Atomic Orbital Energy - Really Confused.

[daFendi]

So, I'm really confused about questions regarding atomic orbitals; specifically regarding higher shell p and d orbitals.

I've encountered a lot of practice questions that ask about the "highest" energy orbital, or to write out the electron configuration of an atom, and every time I see, for example, that a 4s orbital is said to have a higher energy than a 3d orbital...but this doesnt agree with anything of learned at uni, or online.

Can someone please clarify this stuff for me, and what we're meant to think going into the mcat.

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[Jepstein30]

So, I'm really confused about questions regarding atomic orbitals; specifically regarding higher shell p and d orbitals.

I've encountered a lot of practice questions that ask about the "highest" energy orbital, or to write out the electron configuration of an atom, and every time I see, for example, that a 4s orbital is said to have a higher energy than a 3d orbital...but this doesnt agree with anything of learned at uni, or online.

Can someone please clarify this stuff for me, and what we're meant to think going into the mcat.

3d is higher energy than 4s.

1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
etc.

Energy level is given by diagonal arrow starting from 1s.

1s - 2s - 2p - 3s - 3p -4s - 3d 4p -5s - 4d - 5p - 6s etc.

Also, just take a look at the periodic table. All of the transition metals in the 4th row have filled 4s orbitals before they begin filling their 3d orbitals. Hund's rule (I think) states lowest energy orbitals filled first..

 

[daFendi]

3d is higher energy than 4s.

1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
etc.

Energy level is given by diagonal arrow starting from 1s.

1s - 2s - 2p - 3s - 3p -4s - 3d 4p -5s - 4d - 5p - 6s etc.

Also, just take a look at the periodic table. All of the transition metals in the 4th row have filled 4s orbitals before they begin filling their 3d orbitals. Hund's rule (I think) states lowest energy orbitals filled first..

Then why does every question I encounter...e.g. this one : http://www.mcatquestion.com/findquestion.php?arg1=1487
have the S orbital as the highest in energy?

and why do EK questions regarding configuration give answers that end in 3d^64s^2?

 

[Jepstein30]

Then why does every question I encounter...e.g. this one : http://www.mcatquestion.com/findquestion.php?arg1=1487
have the S orbital as the highest in energy?

and why do EK questions regarding configuration give answers that end in 3d^64s^2?

Umm.. you aren't reading the question right.

Its asking which orbitals exist in the HIGHEST energy level of Chromium. Chromium is 4s^2, 3d^4 , not which orbital has higher energy.

4 is the highest energy level, 3d is the highest energy orbital however for Chromium.

And no clue why they give configuration like that. 3d and 4s are pretty interchangeable.. you'll have a lot of transition metals that favor a half filled d orbital over a filled s orbital, etc.

 

[BB8730]

Umm.. you aren't reading the question right.

Its asking which orbitals exist in the HIGHEST energy level of Chromium. Chromium is 4s^2, 3d^4 , not which orbital has higher energy.

4 is the highest energy level, 3d is the highest energy orbital however for Chromium.

And no clue why they give configuration like that. 3d and 4s are pretty interchangeable.. you'll have a lot of transition metals that favor a half filled d orbital over a filled s orbital, etc.

That's not correct.

Cu, Au, Ag, Cr, and Mo are exceptions to Madelung's rule. Half filled or completely filled subshells are more stable. Chromium's configuration is [Ar] 4s^1 3d^5.

So, I'm really confused about questions regarding atomic orbitals; specifically regarding higher shell p and d orbitals.

I've encountered a lot of practice questions that ask about the "highest" energy orbital, or to write out the electron configuration of an atom, and every time I see, for example, that a 4s orbital is said to have a higher energy than a 3d orbital...but this doesnt agree with anything of learned at uni, or online.

Can someone please clarify this stuff for me, and what we're meant to think going into the mcat.

The quantum number is the energy level, so the 4s orbital does have higher energy than 3d. The reason why the 4s orbital fills before the 3d orbital is complicated and it has to do with the electrons in the different orbitals interacting and changing the energy levels - not something you need to know for the MCAT.

 

[daFendi]

That's not correct.

Cu, Au, Ag, Cr, and Mo are exceptions to Madelung's rule. Half filled or completely filled subshells are more stable. Chromium's configuration is [Ar] 4s^1 3d^5.



The quantum number is the energy level, so the 4s orbital does have higher energy than 3d. The reason why the 4s orbital fills before the 3d orbital is complicated and it has to do with the electrons in the different orbitals interacting and changing the energy levels - not something you need to know for the MCAT.

So exactly what is the reason that the S orbital is the highest energy orbital in chromium? is it because the half-filled 3d orbital is more stable than a typical 3d orbital?

 

[BB8730]

So exactly what is the reason that the S orbital is the highest energy orbital in chromium? is it because the half-filled 3d orbital is more stable than a typical 3d orbital?

The orbital with the higher quantum number will always have the higher energy. What makes chromium different is that it prefers a half-filled 4s orbital in order to have a half-filled 3d orbital.

 

[daFendi]

The orbital with the higher quantum number will always have the higher energy. What makes chromium different is that it prefers a half-filled 4s orbital in order to have a half-filled 3d orbital.

ok, so just to clarify the idea, say we take something like an Fe atom. Is its 4s orbital higher in energy than its non-half filled 3d orbital?

Edit: if higher quantum number = higher energy, why are 3d orbitals filled after 4s orbitals? ie. as on the period trends of transition metals.

 

[BB8730]

ok, so just to clarify the idea, say we take something like an Fe atom. Is its 4s orbital higher in energy than its non-half filled 3d orbital?

Yes.

This is also the reason why electrons are removed from the 4s shell before the 3d shell when electrons are lost. For example, Zn = [Ar] 4s^2 3d^10 and Zn2+ = [Ar] 3d^10

Edit: if higher quantum number = higher energy, why are 3d orbitals filled after 4s orbitals? ie. as on the period trends of transition metals

Look at my post #5 above

 

[daFendi]

Yes.

This is also the reason why electrons are removed from the 4s shell before the 3d shell when electrons are lost. For example, Zn = [Ar] 4s^2 3d^10 and Zn2+ = [Ar] 3d^10


Look at my post #5 above

as per post #5, so in cases where electron repulsion in the 3d orbitals causes a change in energy, the 3d orbitals do actually have a higher energy than the 4s? but as you mentioned, we do not need to know this for the mcat.

 

[BB8730]

as per post #5, so in cases where electron repulsion in the 3d orbitals causes a change in energy, the 3d orbitals do actually have a higher energy than the 4s? but as you mentioned, we do not need to know this for the mcat.

Yeah basically. When the 3d and 4s orbitals are unfilled, the 4s orbital has the lower energy, which is why it fills first. When there are electrons in the 3d and 4s orbitals, the interaction makes the 4s orbital have a higher energy. It's way beyond what we would need to know for the MCAT.

Just remember that the higher quantum number = higher energy, and that if electrons are lost, they will be pulled from the orbital with the higher quantum number. BUT when filling orbitals, fill them by convention - 4s first, then 3d.

 

[daFendi]

Thanks very much for the clarification.

 

[BB8730]

You're welcome. 🙂