Question

what is the purprose of a proton?

Answer 1

Protons have a positive charge, and they balance out the negative charge of electrons.

Also, the number of protons determines the atomic number.

Answer 2

Protons make up the core of the atom (alongside the neutron, which is actually "only" there to negate the separating force of the positive charges of the protons, that occurs because they are crammed together in such a small space. The number of protons tells you which element your dealing with, since a change in the number of protons would change the element, whereas a change in the number of electrons does not change the element, but just it's electrostatic state. A change in the number of neutrons changes the isotope of the element and can cause radioactivity in certain elements (namely elements with atomic numbers higher than that of iron)