Number of unpaired electrons question

[johnny13oi]

This is from the sample problems provided by the ADA on their website

1s2 2s2 2p6 3s2 3p2. What is the number of
unpaired electrons in this atom?

I thought it would be 2 but the answer key lists it as 3. I don't understand why.

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[dmdluffy]

It looks like one of the exceptions where an electron from the s subshell gets promoted to the p subshell because half filled subshells are more stable than not half filled. So the s2p2 would end up being s1p3 which would make two half filled subshells which is more stable than one filled and one partially filled. Hope that helps!

 

[dmdluffy]

In retrospect, that still isn't 3 unpaired electrons soooo maybe ADA is wrong?

 

[predental89]

It looks like one of the exceptions where an electron from the s subshell gets promoted to the p subshell because half filled subshells are more stable than not half filled. So the s2p2 would end up being s1p3 which would make two half filled subshells which is more stable than one filled and one partially filled. Hope that helps!

Yea I agree with you except that leaves you with 4 unpaired! ADA might be wrong on this one.

 

[buckey]

you have to be careful to trust the ada answer key as there is usually errors. this is one of them. there are two unpaired electrons as written by that electron configuration. the confusion may stem from the fact that (edit) certain exceptions can have an s electron promoteded to a d subshell to make both of the orbitals half filled. This question involves s and p subshess, but even in that idea were enacted the answer would be 4, not three. exceptions to know would be chromium, copper, molybdenum, and silver

 

[SEAirLand]

And to add to the confusion, this trend is likely not part of the p orbitals.

Cr, Mo and Cu, Ag, Au do this.

And as noted above, even if it did, it would be 4 unpaired electrons, not 3.

 

[crmdental]

Its the e- configuration for Silicon (Si), it isn't one of the exception so it should be 2.

 

[AlbinoPolarBear]

Its the e- configuration for Silicon (Si), it isn't one of the exception so it should be 2.

+1

Answer is wrong on the ADA answer key. Correct answer is 2.

 

[DrRoyal Pains]

Theres a thread on here somewhere that designated all the problems that have errors.

 

[johnny13oi]

Just to make sure though, the s electrons would not be promoted in this case right? I thought the electrons being promoted usually occurred only when trying to fill the D orbitals and even then it only happens when trying to fill it with 5 electrons with one spin or 10 electrons paired in the d orbital and all other cases it would not be promoted. Is this correct?

 

[needzmoar]

Just to make sure though, the s electrons would not be promoted in this case right? I thought the electrons being promoted usually occurred only when trying to fill the D orbitals and even then it only happens when trying to fill it with 5 electrons with one spin or 10 electrons paired in the d orbital and all other cases it would not be promoted. Is this correct?

Yes

 

[johnny13oi]

Thanks, I was starting to doubt my Chemistry knowledge even though I have a BS in Chemistry lol.

 

[needzmoar]

Thanks, I was starting to doubt my Chemistry knowledge even though I have a BS in Chemistry lol.

no worries

 

[johnny13oi]

Okay so if the element was say Oxygen. Would there be 4 unpaired electrons? Does an electron from 2s get promoted to 2p to make it 3 unpaired in the p shell and one unpaired in the s shell? I just glossed over this in the Kaplan book and it confused me because I thought the promotion deal does not happen unless for d orbitals.

 

[needzmoar]

Okay so if the element was say Oxygen. Would there be 4 unpaired electrons? Does an electron from 2s get promoted to 2p to make it 3 unpaired in the p shell and one unpaired in the s shell? I just glossed over this in the Kaplan book and it confused me because I thought the promotion deal does not happen unless for d orbitals.

For a neutral oxygen atom in its ground state, no, it shouldn't. I mean, think about it, if there were for 4 unpaired electrons we would expect oxygen to be tetravalent like carbon, but it's not.

 

[buckey]

to elaborate on the oxygen, the p orbital has 3 electron domains and oxygen has 4 p electrons. the first electron adds to the first domain, the second to the second, the third to the third, and the fourth adds to the 1st domain pairing up that first electron leaving 2 electrons remaining unpaired.

 

[johnny13oi]

Oh I am so sorry, I meant carbon not oxygen. I have been studying quite a lot and got them mixed up. But yeah in my question above does carbon promote the 2s electron to the 2p?

 

[buckey]

in carbons ground state it has only 2 unpaired electrons as the other two are in the s subshell. carbon does promote the electrons in the sense that 1 is in the s shell and 3 are in the p shell but rather blends the s subshell and p subshell together to form an sp3 subshell in which all of the electrons are of the same energy level and the four orbitals have one electron in each.

 

[UCB05]

keep in mind not to read so deeply for the exam. The question is to identify the number of unpaired electrons in
3s2 3p2 - which is 2

If you answer 4 because an s orbital electron might be promoted so that you have
3s1 3p3, you will be marked wrong no matter how much you want to argue, because that is not the electron configuration you were given.