Which element's configuration is [Ar] 4s² 3d¹?

The correct configuration of the element in question is achieved by examining the electron configuration step by step. The notation [Ar] represents the electron configuration of Argon, which accounts for the first 18 electrons. After Argon, the configuration continues with 4s², indicating that there are two electrons in the 4s subshell, followed by 3d¹, which means there is one electron in the 3d subshell.

To determine which element this corresponds to, we can calculate the total number of electrons represented by this configuration. Starting from Argon (18 electrons), adding the 2 electrons from 4s and then the single electron from 3d results in a total of 21 electrons. The atomic number of the element is 21, which matches Scandium (Sc).

Thus, Scandium's electron configuration is [Ar] 4s² 3d¹, confirming that this is the correct answer based on the total electron count and subshell filling order.