When writing noble gas electron configurations, start with the noble gas from the previous period. The symbol of the noble gas is placed in brackets, in this case #["Ar"]#, which represents the electron configuration of atoms of the noble gas. The atoms of the elements in the next row continue to fill the sublevels in order of increasing energy.
In the case of copper, Argon has the atomic number #18#, and copper has the atomic number #29#. In neutral atoms, the atomic number (no. of protons) equals the number of electrons. There are #11# more electrons in copper atoms than in argon atoms. So the rest of the electron configuration includes the #11# additional electrons in the copper atom, which have the configuration #"3d"^10"4s"^1#.
