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

The electron configuration for $Co^{2+}$ is $$Co^{2+}: [Ar]3d^7$$ $Co^{2+}$ does not have noble-gas configuration.
*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 orbital with the lowest value of $n$. 1) The electron configuration of a neutral $Co$ atom is $$Co: [Ar]3d^74s^2$$ 2) Here we need to find the electron configuration for $Co^{2+}$, an ion having 2 electrons removed from a neutral $Co$ atom. Therefore, according to the rules, when electrons are removed, we would look for the orbital with the largest $n$. Here, orbital $4s$, with $n=4$, has the largest $n$. There are 2 electrons occupying orbital $4s$ in a neutral $Co$ atom. Therefore, to make $Co^{2+}$, these 2 electrons would be removed. That means the electron configuration for $Co^{2+}$ is $$Co^{2+}: [Ar]3d^7$$ The above configuration is not a noble-gas configuration, so $Co^{2+}$ does not have noble-gas configuration.