And you find that for copper, #Z_"the atomic number"=29#.
Now #Z# represents the CHARACTERISTIC number of protons, massive, positively-charged nuclear particles that are found in an element's nucleus. For copper #Z=29#. Of course, the nucleus also contains 30-35 neutrons, massive, NEUTRALLY-charged nuclear particles, which gives rise to an isotopic distribution, but these nuclear phenomena do not concern us here.
If the atom contains 29 protons, there must ALSO be 29 electrons, whizzing around the nuclear core IN THE NEUTRAL ATOM. Why so?
And we follow a modified #"aufbau scheme:"# #Cu,1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(1)3d^10#...note that normally #4s# would fill before #3d^10#...copper has an unusual configuration in this respect, and you simply have to learn it (also see the configuration for #Cr#, #Z=24,1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(1)3d^5#).
Copper metal commonly exhibits both #Cu(+I)# and #Cu(+II)# oxidation states.
