Why do all atoms of an element have the same atomic number, although they may have

different mass numbers?

Here's what I get.

Explanation:

The atomic number is the number of protons in the nucleus of an atom. The number of protons determines the number of electrons. The number of electrons determines the chemical properties of the element,

Thus, the atomic number determines the identity of the element.

The atomic mass does not affect the chemical properties, so different isotopes of an element behave alike.