Isotope

AP Physics 2 fixes both halves in Topic 15.2. Statement 15.2.A.2: each atomic element has a unique number of protons. Statement 15.2.A.2.ii: the total number of neutrons and protons identifies the isotope of an element. So $Z$ names the element and $A$ then names which isotope of it you have, with the neutron count $A - Z$ carrying the difference.

Chemistry does not notice. A neutral atom has as many electrons as protons, and 15.2.A.2.i says the number and arrangements of electrons affects how atoms interact, so two isotopes of one element bond and react the same way. The extra neutrons change the mass, since 15.2.A.2.iii puts the mass of an atom in its protons and neutrons, and change nothing an electron cares about.

The nucleus notices a great deal. Statement 15.8.A.3 is blunt: the type of decay exhibited by a given nucleus is determined by the isotope of the element. One isotope can be stable while another of the same element is not, and 15.7.B.3 says half-lives across different unstable elements and isotopes range from fractions of a second to billions of years.

None of which you have to memorise. The Topic 15.8 boundary statement says AP Physics 2 does not expect students to know the processes by which specific isotopes decay or the half-lives of specific isotopes. Any question needing that data prints it.