Here's why:
* Atomic Number: The atomic number defines the element. It's the number of protons in the nucleus. All atoms of the same element have the same atomic number.
* Neutrons: The number of neutrons in an atom can vary. This variation creates isotopes.
* Mass Number: The sum of protons and neutrons in the nucleus is called the mass number. Since isotopes have different numbers of neutrons, they also have different mass numbers.
Example: Carbon-12 and Carbon-14 are isotopes of carbon. They both have 6 protons (atomic number = 6) but Carbon-12 has 6 neutrons while Carbon-14 has 8 neutrons.
