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Answer 1

The answer is that, first, different atoms of the same element will not have different atomic numbers because the identity of an element is determined by the number of protons in its nucleus. And the number of protons of an atom is its atomic number. Pick any atom of that element and it will have that unique number of protons. Elemental identity and proton count are inseparably linked. Said another way, all of the atoms of that element will have the same atomic number - the same number of protons. Electrons can come and go, being borrowed or loaned out in chemical activity, but the proton count remains constant for a given element. Now to the Atomic Mass.

It turns out that for a given atom (and that atom will have a fixed number of protons), the number of neutrons can vary. Said another way, the number of neutrons in any atom of a given element can be different from one atom of that element to another atom of that element. We say that a given element has different isotopes. It has its fixed number of protons, but the number of neutrons isn't fixed. Each isotope has that same characteristic number of protons, but the number of neutrons can vary from one atom to the next. Let's take an example.

Lithium has 3 protons in its nucleus. Every atom of lithium is like this. Its atomic number is three. Always. But what about neutrons? It turns out that there are two stable configurations possible. One has 3 neutrons. Its Atomic Mass is about 6.015 or so. Recall that subtracting the atomic number from the atomic mass gives us the number of neutrons in the nucleus of that atom. The 6.015 minus the 3 yields an answer of 3 for the number of neutrons. There is also a lithium atom that has 4 neutrons in its nucleus. Its atomic mass is about 7.016 or so. Same math. Take the 7.016 and subtract the 3 and get 4 as the answer - and the number of neutrons in the nucleus of that atom. Those are the two stable isotopes of lithium. (There are a number of other isotopes of lithium, but they're all unstable and disappear fairly quickly after being made in the physics lab.)

Pick any other element and it will have a unique identity based on the number of protons in the nucleus. But it will have different numbers of neutrons as you look around at different atoms of that element. The different numbers of neutrons account for the different atomic masses of given isotopes of that element.