Electron configuration - newbie question

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Hi there. I apologise in advance if this question seems stupid and simple, I am very much a novice at this and I couldn't find any site that had the answer I was looking for, or at least not in a manner I could interpret.

In regards to electron configuration, I understand that the order of shells and orbitals is in the form 1s 2s 2p 3s 3p 4s 3d 4p etc.. however there is something I don't understand.

Take for example the element Krypton. Going by the order of electron configuration it would seem the element should have the configuration of

[Ar]4s2 3d10 4p6, yet every site I check it seems to have the form [Ar]3d10 4s2 4p6.

I am curious as to why the 3d10 appears before the 4s2, out of order?

Many thanks.

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from wiki:

Exceptions in 3d, 4d, 5d

A d subshell that is half-filled or full (ie 5 or 10 electrons) is more stable than the s subshell of the next shell. This is the case because it takes less energy to maintain an electron in a half-filled d subshell than a filled s subshell. For instance, copper (atomic number 29) has a configuration of [Ar]4s1 3d10, not [Ar]4s2 3d9 as one would expect by the Aufbau principle. Likewise, chromium (atomic number 24) has a configuration of [Ar]4s1 3d5, not [Ar]4s2 3d4 where [Ar] represents the configuration for argon.

I hope you find that useful!

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from wiki:

I hope you find that useful!

so are you saying that in Krypton, the order of the subshells is altered, leaving us with the 3d and 4s the right way round, unlike the other elements?

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remember always that 4s is always filled first regardless of the arragement.

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remember always that 4s is always filled first regardless of the arragement.

that's the opposite of what the electron configuration for krypton suggests

I have to admit i don't know the answer to the original question

I think most likely it either means that when the subshells are FULL, they exist in the order noted, and they only overlap when not entirely filled. OR it means that it's just customary to write the shells in their "logical" order (keeping all subshells of a shell together) when they're full, since their relative positions becomes less important.

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that's the opposite of what the electron configuration for krypton suggests

Look in krpton as in other atom 4s is filled first because its lower in energy, arragement can differ though.

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not as simple as you say it is....

Have a look at this link

Start at element 36 and add electrons until you get to xenon (54)

you'll see that indeed the 5s electrons get added first, but then one gets taken away again as you get to niobium (element 41) and then when you get to palladium (element 46), another electron si removed from 5s, leaving it empty. THEN it gets filled up again and the 5p block behaves itself properly.

If you continue, you'll see that the f-block has all sorts of crazy twists and turns.

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so are you saying that in Krypton, the order of the subshells is altered, leaving us with the 3d and 4s the right way round, unlike the other elements?

The difference stands (i think) on the level of energy needed to maintain the electron in the shell. You need more energy to keep the electron in the filled s subshell than in half-filed d. That is because the half-filled or full-filled d subshell is more stable than s! It is not the same as the old model, by which first you need to complete s than d!

example:

Chromium-24 $1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^5$

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The difference stands (i think) on the level of energy needed to maintain the electron in the shell. You need more energy to keep the electron in the filled s subshell than in half-filed d. That is because the half-filled or full-filled d subshell is more stable than s! It is not the same as the old model, by which first you need to complete s than d!

example:

Chromium-24 $1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^5$

yes very true. there are a lotf of examples of this kind of thing but it doesnt apply to krypton

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Thanks very much for the explanations. I sort of understand the concept now of how the electron configuration of some atoms may differ from the set pattern, but is krypton one of the elements that does differ? If so, does that mean to say that any element higher than krypton beginning with the abbreviation [Kr] would have the same inconsistency in its electron configuration? For example would Rubidium, being [Kr] 5s1 be equivelant to [Ar]3d10 4s2 4p6 5s1? And would the same principle apply to each subsequent element before the next noble gas, thus breaking the electron pattern?

Or perhaps I've totally misinterpreted the way the electron pattern is supposed to be! Thanks once again, this is all helpful information.

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• 3 weeks later...

The reason the electron configuration is listed 3d10 4s2 4p6 is that they list the orbitals in numerical order, not by energy levels or filling order. This also helps newbies remember that the n = 3 shell consists of 3s, 3p, and 3d orbitals. Hope this helps!

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