The mass number of an atom is found by adding the number of protons and neutrons.

Here's why:

* Protons are positively charged particles located in the nucleus of an atom. They determine the element's identity.

* Neutrons are neutral particles also located in the nucleus. They contribute to the atom's mass but not its charge.

* Electrons are negatively charged particles that orbit the nucleus. They are much lighter than protons and neutrons and are not included in the mass number calculation.

Let me know if you have any other questions!