[Ar] 3d¹ 4s²

Here's how to break it down:

* [Ar] represents the electron configuration of the preceding noble gas, Argon (Ar). This means that the first 18 electrons of scandium are arranged in the same way as Argon.

* 3d¹ indicates that there is one electron in the 3d subshell.

* 4s² indicates that there are two electrons in the 4s subshell.

This configuration represents the arrangement of the 21 electrons in a scandium atom.