Here's why:

* Atomic Number: Represents the number of protons in an atom's nucleus. Protons are positively charged particles that define the element's identity. Every atom of a specific element always has the same number of protons.

* Mass Number: Represents the total number of protons and neutrons in an atom's nucleus. While it influences an atom's overall weight, it doesn't define the element itself.

Example:

* Carbon has an atomic number of 6. This means every carbon atom has 6 protons.

* Carbon can have different mass numbers (e.g., Carbon-12 with 6 neutrons, Carbon-14 with 8 neutrons). These are called isotopes, but they are still carbon because they all have 6 protons.