* Atomic number (number of protons): This is the defining characteristic of an element. Copper has an atomic number of 29, meaning every copper atom has 29 protons.
* Number of electrons (in a neutral atom): In a neutral atom, the number of electrons is equal to the number of protons. So a copper atom also has 29 electrons.
While the number of neutrons can vary (leading to isotopes), the number of protons and electrons are constant and define what makes an atom copper.
