## Chemistry: The Central Science (13th Edition)

The electron configuration for $As^{3-}$ is $$As^{3-}: [Ar]3d^{10}4s^24p^6$$ $As^{3-}$ has the configuration of noble gas $Kr$.
*RULES TO WRITE ELECTRON CONFIGURATIONS FOR IONS: - When electrons are removed to form cations, those of the orbitals that have the largest principal quantum number $n$ will be removed first. - If there are more than one subshell for a given $n$, electrons of the subshell with the highest value of $l$ will be removed first. - When electrons are added to form anions, they would be added to the empty or partially occupied subshell with the lowest value of $n$. 1) The electron configuration of a neutral $As$ atom is $$As: [Ar]3d^{10}4s^24p^3$$ 2) Here we need to find the electron configuration for $As^{3-}$, an ion having 3 electrons added to a neutral $As$ atom. Therefore, according to the rules, when electrons are added, we would look for the empty or partially occupied subshell with the lowest $n$. All the inner orbitals are fully occupied, so we don't care about them. Subshell $4s$ is already occupied by 2 electrons, so it is also fully occupied. Subshell $4p$, on the other hand, is occupied by only 3 electrons, while its maximum capacity is 6 electrons. So, electrons would be added to here first. 3) 3 electrons need to be added to a neutral $As$ atom to make $As^{3-}$. Subshell $4p$ also have 3 spots left for 3 electrons, so these 2 electrons would be added here. That means the electron configuration for $As^{3-}$ is $$As^{3-}: [Ar]3d^{10}4s^24p^6$$ The above configuration is the configuration of noble gas $Kr$, so $As^{3-}$ has the configuration of noble gas $Kr$.