The Electron Configurations: Exceptions: Videos & Practice Problems

Understanding electron configurations is crucial in chemistry, and certain elements exhibit exceptions to the expected patterns due to their drive for stability. Electrons in p and d subshells are most stable when these orbitals are either half-filled or completely filled, which is related to the symmetry and stability of these configurations. Notably, chromium and copper are among the six elements that commonly demonstrate exceptions to their electron configurations. Chromium, for example, would ideally have a 4s² 3d⁴ configuration, but it actually promotes an electron from the 4s² orbital to the 3d⁵ to achieve a half-filled 3d⁵ configuration, resulting in a more stable 4s¹ 3d⁵ state. Similarly, copper adjusts its electron arrangement from an expected 4s² 3d⁹ to a more stable 4s¹ 3d¹⁰ configuration, completely filling the d subshell. This tendency to achieve half-filled or fully-filled d orbitals is a key factor in the exceptions observed in electron configurations of certain transition elements.

Electron Orbital Stability

Before we can talk about the exceptions to the electron configurations, it's important to understand why they exist. We're going to say here that P&D, subshell or sublevel orbitals are most stable when they are half filled or totally filled with electrons because of symmetry. So here for P&D when they're half filled they're stable in this way.

So remember following off bound principle, Not off bound principle, but the following the Huns rule, electrons that are degenerate or have the same energy, they're half built, so up, up, up up up up up. So when P sub levels or subshells are filled halfway, they're stable. When D are half filled, they're also stable. Now when they're totally filled in, they're also stable.

So up, up, up and come back, around, down, down, down, up, up, up, down, down, down, down and down. So just remember it's this drive to either be half build or totally filled by the P&D subshells or sub levels that causes some of these exceptions to the electron configurations that we'll see later on.

Electron Configuration Exceptions

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 two and four.

We're going to say that the exceptions happen with these two elements and with these four elements, so two and four. We're going to say here 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. So we start out with chromium and you skip next four, 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 six 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've know that these are the six that we have to deal with, let's see how these exceptions arise. So click on the next video and let's see what happens.

d^4 Elements

So remember, exceptions start with chromium. So let's look at chromium. If we're 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 3D with four electrons within it. But remember earlier we said that S&D subshells or sub levels 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 D 4 electrons. So what we're saying here is if you're doing the electron configuration of chromium, you're going to end with AD⁴. 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 orbital will be half filled.

So what's going to happen here is we're going to take one electron from the 4S and donated over here. So our 3D⁴ is going to become AD⁵ 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 up D orbitals.

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.

d^9 Elements

So if we look at the second column, let's look at copper. Now copper, if we're to look at the periodic table, we'd initially think it's argon 4S²3D⁹. But if we look at the three D⁹ 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&D sub levels 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 D⁹ elements. Here copper is a D⁹ element. It ends with D⁹. If you could just get one more electron, it can become D¹⁰ 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 4 S and this now becomes totally filled in and therefore more stable.

So now this is 4S¹3 D¹⁰. So just remember we have the six elements we discussed previously in the videos. They have this drive where we're going to take one electron from an S orbital and promote it to one of the D orbitals so that we can create either 1/2 build D orbitals or totally filled, completely filled D orbitals. Remember this driving force that causes the exceptions within these neutral elements.

Electron Configuration Exceptions Example

Here in this example question it says based on the exceptions provide the condensed electron configuration for this silver atom. So we're going to say here that silver is AG. 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 D⁹ element. If it can become D¹⁰, those orbitals will be completely filled. In order to do this we're going to take one 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 one 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 gonna write down what we see initially. What we see here is Krypton Kr5s2, 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 gonna 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 Kr5s14d5. So now we're gonna 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 the molybdenum atom.

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

• A.
• B.
• C.
• D.
• E.

Answer: (D)

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.

Answer: (C)

Here’s what students ask on this topic:

What are the exceptions to electron configuration?

Electron configuration refers to the distribution of electrons in an atom's orbitals. However, there are exceptions to the predicted order, which usually follow the Aufbau principle, Hund's rule, and the Pauli exclusion principle. These exceptions often occur due to the closeness in energy between the s and d orbitals or the p and d orbitals in the higher energy levels.

One common exception is seen in elements such as chromium and copper. In the case of chromium, instead of the expected electron configuration of [Ar] 4s² 3d⁴, it actually has a configuration of [Ar] 4s¹ 3d⁵. This is because half-filled d subshells are more stable due to electron repulsion and exchange energy. Similarly, copper is expected to have a configuration of [Ar] 4s² 3d⁹ but instead has [Ar] 4s¹ 3d¹⁰, as a completely filled d subshell is also more stable.

These exceptions generally occur because atoms seek the most stable electron configuration, which sometimes means deviating from the expected order to achieve a lower energy state. It's important to remember that the stability of electron configurations can be influenced by several factors, including electron-electron interactions and the specific energies of different orbitals.

Why are there exceptions to electron configuration?

Electron configurations typically follow a standard order based on the Aufbau principle, which states that electrons fill orbitals starting with the lowest energy levels before moving to higher ones. However, there are exceptions to these configurations primarily due to electron-electron interactions and the stability associated with half-filled and fully filled subshells.

In certain atoms, electrons will rearrange themselves to achieve a more stable electron configuration. This is often seen in the d and f blocks of the periodic table. For example, chromium (Cr) and copper (Cu) are two classic cases where we see exceptions. Chromium has the electron configuration [Ar] 3d⁵ 4s¹ instead of the expected [Ar] 3d⁴ 4s². This is because a half-filled d subshell (with five electrons) offers extra stability. Similarly, copper has the configuration [Ar] 3d¹⁰ 4s¹ as a fully filled d subshell (with ten electrons) is more stable than the expected [Ar] 3d⁹ 4s².

These exceptions occur due to the subtle balance between the energies of the orbitals and the specific electron configurations that confer extra stability to the atom.

What elements are exceptions in electron configuration?

In electron configuration, exceptions often occur due to the stability provided by half-filled or fully filled subshells. The most common exceptions are found among the transition metals, particularly in the chromium (Cr) and copper (Cu) families.

For chromium, which has an atomic number of 24, one would expect the electron configuration to be [Ar] 4s² 3d⁴. However, it is actually [Ar] 4s¹ 3d⁵. This is because having a half-filled d subshell (with five electrons) provides additional stability.

Similarly, copper, with an atomic number of 29, is expected to have the electron configuration [Ar] 4s² 3d⁹. Instead, its actual configuration is [Ar] 4s¹ 3d¹⁰. In this case, a completely filled d subshell (with ten electrons) is more stable than having a filled 4s and a partially filled 3d subshell.

These exceptions are due to subtle energy differences between the 4s and 3d subshells, and the stability that comes from symmetrical or fully filled electron arrangements. Other elements that show exceptions typically reside in the d-block and f-block of the periodic table, where similar stability considerations come into play.