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Chemistry electron configuration question


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#1 Kendall

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Posted 19 June 2017 - 06:15 PM

Orbital Filling Chapter 10

1.      In  a principal energy level that has d orbitals, the s orbital from the next level fills before the d orbitals in the current level.

 

Then in the partial electron configurations chart for elements numbered

24, 29, 41-47, 78,79,

The chart shows 4s1 or 5s1 or 6s1 and then 3d or 4d or 5d respectively, except for element 46 which shows only 4d.

 

I’m sure these constitute some exception, but I don’t see where in the text this is explained (Introductory Chemistry:A foundation by Zumdahl chapter 10). 

 

This is not an honors course. Should I just avoid  giving them those elements to give the electron config for? Or is it important at this level for them to get these strange ones?

 

Thank you,

Kendall

 


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#2 Twigs

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Posted 20 June 2017 - 01:43 PM

I tutor General Chemistry at the local commuity college, so that is my perspective.

 

I've quoted a couple of college level chemistry texts below (bolding mine). Our students need to know the exceptions for Cr, Cu, Mo, and Ag.

 

Cr and Mo are in the 4th column of the D block, so we think the electron configuration should be (n)s2 (n-1)d4, but the configuration  (n)s1 (n-1)d5 is more stable.

 

Cu and Ag, in the 9th column of the D block, we think the configuration should be (n)s2 (n-1)d9, but is (n)s1 (n-1)d10.

 

This is not an honors course. Should I just avoid  giving them those elements to give the electron config for? Or is it important at this level for them to get these strange ones?

If I were in your situation, I would point out the exceptions and give the explanation from OpenStax. Noticing that the examples I used are from the 4th and 9th columns of the D block make those easy to remember, but for high school level chemistry, I would not require the exceptions to be memorized.

 

If you are requiring outside verifyification (such as SATII or CLEP), you will need to check out the scope for those exams.

 

From OpenStax Chemistry (textbook page 313) 

In the case of Cr and Cu, we find that half-filled and completely filled subshells apparently represent conditions of preferred stability. This stability is such that an electron shifts from the 4s into the 3d orbital to gain the extra stability of a half-filled 3d subshell (in Cr) or a filled 3d subshell (in Cu). Other exceptions also occur. For example, niobium (Nb, atomic number 41) is predicted to have the electron configuration [Kr]5s 24d 3 . Experimentally, we observe that its ground-state electron configuration is actually [Kr]5s 14d 4 . We can rationalize this observation by saying that the electron–electron repulsions experienced by pairing the electrons in the 5s orbital are larger than the gap in energy between the 5s and 4d orbitals. There is no simple method to predict the exceptions for atoms where the magnitude of the repulsions between electrons is greater than the small differences in energy between subshells.

 

 

General Chemistry (textbook page 303) (also, in this text, see Appendix D for a complete list of Electron Configurations…)

As we have said, the building-up principle reproduces most of the ground-state configurations correctly. There are some exceptions, however, and chromium (Z 24) is the first we encounter. The building-up principle predicts the configuration [Ar]3d4 4s 2 , though the correct one is found experimentally to be [Ar]3d5 4s 1 . These two configurations are actually very close in total energy because of the closeness in energies of the 3d and 4s orbitals (Figure 8.8). For that reason, small effects can influence which of the two configurations is actually lower in energy. Copper (Z 29) is another exception to the building-up principle, which predicts the con- figuration [Ar]3d9 4s 2 , although experiment shows the ground-state configuration to be [Ar]3d104s 1 .

 

 

Los Alamos Periodic Table with electron configurations.

 

Best wishes.


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#3 Kendall

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Posted 20 June 2017 - 08:11 PM

Thank you.  This is very helpful and has confirmed that the text left that out, and I shouldn't then expect them to figure it out from what they read in the text. I will follow you advice and share it with them, but not require that they memorize it at this level.


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