The number of protons in an atom's nucleus determines its

atomic mass.

number of neutrons.

identity as an element.

total charge as an ion.

The answer to this question would be: identity as an element.

The number of protons is called atomic number unique for each element. That means there is only one element that has that number of protons, so it can be called as the identity of the element. Atomic mass is made from adding the total amount of proton with neutrons. The charge is made by subtracting the number of protons with the electrons.