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Page 12 of the Study Guide for Campbell Biology, 11th Edition has the following question:

  1. A carbon atom has 6 neutrons. How many protons are present in the nucleus of a carbon atom?

    A. 12
    B. 8
    C. 6
    D. Can't tell; more information is required.

The answer given on page 431 is D. I think the answer should be C since carbon has 6 protons.

What type of additional information might be required?

I understand that elements are defined by their number of protons, but maybe there is some strange edge case like a moment during nuclear fusion? Is there a case when a carbon atom does not have 6 protons?

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    $\begingroup$ The answer may technically be (d), because without any external knowledge of the Periodic Table, there is no way to infer the number of protons from the number of neutrons... $\endgroup$ – orthocresol Oct 21 '18 at 21:38
  • $\begingroup$ @orthocresol That's an interesting idea, thanks. $\endgroup$ – freeradical Oct 21 '18 at 21:46
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    $\begingroup$ @orthocresol it still seems to be a poor question because they do include the reference to carbon. If they just said an atom, that would be better. $\endgroup$ – Tyberius Oct 22 '18 at 1:46
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    $\begingroup$ All isotopes of carbon have 6 protons. That of course is what makes the atoms carbon. The number of neutrons can, and does, vary. Pure and simple the question has a horribly wrong answer. $\endgroup$ – MaxW Oct 22 '18 at 4:55
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  1. A carbon atom has 6 neutrons. How many protons are present in the nucleus of a carbon atom?

Obviously this wording would make any astute student say the answer is 6 since carbon by definition has 6 protons. You are correct, elements are defined by their number of protons. Key word being defined, changing the number of protons changes the element.

I believe the key here that your book is getting at is you only know that you have an atom from a carbon sample that has 6 neutrons but nothing else. I think the question is trying to evaluate if you are aware that proton numbers are independent of neutron numbers thus knowing there are 6 neutrons does not help you determine protons, especially given one of the options is:

D. Can't tell; more information is required.

If this is what they wanted then it was a very poor choice of words to say a carbon atom in the question then ask for number of protons. They should have said:

Given an atom (or nucleus) with 6 neutrons, how many protons are present in the nucleus of the atom?

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  • $\begingroup$ Maybe an even more well-formulated question would be "atom A has 6 protons, and atom B has 6 neutrons, which elements do A and B correspond to?", which has the bonus of reinforcing the notion that only proton count matters. I presume all chemistry tests/study guides come with a periodic table. $\endgroup$ – Nicolau Saker Neto Oct 24 '18 at 21:46

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