the number of valence electrons the number of protons the charge of the atom the size of the atom
I gave my students this multiple choice question: In order to determine if two atoms are copper (Cu) atoms, what must be the same for each?

Only one answer is correct among these 4 choices:
- the number of valence electrons
- the number of protons
- the charge of the atom
- the size of the atom

Could you tell me which one is correct?

the number of protons