Protons will ALWAYS determine what the element is, always, no matter what. If I change the number of protons, I’m going to get a new element. Taking Hydrogen (with it’s one proton) and adding a second proton won’t make it hydrogen anymore- it becomes helium. The atomic number just shows you how many protons and electrons this element has. The symbol is only going to tell you what element this is. The weight? Well, that just tells you how many amu the element is, so it’s no help here. Electrons are always equal to the number of protons, and because I can’t change the protons without getting a new element, they also won’t help.
The answer for this guy is neutrons. It’s helpful to know the definition of an isotope- the same element with different number of neutrons.
