Electron config 3d 4s or 4s 3d

So ive been taught that the electron configuration order is 1s 2s 2p 3s 3p 4s 3d 4p etc. But ive noticed that in my revision guide they say that is the correct order but then still write electron configurations as 3s 3p 3d 4s. So, now i am confused - for what elements does the 3p 4s 3d rule apply? Also for what transition metals or other elements does the electron go into the next energy level e.g. Ive noticed that for chromium the electron config ends in 3d5 4s1 rather than 3d6 but in the book it says it is 3d5 4s1 rather than 3d4 4s2 (isnt it supposed to be 3d6 because 3d can hold max. 10 electrons?) Im so confused could someone please explain?

So ive been taught that the electron configuration order is 1s 2s 2p 3s 3p 4s 3d 4p etc. But ive noticed that in my revision guide they say that is the correct order but then still write electron configurations as 3s 3p 3d 4s. So, now i am confused - for what elements does the 3p 4s 3d rule apply? Also for what transition metals or other elements does the electron go into the next energy level e.g. Ive noticed that for chromium the electron config ends in 3d5 4s1 rather than 3d6 but in the book it says it is 3d5 4s1 rather than 3d4 4s2 (isnt it supposed to be 3d6 because 3d can hold max. 10 electrons?) Im so confused could someone please explain?

Wtf is this
what happened to 2, 8, 8 :'(

Are you doing a level? - If you havent got to this part yet then good luck i miss the 2.8.8 😕 but dont worry the basic idea of 1s 2s 2p etc. is pretty simple to understand.

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nah I'm just year 11, they did say "well it's not this simple but this is all you have to know so far" so yeah I'm guessing they meant what you're talking about now?

Ahh right - .... y11 - seems so easy at gsce 😭 now. Anyway if you do a level chem they're basically like so what you learnt at GCSE was wrong so forget this forget that blah blah blah. I remember being told you only need to know it up to 2.8.8.8 i think? Are you doing triple?

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I think for Chromium the electron configuration is [Ar] 3d5 4s1 because the energy level difference between 3d and 4s is so small that it is actually favorable just to have the one electron in the last electron shell of 3d. Pairing the electrons like 3d4 4s2, due to repulsion raises the energy of the system more than it would to just put a single electron in the last orbital of 3d and the 4s orbital, thus it's energetically favorable.

In terms of what order to write electron configuration down in, I don't think it matters a huge amount, I don't think you'll be penalised in an exam as long as the actual orbitals are right. The transition metal "exceptions" are for Copper and Chromium. The reason why this happens is because even though the 4s shell is lower energy than the 3d shell (which is why it normally fills up before), the electron density in the 4s shell is closer to the nucleus than the 3d shell which means paired electrons in the 4s shell are closer together than paired electrons in the 3d shell, so they experience more repulsion. This is why in the case of Cu and Cr, it's 4s1 3d5 rather than 4s2 3d4 because it's more energetically stable to have 5 unpaired electrons in the 3d shell and 1 unpaired in the 4s shell than to have paired electrons in the 4s and 4 unpaired electrons in the 3d.

Please read the info at the website I cited above.

I think for Chromium the electron configuration is [Ar] 3d5 4s1 because the energy level difference between 3d and 4s is so small that it is actually favorable just to have the one electron in the last electron shell of 3d. Pairing the electrons like 3d4 4s2, due to repulsion raises the energy of the system more than it would to just put a single electron in the last orbital of 3d and the 4s orbital, thus it's energetically favorable.

Completely irrelevant but im from Yorkshire (noticed your nickname) yay.

Posted from TSR Mobile

So ive been taught that the electron configuration order is 1s 2s 2p 3s 3p 4s 3d 4p etc. But ive noticed that in my revision guide they say that is the correct order but then still write electron configurations as 3s 3p 3d 4s. So, now i am confused - for what elements does the 3p 4s 3d rule apply? Also for what transition metals or other elements does the electron go into the next energy level e.g. Ive noticed that for chromium the electron config ends in 3d5 4s1 rather than 3d6 but in the book it says it is 3d5 4s1 rather than 3d4 4s2 (isnt it supposed to be 3d6 because 3d can hold max. 10 electrons?) Im so confused could someone please explain?

I was also taught that 4s comes before 3d. My teacher said it's right, so I'm assuming so. Also, which revision guide is it that you are using?