Question

all atoms of the same element have the same...
mass.
number of electrons.
number of protons.
number of neutrons.

Explanation:

The atomic number, which is the number of protons in an atom, defines the element. Isotopes of an element have the same number of protons but different numbers of neutrons, and the number of electrons can vary in ions. Mass also can vary due to isotopes. So, all atoms of the same element have the same number of protons.

Answer:

number of protons.