An atom has an atomic number 17 and atomic mass number 35. How many neutrons are present in its nucleus?

Question

frostbite · Answer

Okay I fast want to make the point that: two atoms can only be same if they share the same atomic number, else they are different elements.

So with that established we make the rule that the atomic number is always to the amount of protons in an atom, thereby putting up the condition that, elements are different from each other with the amounts of atoms in the nucleus.

Now the atomic mass number, which we in my country call the "nucleon number" tells you much many "nucleons" there are in the nucleus
Nucleons as you can might imagine, is any particle found in the nucleus aka protons and neutrons. So with this set up we can try do a little math: if we call the atomic mass number/nucleon number for $A$ and the amount of protons for $Z$ and amount of neutrons for $N$, we get the relation:
$$\Large A = Z + N$$
You were interested in the amount of neutrons so we solve for $N$ and get:
$$\Large N=A-Z$$
You got all the information to solve the problem for here.

Good luck.