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:

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.

i just calculated it all out, and not including weekends, and all the different days off. as of this school year (2018-2019) students are spending 2198 hours in school this year