Comments by "EebstertheGreat" (@EebstertheGreat) on "What Are Atoms and Isotopes?" video.
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This is exactly correct. Elements are defined by their "proton number," literally the number of protons in their nucleus. So whether the nucleus is just a single proton (protium), a proton and a neutron (deuteron), or a proton and two neutrons (triton), the element is still hydrogen. Chemistry depends almost entirely on electrical interactions, so these are chemically nearly identical (they are not exactly identical, but the reason for the slight difference is not particularly important here). The same applies to larger elements. For instance, carbon-12 (6 protons and 6 neutrons) and carbon-13 (6 protons and 7 neutrons) are nearly indistinguishable chemically. And for very large elements, this remains true. So uranium-238 (92 protons, 146 neutrons) is basically indistinguishable, chemically speaking, from uranium-235 (92 protons, 143 neutrons). To separate these (and "enrich" uranium), we must distinguish just by their mass, which means spinning samples of uranium at extremely high speeds such that the more massive U-238 atoms tend to move to the outside of the centrifuge while the U-235 atoms tend to remain closer to the center.
Ions however differ dramatically in their chemistry. A positive hydrogen ion (hydrogen "cation") lacks an electron, so it's just a bare nucleus (typically just a proton), whereas a negative hydrogen ion (hydrogen "anion") has an extra electron, so it has two electrons. Unlike neutral atoms, ions have net charges, and this makes them behave very differently. The electrostatic force is so strong that even in a highly charged object like a Tesla coil, only a miniscule percentage of the atoms in the metal are actually ionized. Even that very tiny percentage is enough to make a huge difference in behavior. As an example, consider the difference between sodium cations and chloride anions in common table salt (sodium chloride) from neutral sodium atoms in sodium metal and neutral chlorine atoms in chlorine gas, both of which are quite dangerous and unstable.
But a sodium ion can easily pick up another electron to become a neutral sodium atom. So we don't categorize elements by their electron number but by their proton number, which is much harder to change. To gain or lose a proton, an atom must decay or collide with certain types of radiation (such as beta radiation in electron capture decay). Most hydrogen atoms have remained hydrogen since shortly after the Big Bang without ever transmuting into a different element, whereas most ions have changed several times between charged ions and neutral atoms. And chemical reactions, since they are electrical in nature, can ionize atoms, but they cannot affect their nuclei. So no matter how many chemical reactions you put a sample of carbon-12 through, you will never produce carbon-13. This is a very good reason for our current categories.
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