When we write the electron configuration for an anion, we're adding extra electrons to the electron configuration of the neutral atom because anions have additional electrons. We add the electrons to the lowest energy unfilled orbitals first. For example, chloride has a one-minus charge, so we need to add one electron to the electron configuration for chlorine. Chlorine has five electrons in the 3p subshell, so there's still room for one more. We add the one electron to the 3p subshell, and it becomes fully filled. When we write electron configurations for a cation, we're removing the electrons from the electron configuration of the neutral atom because cations have fewer electrons. We remove the electrons first from the highest principle quantum number, starting with the highest energy orbitals in that shell, which is not necessarily the reverse order of filling. For example, if we create the electron configuration for iron three plus, we need to remove three electrons from the electron configuration of a neutral iron atom. The electron configuration for iron is argon 4s2 3d6. The highest principal quantum number is 4, so we have to remove the 4s electrons first before we remove any 3d electrons. As a result, the electron configuration for iron three plus is argon 3d5, not argon 4s2 3d3.