Here's why:

* Copper's Electron Configuration: The electron configuration of copper is [Ar] 3d¹⁰ 4s¹.

* Outermost Shell: The outermost shell is the 4th shell.

* Electron Count: The 4s¹ part of the configuration indicates that there is one electron in the 4th shell.

Important Note: While copper might seem like it should have two electrons in its outer shell (due to the 4s¹ configuration), the d-orbitals are actually filled first in copper due to its position in the periodic table.