Question

Is it possible for two different substances to contain the same number of protons but different atomic numbers?

Answer 1

Consider `"NH"_3` and `"Ne"`. Both of these have `10` protons:

`"N" -> "7 protons"`, atomic number of `7`
`3 xx "H" -> "3 protons"`, atomic number of `1`
`7 + 3 = bb("10 total protons")` in `"NH"_3`

`"Ne" -> bb"10 protons"`, atomic number of `10`

They are both substances... but they are not the same substance. `"NH"_3` is a smelly gas, and `"Ne"` is an inert noble gas.

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On the other hand, if BOTH substances are the SAME KIND, then if they have the same number of protons, they cannot have differing atomic numbers.

If you have a `"Ne"` atom, it must have `10` protons, no matter what. If you choose an element to have an atomic number of `11`, it must be `"Na"` and have `11` protons, no matter what.

That is because we specifically chose elements, which contain only one kind of atom.