Video: Identifying the Electron Configuration of an Excited Atom in a Set of Electron Configurations

Which of the following could be the electron configuration for an excited atom? [A] 1s² [B] 1s² 2s² 2p⁵ [C] 1s² 2s² 2p⁶ [D] 1s² 2s² 2p⁶ 3s¹ [E] 1s² 2s² 2p⁶ 3s¹ 3p²


Video Transcript

Which of the following could be the electron configuration for an excited atom? A) 1s², B) 1s² 2s² 2p⁵, C) 1s² 2s² 2p⁶, D) 1s² 2s² 2p⁶ 3s¹, or E) 1s² 2s² 2p⁶ 3s¹ 3p².

Let’s start off by thinking about the meaning of excited when it applies to an atom. Let’s imagine we’ve got an atom with electrons in various energy levels. An atom or ion is said to be in its ground state if all its electrons are in the lowest possible energy orbitals. In this example, the first two electrons went into the 1s subshell. The next two electrons went into the 2s subshell because there was no space in the 1s subshell. And the next five went into the 2p subshell, which just happened to be the next lowest energy level. Purely by coincidence, this electron configuration matches answer B. But this is just an example, not the answer. We still haven’t dealt with what an excited atom is.

Now, let’s see what happens when our atom absorbs some energy. Look, an electron has moved from the 2p subshell to the 3s subshell. We’ve now excited our atom, moving an electron from its lowest possible energy orbital into a higher one. So an excited atom is one with at least one electron not in the lowest possible energy orbital. The 2p subshell can fit a further two electrons, up to its maximum of six. So we went from a ground-state electron configuration to an excited-state electron configuration. The giveaway is that we’ve got electron in the 3s subshell when there’s still space in the 2p subshell. So when it comes to finding the electron configuration for an excited atom out of our candidates, we’re looking for the same pattern. Let’s go through them one by one.

An atom or ion with a 1s² electron configuration must be in its ground state because the 1s subshell is the closest to the nucleus and therefore the lowest energy. What about B, 1s² 2s² 2p⁵? The first two electrons are filling the 1s subshell, which can take a maximum of two electrons. The next two electrons are in the 2s subshell, which again is full. The next five electrons are in the 2p subshell, taking up five out of the six slots. But we don’t have any further electrons in higher-energy subshells. So 1s² 2s² 2p⁵ represents a ground-state electron configuration. The next candidate has a very similar story. 1s² 2s² 2p⁶ has all the subshells filled. So this, too, is a ground-state electron configuration.

In the next electron configuration, we only have one electron in the 3s subshell. All the lower-energy levels are full. For the same reasons as for B, this is a ground-state electron configuration. Even though the 3s subshell isn’t full because we don’t have any electrons in higher-energy levels than that, it must be a ground-state configuration. This just leaves us with E. 1s², that’s full, 2s² full again, 2p⁶ absolutely stuffed. But the 3s subshell is missing an electron. And above that, we have two electrons that have the possibility to drop back down and fill that last space. So the electron configuration 1s² 2s² 2p⁶ 3s¹ 3p² is an example of an excited state. So it’s perfectly possible that it could be the electron configuration for an excited atom. If it was an atom, there would be 13 electrons in total. And we’d have an atom of aluminum.

Thankfully, this question was relatively easy. But if we’d had other subshells, like 3d and 4s, we might have had to think a little bit harder. Up to the 3p subshell, the energies continue to increase. However, when we get beyond the 3p subshell, what comes next might surprise you. It’s the 4s subshell and then the 3d, 4p, and 5s subshells. So if we’d had any of these subshells in any of our options, we’d have had to factor in these relative energies in order to figure out whether the state was excited or not.

As it is, we figured out that the only valid electron configuration for an excited atom out of the five is 1s² 2s² 2p⁶ 3s¹ [3p²].

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