The electron configuration for the Fe3+ ion is [Ar] 3d5.
To arrive at this configuration, we start by determining the electron configuration of the neutral iron (Fe) atom, which has 26 electrons. The electron configuration for neutral iron is [Ar] 4s2 3d6. When iron forms the Fe3+ ion, it loses three electrons. The first two electrons are removed from the 4s subshell, followed by one electron from the 3d subshell.
Thus, stripping three electrons from the neutral configuration results in [Ar] 3d5 for the Fe3+ ion. This configuration shows that there are five electrons in the 3d subshell while the 4s subshell is completely empty.