What is the element represented by the electron configuration [Ar] 4s² 3d⁹?

The electron configuration [Ar] 4s² 3d⁹ corresponds to the element Copper (Cu). The reason this configuration is correct lies in the filling order of the orbitals based on the Aufbau principle.

In this configuration, the brackets [Ar] indicate that the first 18 electrons are filled, following the electron configuration of Argon. The next electrons after Argon fill the 4s subshell with 2 electrons, represented by 4s². After filling the 4s subshell, the 3d subshell begins to fill. However, in this case, there are 9 electrons in the 3d subshell, denoted as 3d⁹.

This specific arrangement is characteristic of Copper, which has an atomic number of 29. When you sum the electrons in the configuration ([Ar] = 18 + 2 from 4s² + 9 from 3d⁹), you get a total of 29 electrons, confirming that this configuration indeed describes Copper.

The substitution of one of the d-electrons to achieve a more stable configuration (which could be 3d¹⁰ for Copper, leading to the notable stability) is also a concept related