Which element has the electron configuration [Kr] 5s² 4d⁹?

The electron configuration [Kr] 5s² 4d⁹ corresponds to an element that has filled the krypton core plus two electrons in the 5s subshell and nine electrons in the 4d subshell.

When considering the periodic table, we can assess the position of the elements provided in the options. Gold (Au) has the electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s¹. Silver (Ag) is characterized by the configuration [Kr] 4d¹⁰ 5s¹. Copper (Cu) has the configuration [Ar] 4s¹ 3d¹⁰, and Platinum (Pt) is represented by [Xe] 4f¹⁴ 5d⁹ 6s¹.

Focusing on the element with 4d⁹, Gold's configuration indicates it has filled the 5d subshell with 10 electrons, while Ag has 4d¹⁰, indicating a filled 4d subshell. Copper, with 3d¹⁰, and Platinum, having a filled 5d subshell, do not match the 4d⁹ requirement.