified) Example 1

Here in this example question, it says, based on the exceptions, provide the condensed electron configuration for the silver atom. So we're going to say here that silver is an element. It has an atomic number of 47. Since we're dealing with an atom, it's the neutral form of it, so it has 47 electrons. Now, looking at the periodic table, what we would see initially is we would see krypton 5s²4d⁹. Remember, silver is one of the elements within the second column we discussed, and remember it's a d nine element. If it can become d ten, those orbitals will be completely filled. In order to do this, we're going to take 1 electron from the s orbital and promote it up to the d set of orbitals. Doing this gives us now the correct exception to silver, which is Krypton 5s¹4d¹⁰. So this would be the correct electron configuration for silver. So just keep in mind these six elements we discussed earlier, all of them do this, where we take 1 electron from the s orbital and promote it up, so that we either have half-filled d orbitals or completely filled in d orbitals like silver here.

Illustrate the exception to the electron configuration of molybdenum.

Here it says, illustrate the exception to the electron configuration of molybdenum. Molybdenum is Mo. It has an atomic number of 42, so it has 42 electrons. If you were to look at the periodic table, we're going to write down what we see initially. What we see here is Krypton 5s², and then we see 4d4. Molybdenum is right below chromium, and remember we said that in that column both chromium and molybdenum are exceptions to our typical electron configuration. Here it's a d four element. Remember, d is most stable when it's half-filled. To do that, we're going to take one electron from the s orbital and promote it up to a d orbital, and that way make a half-filled set of d orbitals. So the correct electron configuration for molybdenum is actually Krypton 5s14d5. So now we're going to illustrate that here with this electron orbital diagram. So here this would be Krypton, this is 4s because remember, I'm in 5s because remember, s sublevel has 1 orbital, and then these 5 would be our 4d set of orbitals. So we'd have only 1 here in the 5s orbital, and then we have 5 here in 4d. Following Hund's rule, it'd be up, up, up, up, and up. So this would be the correct illustration for the electron orbital diagram for a molybdenum atom.

Which of the following is the correct electron configuration of gold? 

• A.
• B.
• C.
• D.
• E.

A comparison of the electron configurations of palladium (Pd) and silver (Ag) indicates that: 

• A. Ag has 2 more d electrons and the same number of s electrons as Pd.
• B. Ag has 1 more d electron and the same number of s electrons as Pd.
• C. Ag has 2 more d electrons and 1 less s electron than Pd.
• D. Ag has 1 more d electron and 1 less s electron than Pd.
• E. Ag has 1 more d electron and 1 more s electron than Pd.

Here’s what students ask on this topic:

What are the exceptions to the electron configuration rules?

Exceptions to the electron configuration rules primarily occur in transition metals, starting with chromium (Cr) and copper (Cu). For chromium, the expected configuration is [Ar] 4s² 3d⁴, but it adjusts to [Ar] 4s¹ 3d⁵ to achieve a half-filled d subshell. Similarly, copper's expected configuration is [Ar] 4s² 3d⁹, but it changes to [Ar] 4s¹ 3d¹⁰ for a fully filled d subshell. These adjustments occur because half-filled and fully filled d subshells are more stable due to electron repulsion and symmetry considerations.

Why is the electron configuration of chromium [Ar] 4s¹ 3d⁵ instead of [Ar] 4s² 3d⁴?

The electron configuration of chromium is [Ar] 4s¹ 3d⁵ instead of [Ar] 4s² 3d⁴ because a half-filled d subshell (3d⁵) is more stable. This stability arises from reduced electron-electron repulsion and increased symmetry. By promoting one electron from the 4s orbital to the 3d orbital, chromium achieves a more stable configuration with half-filled d orbitals.

How does Hund's rule explain the stability of half-filled and fully filled d subshells?

Hund's rule states that electrons will fill degenerate orbitals (orbitals with the same energy) singly before pairing up. This minimizes electron-electron repulsion and maximizes total spin, leading to greater stability. In the context of d subshells, half-filled (d⁵) and fully filled (d¹⁰) configurations are particularly stable because they offer symmetrical electron distribution and reduced repulsion, making the atom more energetically favorable.

What is the electron configuration of copper and why does it differ from the expected configuration?

The electron configuration of copper is [Ar] 4s¹ 3d¹⁰ instead of the expected [Ar] 4s² 3d⁹. This adjustment occurs because a fully filled d subshell (3d¹⁰) is more stable than a partially filled one. By promoting one electron from the 4s orbital to the 3d orbital, copper achieves a more stable configuration with a fully filled d subshell, reducing electron repulsion and increasing symmetry.

What is the significance of the atomic number 24 in electron configuration exceptions?

The atomic number 24 is significant because it marks the beginning of notable exceptions in electron configurations, starting with chromium (Cr). Chromium, with an atomic number of 24, has an electron configuration that deviates from the expected pattern to achieve a more stable half-filled d subshell. This pattern of exceptions continues with other elements, such as copper, highlighting the importance of stability in electron arrangements.