# What is the electron configuration of the "As"^(3-)" ion?

The electron configuration for the $\text{As"^(3-)}$ ion is ["Ar"]"3d"^10"4s"^2"4p"^6".
Arsenic, As, has atomic number 33, which is the number of protons in the nuclei of its atoms. A neutral As atom would also have 33 electrons. The electron configuration of a neutral arsenic atom is ["Ar"]"3d"^10"4s"^2"4p"^3".
An arsenic 3- ion $\left(\text{As"^(3-)}\right)$, has gained three electrons, and its electron configuration is ["Ar"]"3d"^10"4s"^2"4p"^6", which is isoelectronic with the noble gas krypton, Kr.