Identify the element with the electron configuration [Ar] 4s² 3d⁸.

The electron configuration [Ar] 4s² 3d⁸ corresponds specifically to the element nickel (Ni). Here's how this configuration breaks down:

• The notation [Ar] indicates that the electron configuration begins with the noble gas argon, which accounts for the first 18 electrons.

• Following argon, there are 2 electrons in the 4s subshell. This fills the 4s orbital.

• The next electrons fill the 3d subshell, where 8 electrons are present.

Nickel is located in period 4, group 10 of the periodic table, and has a total of 28 electrons, consistent with this electron configuration.

Understanding the filling order of orbitals, which follows the Aufbau principle, demonstrates that the 4s subshell is filled before the 3d subshell. After the 4s has 2 electrons, the next electrons go into the 3d subshell, filling it up to 8 electrons for nickel.

Thus, nickel is defined uniquely by this electron configuration, confirming that it is the correct answer.