I'm a big dumb.
According to wikipedia:
>Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons.

>The number of nucleons (both protons and neutrons) in the nucleus is the atom's mass number, and each isotope of a given element has a different mass number.

So if I buy a block of lead from a store, how do you know what type of lead I just bought?
Is it 178Pb, 202Pb, 214Pb?
What's the deal with that?