GS-2 Physical Sciences Question 50

[Guest]

Hi, the answer to #50 is A. --b/c chromium has a half filled d orbital; could you clarify this for me..I thought Mn had a half-filled d orbital...Also, When I clicked end test it quickly showed me a grade and when I closed out of the window and tried to view it in the history CBT exams, it gave a long list of scores that didn't appear to be mine so I closed out of it and went back to my history, and it showed I had never taken the test. Just thought I would let you know my grading has been fairly shaky on most the tests.

[admin]

There are 2 layers to this question.

The last 2 terms for the electronic configuration for Mn is indeed 4s2 3d5 as you suggested. One would expect that Cr would be 4s2 3d4 but because of the increased stability of half filled orbital, the actual configuration for Cr is 4s1 3d5 (2 half filled orbitals, parallel spins, very stable).

Regarding the scoring of your test, since we converted to the new format last week, there have been some bumps which should be ironed out in the next few days.

However, as per the instructions, the tests are only corrected if you click End Test. While any bugs are being addressed over the next few days, if you want to be sure your test is corrected, then before End Test, minimize and make sure that you are logged into MCAT-prep.com then End Test.

[tazotea]

so does that mean that the correct answer is not A then? because both chromium and manganese both have half filled d-orbitals?

[admin]

Good question. In the first ionization potential, Mn does not lose d5 status because it now has the configuration of Cr. Thus Mn remains stable. However, Cr, which was just hanging on to the more stable d5 status, now will lose it. Of the 2 atoms, it is Cr which will have a rather unexpectedly high IE because of loss of stability.

Out of 4 real MCATs, you might get one question like this one.

[kbshah6109]

I know that the electron configuration for Cr is 4s1 3d5
and for Mn is 4s2 3d5.

Looking at this, I would say that it would be more likely for Cr to lose it's outer most electron, bc it will lose the one electron in the s orbital. As for Mn losing an electron, it will lost one from the s orbital as well, but it will want to hold on to that electron tightly because it is a filled orbital so it will have a higher ionization energy.

Thus, I don't think this is a valid questions bc I would say that Cr has a lower first ionization energy than Mn (opposite to the question being asked).

I was curious about this so I looked up the exact numbers and it shows in fact that Mn has a higher first ionization energy than Cr.

????

[admin]

I certainly would not argue about the issue of validity, only to say that according to the AAMC, 1-2 questions on the real exam should be considered test questions and would therefore not be graded. Sometimes these "test questions" are simply unreasonable. Consider this a test question. There is never anything wrong to look at a couple of questions in physical sciences that seem challenging, do your best, don't waste time and just move on. Even with a few mistakes, it is still possible, in the sciences, to score a 14.

Keep up the good work.

[newno558362]

Alright, so I was making notes to conform my thinking to that of the given answer... then i stumbled upon this thread.

So the conclusion for this question is that Mn would, without a doubt, have the higher first ionization energy?

I always just assumed a filled orbital was more stable than a 1/2 filled orbital... therefore, thats why I reasoned Mn would have the higher first ionization energy.

ie. 4s2 has a greater Ionization Energy than 4s1

BUT BUT BUT

Could a 4s1 atom have a higher ionization energy becasue it absolutely does NOT want to lose it's electron.... while an 4s2 atom could potentially lose an electron and still be mostly stable sitting at 4s1??

What about other similar problems, say with 3s2 vs 3s1 etc. ?

I understand what this question was TRYING to do, but in the end it just ended up confusing my "rules" for for this concept!


Thanks

[admin]

The rule about subshells revealed in this problem only refers to d orbitals (not s).

A d subshell that is half-filled or full (ie 5 or 10 electrons) is more stable than the s subshell of the next shell.

See "Exceptions in 3d, 4d, 5d": http://en.wikipedia.org/wiki/Electron_configuration

[Nirfoochil2984]

First of all the question stem contains a false statement. Mn does not have an ionization energy less than chromium.

Mn = 717.3
Cr = 652.9

Here is the explanation that is given for this question.

[i]A half-filled electron shell contains subshells each of which possesses one unpaired electron. All unpaired electrons have parallel spins and this half-filled system confers extra stability to the atom. [b]Thus more energy than would be expected[/b] (from a consideration of the ionization energies of other elements in the same period)[b] is required to remove a valent electron from the neutral atom.[/b][/i]

This explanation is basically saying the more stable atom will have the highest ionization energy. Since Manganese has a half filled d AND a full s, it is the most stable. Chromium, on the other hand, has a[b] [b]half filled s [/b]orbital[/b] and a half filled d. Therefore, since Mn is more stable, the ionization energy would have to be greater (just as explanation states)

I suspect that there was an error with the test

[tjk447963]

Also, according to periodic trends, ionization energy increases as you go to the right of a period. Since Mn is to the right of Cr, its ionization energy is greater.

[dnpgr16513]

Totally agree..

[hua8986059]

[bribercar2809]

The 1st ionization energy of chromium is 652.8 kJ mol-1

The 1st ionization energy of manganese is 717.4 kJ mol-1

I don't understand your answer?

[rjlee886341]