Carbon has the atomic number 6. Find the number of neutrons in carbon-12, carbon-13 and carbon-14.

Isotopes are atoms of the same element, with the same $Z$ and a different $A$. They have the same electrons, so the same chemical properties. Isobars are atoms of different elements with the same $A$ and a different $Z$. The mass of a molecule is the sum of the masses of its atoms.