Which element has the electron configuration [Ar] 4s² 3d⁷?

The correct identification of the element with the electron configuration [Ar] 4s² 3d⁷ corresponds to a specific arrangement of electrons in the atomic orbitals. The notation [Ar] represents the electron configuration of Argon, which accounts for the first eighteen electrons in the element.

Following Argon, the next two electrons fill the 4s subshell, resulting in 4s². After the 4s subshell is filled, the next seven electrons are added into the 3d subshell, leading to the configuration 3d⁷.

When analyzing the periodic table, the element that possesses this arrangement of electrons, particularly in the 3d sub-level, is Cobalt. Cobalt is located in period 4, group 9, and has a total of 27 electrons, which matches the described electron configuration. This configuration reflects that two electrons fill the 4s subshell, while seven electrons occupy the 3d subshell, which aligns perfectly with Cobalt's established electron structure.

Other elements, like Iron, Nickel, and Copper, have different configurations: Iron is [Ar] 4s² 3d⁶, Nickel is [Ar] 4s²