I am in an online chemistry course and its going well except for this particular type of problems. It states letters and tells me to write what isotope it is. Here goes... Write isotopic symbols of the form X-A (e.g., C-13) for each of the following isotopes. the a isotope with n(subscript a) neutrons. That's all it says. Nothing else. So i guessed trying like A-a but that was not it. I clicked show answer and it says it is Ar-40. How does one arrive at that? How does a isotope with n(sub a) neutrons = Ar-40?????? Then it goes b isotope with n(sub b) neutrons, c with n(sub c), and d with n(sub d). I would like to know how this works. It feels like i am playing riddles. There are other similar questions after this group but i think if somebody explains how you go from a isotope with n(sub a) neutrons to Ar-40 i will be able to figure out the rest for myself. I could post an image of the questions if this doesn't make any sense. Thank you.