Oh chemistry. Matter is made of atoms. Atoms are made of subatomic particles. I'll stay simple, so those subatomic particles are protons, neutrons, and electrons. If an atom has a neutral charge, it has the same number of protons and electrons. The number of protons (electrons if a neutral charge) is called the atomic number. It is found in the upper left corner of each element's box on the periodic table of elements. For flourine, the atomic number is 9. That means the atom has 9 protons (and 9 electrons if it is neutral).
Not all atoms of the same element have the same number of neutrons. All atoms of that element will have the same number of protons, but neutrons can vary. That's what an isotope is. Same element, different amounts of neutrons. Let's use flourine again. It's possible that a flourine atom has 9 protons, 9 electrons, and 9 neutrons (mass number is 18; 9 pro + 9 neu = 18). It's also possible that a flourine atom has 9 protons, 9 electrons, and 10 neutrons (mass number is 19; 9 pro + 10 neu = 19). Same element. Different isotopes.
The periodic table indicates what an element's typical isotope is too. Usually at the bottom of the element's box is a number with decimals. That is the atomic mass. It's an average of mass numbers for that element. Flourine's atomic mass is 18.998. That's really really close to 19. Flourine has 9 protons. Mass number of 19 - 9 protons = 10 neutrons. The typical isotope of flourine has 10 neutrons.
Isotopes are the atoms of same elements having same atomic number and different mass number.
Atomic number of an elemnt is equal to the number of protons present in the atom (because an atom is electrically neutral it can also be stated as the number of electrons).
Mass number is the number of nucleons(sum of protons and neutrons present in the atom of the element)
so,in case of isotopes,the atomic number is same but the mass number is different.Example-hydrogen has three isotopes(protium,deuterium,tritium with atomic number 1 and mass numbers 1,2 and 3 respectively.
(HOPE THIS HELPS!!)