In a sample of pure copper, all atoms have atomic numbers which are

All atoms have same atomic number because number of electrons in all atoms are same.

Explanation

Every atom consist of nucleus or a positive center. The protons and neutrons are present with in the nucleus while electrons are present out side the nucleus. All these three subatomic particles construct an atom. The number of protons or number of electrons are the atomic number of an atom while the number of protons and number of neutrons are the mass number of an atom. A neutral atom have equal number of protons and electrons. In other words we can say that negative and positive charges are equal in magnitude and cancel the each other.

For example:

Atomic number of copper is 29 it have 29 electrons and 29 protons.

While its atomic mass is 63.546 and have 35 neutrons.

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