So, my son’s science class is doing a project which requires the students to answer the question, “How many neutrons are in this element’s atom?”
I say this is a poor question… fine if the element is hydrogen or helium; flawed if it’s an element with many common isotopes, and especially flawed if it’s an element with no one “take charge” isotope.
Or am I being nitpicky?
Um, well, what IS the element? Do they all chose different ones or what?
I thought they were going to be asked “how many neutrons in a 1kg lump of [whatever]” 
I’d say, if there’s only one marginally stable isotope, then fine, that one. If not, I agree it’s poorly worded: does it mean an average, or most common? Whether or not it’s a big deal depends on the teaching – will anything where a reasonable answer is given be ok, or will the teacher play the “AHA! This time I meant ‘average’ and you incorrectly guessed I was thinking of the most common isotope YOU MORON. You will never amount to anything!” game?
You’re overreacting. School will be full of ambiguous questions like this. It’s not hard to put down the answer that the teacher is looking for. If possible you can put down a better answer and explain it. But unless your son puts down a reasonable answer and has it marked wrong, there isn’t anything worth reacting to.
This is presented in connection with a module on the periodic table, atomic numbers, and atomic weights, no?
So the question presented can be read noscitur a sociis with this lesson.
It’s the element’s atomic weight (rounded to the nearest whole number) less the atomic number. Which you knew. Perhaps you can have your son include a footnote upon the nature of isotopes. (“Carbon predominately occurs with 6 neutrons, but see, e.g., carbon-13 (7 neutrons) and carbon-14 (8 neutrons).”)
The specific assignment was to create a “trading card” of the element, with it’s weight, and spaces to fill in the number of protons, neutrons, and electrons it has.
I agree this is uncomplicated for carbon.
What’s the right answer for promethium? For ruthenium?
Atomic weights of 145 and 101, respectively.
Was he assigned a specific element? Otherwise, use this a teachable moment for the lesson “Just pick carbon already and let’s call it a [fuckin’] day.” It’s an important lesson to learn, possibly even more important than learning how to calculate the number of neutrons given atomic numbers and weights.
Use the number of protons and an asterisk, or just an asterisk. The trading card format gives the opportunity to add an explanation.
And anyone would prefer a Ruthenium card to a Bondsenium no matter how many neutrons are listed.
Right there with you, but he was assigned both elements.
Word.
I vote not over-reacting. Well, I guess that depends on how the OP is reacting, he doesn’t say. If your going to march to the school and threaten you set yourself on fire unless the teacher is run out of town, your probably overdoing it. But assuming your reaction is “mild annoyance followed by kivetching on the Dope”, I say not over-reacting.
But anyways, the fact that an element has a fixed number of protons but variable number of neutrons is one of maybe three things a fifth grader has to learn about the periodic table. By phrasing the question so it seems like that’s not the case, the teacher is screwing up basically the essential point of the lesson.
Sure Bricker can set Jr. straight and they can add a note to the assignment acknowledging other isotopes, but I’m sure plenty of students have less engaged parents, or parents that don’t remember how the periodic table works.
Thanks for phrasing it so succinctly. This is it, exactly – the idea that atoms of an element can have different numbers of neutrons and still be chemically identical.
Frankly, I wouldn’t be surprised if the teacher didn’t know how the periodic table works. One of my big gripes about elementary school in this country is the fact that teachers are generally responsible for subject area content but aren’t required to take many (if any) courses in that content.
I’d say over-reacting. What’s wrong with explaining to your son that there are different isotopes, and that giving either the number for the most abundant isotope, or giving a range of possible values, is OK? Just specify which on the card.
FWIW, NIST has a list of Isotopic Compositions for elements, with their relative abundances. Can’t argue with NIST.
ETA: Or he could use the abundances and number of neutrons, and calculate an average.
I honestly have never understood why schools like to teach kids incorrectly at first and then fix it later, but it is common, and I think it’s probably a good idea for Bricker Jr to learn this sooner rather than later.
And I’d definitely put the average on there, with several decimal points. I was all about that sort of thing in school. And, despite what you’d think, I was also always the teacher’s pet.