The following question is from "The Official Guide to the HiSET Exam, Second Edition 2nd Edition", published by McGraw Hill, 2nd edition (March 25, 2016).2016 text, p. 75 (ISBN10: 1259640795).

Along with the question, students are given a periodic table (going up to element 109, Mt / meitnerium) that only contains element symbols (without atomic numbers or any other numbers).

Which group of elements has valence electrons in the same energy level?

A. B, Si, As, Te
B. Ga, As, Se, Kr
C. H, Li, Na, K
D. La, Pr, Os, Hg

The correct, given answer is B. I get this as all elements are on energy level 4.

But why is choice D not correct? To my understanding, all of these elements are on energy level 6 (and all seem to have valence electrons on 6th level).

Is the book incorrect or am I missing something?

I'm good with all the other chemistry questions and can explain them to the students. I'm stuck on this one and text fails to explain the answer. Again I think I understand why B works, but don't understand fully why D can't. Both answers feature elements each a part of the same respective horizontal row or energy level / period.

  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Chemistry Meta, or in Chemistry Chat. Comments continuing discussion may be removed. $\endgroup$
    – Buck Thorn
    Nov 25 at 4:21

2 Answers 2


Lanthanides are "f-block" elements, filling the 4f shell in series.

Mercury and Osmium have filled their f shells with 14 electrons.

Valence electrons are not always the outermost.

  • $\begingroup$ I had edited the answer to include the electronic configurations of the elements and explain what was the valence shell actually by Hund's rule. It was rejected though and I have submitted yet again. For those in the privileged team, please do know that this is quite relevant only and not deviating from the original interest of DrMoishe Pippik. I don't seem to find a reason why my edit should be irrelevant. $\endgroup$ Nov 25 at 14:30
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    $\begingroup$ @HarikrishnanM, sorry, I'd neither approved nor rejected it. If you'd like, make it an additional answer, shedding more light on this! $\endgroup$ Nov 25 at 23:17
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    $\begingroup$ @HarikrishnanM please write a new answer. Your edit guess further than the original. $\endgroup$ Nov 26 at 12:46
  • $\begingroup$ @Martin-マーチン even if I write my new answer most of the points will match DrMoishe Pippik's answer except for the electronic configs added in. I don't seem to have a reason for my answer to be unique and it's better off I edited Dr Moishe Pippik's answer as 2 answers with only EC added is gonna be redundant. And given ChemSE's strict intolerance to that, I don't wish to risk getting bad remarks from everyone. $\endgroup$ Nov 26 at 14:11
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    $\begingroup$ @HarikrishnanM, No risk - if your answer is unappreciated, delete it. But get credit yourself for making this more clear! $\endgroup$ Nov 26 at 18:35

Hg : [Xe] $\ce{4f^14 5d^10 6s^2}$

Os : [Xe] $\ce{4f^14 5d^6 6s^2}$

Pm : [Xe] $\ce{4f^3 6s^2}$

La : [Xe] $\ce{5d^1 6s^2}$

Over here, we see that in Hg, Os and La 5d shell is the last filled shell and in Pm 4f is filled last by Hund's rule. Thus Pm is in a lower energy level than the others.

Moral of the story is that valence electrons are not always the outermost.

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    $\begingroup$ This is very helpful. When looking at a more detailed periodic table that includes electron configurations I noticed that the final shell for the 4 elements never changed (always 6s^2) compared to a contrasting row like row 4 where we have Ga, As, Se, Kr (Gallium, Arsenic, Selenium, Krypton) where the final shell does change in number (4p^1, 4p^3, 4p^4, 4p^5). Intuitively I had wondered if that might be a clue--but couldn't conceptualize it--as I was assuming valence electrons were always of the outmost shell. Now I know otherwise. MUCH appreciated. $\endgroup$ Nov 27 at 15:41
  • $\begingroup$ @Sail2DeepBlue - do remember, always follow Hund's rule. Whatever comes last in Hund's rule is the valence orbital. $\endgroup$ Nov 27 at 16:05

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