The answer is protons.
Because their are different isotopes that elements have, the number of neutrons varies from atom to atom.
Because ionic bonding is the result of the transfer of electrons, the number of electrons changes in an atom.
With this being said, the only thing that doesn't change about an atom is the number of protons, therefore making them key to the atom's identity.
Again, PROTONS is the answer.
I hope this helps!
Best wishes :D
