Ionization energy: Inconsistent logics for determining which IE is larger

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lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

Ionization energy: Inconsistent logics for determining which IE is larger

Post by lotus0618 » Thu Jan 16, 2020 8:43 pm

What I learned from class regarding the first ionization energy and 2nd IE is that the energy depends on whether the element is para or diamagnetic. For instance, removing the first electron from a pair of electrons requires more energy than removing the second electron (cause now the element is unpaired).
However, it's confusing to me the video explains that the second ionziation energy ALWAYS LARGER than the first ionization energy. I disagree with this cause it's not always true. Can you please clarify this for me?

2) With that being said, to the question "how does the first ionziation energy of oxygen compare to that of nitrogen" (attached), why is my answer "the first IE of oxygen is larger than of nitrogen" wrong when we know oxygen has 6 electrons in its p orbital while nitrogen has 5 valance electrons. Thus it's harder to remove the first electron from oxygen.

3) Then the other question "which 3 of the following... characterize S in its uncharged state" has the correct answer as "large first ionization energy". This makes sense using my logic. But I 'purposely' picked 'small first ionization energy' using the video's logic "the second ionization energy ALWAYS LARGER than the first ionization energy".
Basically, which logic is correct?

Thank you!
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ionization energy WTH.png
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ionization energy WTH.png 2.png
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sulfur.png
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NS_Tutor_Mathias
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Joined: Sat Mar 30, 2019 8:39 pm

Re: Ionization energy: Inconsistent logics for determining which IE is larger

Post by NS_Tutor_Mathias » Thu Jan 16, 2020 11:33 pm

1) The second ionization energy will always be larger than the first for any given element: The electron will be feeling a greater effective nuclear force, since it is already a +1 cation. There aren't a lot of other concerns here. Para- versus diamagnetism doesn't factor into it enough to break this trend and shouldn't be a concern for predicting ionization energies on the MCAT.

2) This is a very mean question. The half-filled orbitals of nitrogen are relatively more stable. So even though oxygen exerts a greater effective nuclear force and would be expected to have a higher first ionization energy, it does not. I wouldn't get hung up on rare exceptions like this, for the most part the MCAT doesn't expect you to be this wily.

3) Whether a first ionization energy is small or large means implicitly compared to all other first ionization energies. That is, compared to the first ionization energies of other elements. And the first ionization energy of sulfur is larger than that of many other elements because it is far right on the periodic table (suggesting that a fairly large amount of effective nuclear force is present and that it is hard to strip off even that first electron).
lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

Re: Ionization energy: Inconsistent logics for determining which IE is larger

Post by lotus0618 » Wed Jan 22, 2020 10:24 pm

omg. This is so crazy I wasn't even aware that I made my understanding was based on the assumption of comparing the first ionization energy of one element to the other's. It makes total sense now that obivously on elements are neutral on periodic table so second IE is always larger than the first IE. Now if compare Sc's first ionization energy to Ca's, then the Sc's first IE is lower than Ca's.
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