Question

Changing the number of _____ would change an atom into an atom of a different element

Answer 1

Electrons - each atom has its own unique #

Answer 2

Changing the number or protons would change the element is an element loses an electron it becomes an ion but if it loses a proton the element its self-changes
eg. calcium has 20 protons if it loses on it becomes potassium that has 19 protons.

Hope that helps :)