Answer
$Cu:[Ar]4s^{1}3d^{10}$
Therefore, there are ten 3d electrons.
Work Step by Step
Write the electron configuration for copper using noble gas notation. Then determine the number of electrons for the specified subshell. Note that copper is one of the excetion to the rule. Even though it is the ninth element of the 3d subshell, it borrows an electron from the 4s subshell to form a completely filled 3d subshell.
