hjkiire
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when does 3d come before 4s? are there specific elements that have this...? why doesn't the 3d shell fill before using 4s?
i have a lotttt of questions about it so if someone could sorta explain electron configuration to me that would be abSolUTEly ammazzinngg
cheers lads (Btw i do aqa as chemistry.. idk if that changes things but if it does then i guess that might be useful?)
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cmsn
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4s subshell always fills before the 3d subshell except some exceptions.

For Chromium, instead of [Ar]4s2,3d4, the 4s subshell loses an electron to the 3d subshell, so chromium is actually [Ar]4s1,3d5.
The same happens for copper, which is [Ar]4s1,3d10 rather than [Ar]4s2,3d9

When forming ions, electrons are lost from the 4s subshell first. For example the ion Cu+ would be [Ar]4s0,3d10 (you can just not write the 4s if you like) as opposed to [Ar]4s1,3d9
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hjkiire
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ahhh ye cool, that makes sense now
thank you!!
(Original post by cmsn)
4s subshell always fills before the 3d subshell except some exceptions.

For Chromium, instead of [Ar]4s2,3d4, the 4s subshell loses an electron to the 3d subshell, so chromium is actually [Ar]4s1,3d5.
The same happens for copper, which is [Ar]4s1,3d10 rather than [Ar]4s2,3d9

When forming ions, electrons are lost from the 4s subshell first. For example the ion Cu+ would be [Ar]4s0,3d10 (you can just not write the 4s if you like) as opposed to [Ar]4s1,3d9
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cmsn
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(Original post by hjkiire)
ahhh ye cool, that makes sense now
thank you!!
You’re welcome
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Interea
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I always remember it using this diagram for filling the subshells, easy to construct in exams if you need a reminder!
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cmsn
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(Original post by Interea)
I always remember it using this diagram for filling the subshells, easy to construct in exams if you need a reminder!
That’s cool I’ve never seen that before

A level you only need up to 4p6 so would be even simpler than that image to reproduce in exam if you forgot
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Interea
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Haha yep, it has saved me in many an A level mock
(Original post by cmsn)
That’s cool I’ve never seen that before

A level you only need up to 4p6 so would be even simpler than that image to reproduce in exam if you forgot
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cmsn
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(Original post by Interea)
Haha yep, it has saved me in many an A level mock
The only questions they ever ask about it are the exception cases of chromium and copper to try and catch you out so you would need to remember these as the diagram doesn’t do these exceptions
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Interea
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Very true, that sneaky repulsion always used to catch me out when we first learned it
(Original post by cmsn)
The only questions they ever ask about it are the exception cases of chromium and copper to try and catch you out so you would need to remember these as the diagram doesn’t do these exceptions
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cmsn
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(Original post by Interea)
Very true, that sneaky repulsion always used to catch me out when we first learned it
It just becomes second nature
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David Tan
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(Original post by hjkiire)
when does 3d come before 4s? are there specific elements that have this...? why doesn't the 3d shell fill before using 4s?
i have a lotttt of questions about it so if someone could sorta explain electron configuration to me that would be abSolUTEly ammazzinngg
cheers lads (Btw i do aqa as chemistry.. idk if that changes things but if it does then i guess that might be useful?)
From potassium onward, electron has to fill the 4s orbital first before filling the 3d orbital. That is because the 4s orbital has a slightly lower energy level than the 3d orbital. However the moment an orbital contains electrons, the energy level of that electron-containing orbital will be raised relative to the orbital that doesn't house an electron. Hence when a 4s orbital is filled with an electron, its energy level will be raised. However, due to the very small energy difference between 4s and 3d orbitals, the 4s orbital now has a HIGHER energy level than the 3d orbital. That explains why school teachers always tell their students to "fill up the 4s orbitals with electron first, but at the same time remove electron from the 4s orbital first as well", which seems counter-intuitive at first sight. Since the 4s orbital (containing electrons) has a higher energy level, it also implies that it is further away from the nucleus and its electron can be removed more easily. This "orbital switching" doesn't occur for 1s and 2s/ 2s and 2p/ 2p and 3s/ 3s and 3p etc because the energy gap is too big to be overcomed.

Let's start with the terminology first: there is a difference between principal quantum shell, subshell and orbital. We only learn 4 types of subshells and they are s, p, d and f. There is 1 orbital in a s-subshell, 3 orbitals in a p-subshell, 5 orbitals in a d-subshell and 7 orbitals in a f-subshell.

Some school teachers teach their students a diagram with diagonal arrows that show the order of electron filling. Actually, that diagram is not necessary. We can just refer to the Periodic Table and read off the electronic configuration directly. Hope it helps.
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