Question

Sulfur has an atomic number of 16. You have found an isotope that has a mass of 35. What is different about this isotope than a sulfur atom with a mass of 32?

Answer 1

An atom contains 2 sorts of massive particles, which are both located in the nucleus.

Explanation:

Positively charged nuclear particles, are termed `"protons"`, and the number of protons `=` `Z`, the atomic number. The nucleus will also contain varying numbers of neutrons, massive, neutrally charged particles. The neutrons and protons are bound together by the strong nuclear force that at such short ranges (i.e. nuclear distances) overcome electrostatic repulsion.

In your problem, you have two sulfur isotopes. Because it is `S`, the number of protons is `16`; ordinarily you would not be given this info because you are expected to utilize the Periodic Table. If the isotopic mass is `35`, this sulfur isotope must contain `19` neutrons. The other sulfur isotope contains `16` neutrons. So the isotopes necessarily differ in the number of neutrons.

We would represent the isotopes as `""^35S` and `""^32S`. Why don't I have to specify `Z`, the atomic number?

See here for a prior answer.