Which subshell does the 30th electron belong to?

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"The 30th electron of 35Br belongs to 4s."

I saw this sentence as a part of a question in my high school chem book, but I don't know whether it's correct or not.

It's about the order of filling subshells with electrons;

As far as I know, the 29th electron goes to 3d & makes it full, so the next one should go to 4s:

28: 3d⁸ 4s²

29: 3d¹⁰ 4s¹

30: 3d¹⁰ 4s²

But if that's correct, why exactly does 30Zn (or some other elements like 48Cd or 80Hg) belong to the d-block?! (While their last electron belongs to s subshell.)

I would really appreciate it if you could answer as soon as possible.

AM

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Actually, the 4s is filled before the 3d. I suggest looking at the Aufbau principle to help with this, but you can also figure out out by looking at the periodic table if you know where the s, p, d and f blocks are.

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Yes i know about the Aufbau principle, but as I've expressed in my first post, Zn, Cd and Hg are exceptions (also 24Cr, 42Mo & 74W);

For example when an atom has 20 electrons (Ca), its 4s is filled while the 3d is empty, and in an atom with 21 electrons, the 21th e belongs to 3d; so we expect the next atoms to have a filled 4s subshell while their 3d is not filled yet, but when we reach Z=24 (Cr), we'll realize that its electron configuration is:

24Cr: [Ar] 3d⁵ 4s¹ (instead of [Ar] 3d⁴ 4s²)

And this is also true for 29Cu; which has a filled 3d but a half filled 4s:

29Cu: [Ar] 3d¹⁰ 4s¹

(I think it might happen due to the more stability of filled & half filled subshells...)

So, I really expect the 30th electron to go to the 4s subshell and fill it, as we see in Zn's electron configuration:

30Zn: [Ar] 3d¹⁰ 4s²

Thus, if the last electron of Zn (30th) belongs to s subshell, why does zinc belong to the d-block?!

How elements are separated to s p d f blocks, according to my high school book:

If the last electron of an atom belongs to a special subshell, the atom would be in the corresponding block.

(I know it might be simplified, that's a high school book afterall.)

So I feel a contrast between the subshell to which the 30th electron belongs, and the definition of blocks.

Please let me know if I am wrong

& Sorry for non scientific terminology or grammatical mistakes, I am not a native English speaker you know

Edited by NerdShift
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From Scandium [Ar] 3d^1 4s^2 to Zinc [Ar] 3d^10 4 s^2 the d-Orbitales are filled. So all of them are d- Block Elements. In some cases half shell and very close to full shell s- electrons move to d-block. Consider all have a s^2 shell and free d Orbitals. The 30th. electron fill the d-Orbital Check, chromium, copper and palladium for irregularities.

Edited by chenbeier
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On 9/30/2020 at 2:57 AM, NerdShift said:

Yes i know about the Aufbau principle, but as I've expressed in my first post, Zn, Cd and Hg are exceptions (also 24Cr, 42Mo & 74W);

For example when an atom has 20 electrons (Ca), its 4s is filled while the 3d is empty, and in an atom with 21 electrons, the 21th e belongs to 3d; so we expect the next atoms to have a filled 4s subshell while their 3d is not filled yet, but when we reach Z=24 (Cr), we'll realize that its electron configuration is:

24Cr: [Ar] 3d⁵ 4s¹ (instead of [Ar] 3d⁴ 4s²)

And this is also true for 29Cu; which has a filled 3d but a half filled 4s:

29Cu: [Ar] 3d¹⁰ 4s¹

(I think it might happen due to the more stability of filled & half filled subshells...)

So, I really expect the 30th electron to go to the 4s subshell and fill it, as we see in Zn's electron configuration:

30Zn: [Ar] 3d¹⁰ 4s²

Thus, if the last electron of Zn (30th) belongs to s subshell, why does zinc belong to the d-block?!

How elements are separated to s p d f blocks, according to my high school book:

If the last electron of an atom belongs to a special subshell, the atom would be in the corresponding block.

(I know it might be simplified, that's a high school book afterall.)

So I feel a contrast between the subshell to which the 30th electron belongs, and the definition of blocks.

Please let me know if I am wrong

& Sorry for non scientific terminology or grammatical mistakes, I am not a native English speaker you know

Yes, sorry, I was writing on my phone so I didn't have time for a full reply.

The two exceptions you mention are actually a little misleading. I believe that in reality, they do fill the 4s orbital first, then as electrons start filling up the 3d orbitals one of the 4s electrons moves into the 3d to give 3d54s1 / 3d104s1. It is not a case of only putting one electron into the 4 s and then filling up the 3d orbitals, which is maybe where you are confused with Zn. The reasons for this come down to stability, as you mentioned. It is more favourable to have a completely full or half full set of d orbitals.

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Quote
On 9/30/2020 at 7:07 PM, chenbeier said:

all of them are d- Block Elements. In some cases half shell and very close to full shell s- electrons move to d-block.

Quote
19 hours ago, hypervalent_iodine said:

believe that in reality, they do fill the 4s orbital first, then as electrons start filling up the 3d orbitals one of the 4s electrons moves into the 3d

Thank you both for your replies.

But I'm still a little confused;

Both of you mentioned that a '4s electron' moves to 3d subshell. So, it isn't a S electron any longer, or is it?!

I don't have a problem with the movement of that electron, I can't realize what exactly happens after that; let's say the 29th electron goes to 3d, so we have 3d⁹ but it isn't stable so one 4s electron moves to the 3d subshell and makes it 3d¹⁰4s¹.

But how about the 30th electron?!

I can only imagine 2 possible ways:

1) Because the 3d is already filled (3d¹⁰4s¹), the 30th electron goes to 4s to fill it.

2) that 4s electron which had moved to 3d to fill it, will return to the 4s and the 30th electron goes to 3d.

I think the 1st one is more logical, but the second one is the only way to convince Zn belongs to d block (according to my book).

Which one is correct?

Or if none of them, where exactly does the 30th electron go?!

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Electrons are not different,  so it doesn't matter if the d Orbitale will filled or the s Orbital.

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10 minutes ago, chenbeier said:

Electrons are not different

Yes, i know chenbeier. Electrons are not actually distinguishable; but it was the question i had to answer;

Do you think it is basically wrong?

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Nothing will be filled, because copper and zinc individually elements.

Copper has stabil configuration 3d10 4s1 and zinc has 3d10 4s2

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So my first question is basically wrong?!

Yes

Ok, thnx

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• 10 months later...

There are several Anomalous Elements in the Transition Metals whose Electrons take from the S Orbital or add to the S Orbital. This is illustrated in the image of the Periodic Table attached, using simple lines to show that these D-Block Elements actually add or take Electrons to/from the S Orbital.

Also, 74W Tungsten is NOT one of the Anomalies/Exceptions as it's Electron Configuration is [Xe] 6s2,4f14,5d4 which is exactly what is expected given its position in the Periodic Table.

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The energy level of each s, p d f etc orbital varies with atomic number and there are some crossovers.

Here is a plot, showing how complicated it gets as Z increases

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