Section Bank, Physical Sci #25

[GCSDOC]

Can someone help me understand why option B is not the correct answer for #25 in the Physical Science section of the official Section Bank? Thanks in advance!

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[aldol16]

Want to provide more details about the question?

 

[NextStepTutor_3]

Oh, I actually happen to have the section bank open so here you go:

The question was "What is the electron configuration of the Co(II) center found in vitamin B12"? The answer was [Ar] 3d^7 as opposed to [Ar] 4s^2 3d^5.

To answer, we have to remember the difference between the order in which orbitals are filled and the order in which electrons are removed from them. Electrons fill orbitals according to the Aufbau principle, which states that 4s fills before 3d.

But a 3d orbital is not higher-energy than a 4s one, and it is the 4s orbital that is located farther from the nucleus, with a higher principal quantum number. When finding the electron configuration of a cation, you want to remove electrons from the outermost orbitals first. Here, you take two electrons from the 4s orbital before taking any from 3d. You'd do the same if the distinction was, say, between 5s and 4d - you take the electrons from 5s first.

Others can definitely go into more detail here - you can study quantum mechanics forever if you want to. But this is sufficient for the MCAT.

Good luck 🙂

 

[deleted917327]

Oh, I actually happen to have the section bank open so here you go:

The question was "What is the electron configuration of the Co(II) center found in vitamin B12"? The answer was [Ar] 3d^7 as opposed to [Ar] 4s^2 3d^5.

To answer, we have to remember the difference between the order in which orbitals are filled and the order in which electrons are removed from them. Electrons fill orbitals according to the Aufbau principle, which states that 4s fills before 3d.

But a 3d orbital is not higher-energy than a 4s one, and it is the 4s orbital that is located farther from the nucleus, with a higher principal quantum number. When finding the electron configuration of a cation, you want to remove electrons from the outermost orbitals first. Here, you take two electrons from the 4s orbital before taking any from 3d. You'd do the same if the distinction was, say, between 5s and 4d - you take the electrons from 5s first.

Others can definitely go into more detail here - you can study quantum mechanics forever if you want to. But this is sufficient for the MCAT.

Good luck 🙂

OK this makes sense, but where does the rule that states elements prefer a half filled 3d orbital come into play. Like OP I picked B because I was under the assumption that elements were more stable when they had a half filled 3d orbital. Is this only when electrons are filling the orbitals and not being removed? Can you provide an example for the phenomenon that I am talking about?

 

[pretty_positron]

OK this makes sense, but where does the rule that states elements prefer a half filled 3d orbital come into play. Like OP I picked B because I was under the assumption that elements were more stable when they had a half filled 3d orbital. Is this only when electrons are filling the orbitals and not being removed? Can you provide an example for the phenomenon that I am talking about?

Half-filled d-orbitals are the ones with d4 that want to be d5, and then the opposite is true for d9 who want to be d10.