[Ar] 3d¹⁰ 4s¹

Here's why this is the case:

* [Ar] represents the electron configuration of Argon, the noble gas that precedes copper on the periodic table. This means copper has the same inner electron configuration as Argon.

* 3d¹⁰ indicates that the 3d subshell is completely filled with 10 electrons.

* 4s¹ indicates that the 4s subshell has only one electron.

Why not [Ar] 3d⁹ 4s²?

You might expect copper to have this configuration based on the standard filling order. However, a completely filled d subshell is more stable than a partially filled one. The extra stability gained by having a full 3d subshell outweighs the slight energy disadvantage of having only one electron in the 4s subshell.