Why is atomic number unique? [closed]

We all know that atomic number of an atom is "unique" just like fingerprint. And no two elements can have same atomic number. But how can we be So sure that no two elements can have same atomic number?

closed as unclear what you're asking by Waylander, Todd Minehardt, Jon Custer, Poutnik, Mathew MahindaratneSep 24 at 14:47

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• Well, if they do, they would be the same element by definition. – Ivan Neretin Sep 24 at 11:39
• But what if we find two elements whose atomic number are same but their chemical properties are completely different from each other. So are we still gonna call them same elements. – Shekhar Sep 24 at 11:59
• Well, chemical properties depend on the electron orbitals, which in turn depend on the nuclear charge, which is the same as atomic number. – Ivan Neretin Sep 24 at 12:03
• Let's deal with it when we face it. When we face a double head unicorn, we will then deal how to treat it. – Poutnik Sep 24 at 12:11
• It has nothing to do with any other properties - all nuclei containing only equal amount of protons and equal or different number of neutrons are nuclei of the same element. That's it. – Mithoron Sep 24 at 13:22

Recall that atomic number is the number of protons in the nucleus of the element. Also recall that species having the same number of protons and different number of neutrons are termed as isotopes. Isotopes are all considered the same element. $$\ce{^{12}C}$$ and $$\ce{^{13}C}$$ are both very much carbon.