In a sample of pure copper, all atoms have the same atomic number.

Here's why:

* Atomic number defines the element. It represents the number of protons in an atom's nucleus. Copper always has 29 protons, making it copper.

* Mass number can vary within a sample of an element due to isotopes. Isotopes of the same element have the same number of protons but different numbers of neutrons.

So, while all copper atoms have the same number of protons (atomic number), they might have different numbers of neutrons, leading to different mass numbers.