Electron Configuration of Ions

Anions gain electrons to acquire a negative charge. Cations lose electrons to become positively charged.

Anions

When adding electrons to an atom, the typical rules apply to the filling order. Electrons will be placed in the lowest energy orbitals according to the aufbau principle, only two electrons per orbital with opposite spins as Pauli said, and degenerate orbials will be filled singly with parallel spins according to Hund.

Example:

Oxygen (O): $1s^22s^22p^4$ Oxygen anion (O$^{2-}$): $1s^22s^22p^6$

Cations

When removing electrons from orbitals, you take first from the highest n level (outer electrons). When there are multiple subshells in the highest principle shell, electrons are removed from the orbital with the highest l value (i.e. If both the 3s and 3p subshells contain electrons, they will be removed from the 3p first because l = 1 is higher than the 3s where l = 0.).

For main group elements in my s and p blocks, this is straight forward because electrons will be removed in the order they were added. In magnesium (below), the electrons are lost from the 3s orbital to give a complete n = 2 shell. However, for those pesky transition metals, one must be careful. Because we take from the highest n level, the electrons lost in forming an ion will not be the last added. Take titanium in the example below. The highest n orbital filled is the 4s, so we will lose from here first. The reason the 4s electrons are lost first is complex. Once there are electrons in the 3d shell, it becomes lower in energy than the 4s and will therefore shield the 4s electrons from the nuclear charge. They will be less attracted to the nucleus and be easier to lose.

Examples:

Magnesium (Mg): $1s^22s^22p^63s^2$ Magnesium ion (Mg$^{2+}$): $1s^22s^22p^6$

Titanium (Ti): $1s^22s^22p^63s^23p^64s^23d^2$ Titanium ion (Ti$^{2+}$): $1s^22s^22p^63s^23p^63d^2$

Common ions

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Most elements form a predictable ion. The ions formed generally have complete shells (main group s and p blocks) or subshells (transition metals). The position on the periodic table gives us an easy way to "see" this pattern. Common ions are listed in the figure to the right from your ebook. Groups 1, 2, 6, and 8 form just one type of ion. Many transition metals can form a variety of ions.