D4.5 Electron Configurations of Monoatomic Ions

When atoms lose electrons or gain electrons, ions form. It is useful to know what kinds of ions form and what their properties are. A monoatomic ion is a single atom that has gained or lost one or more electrons. A positively charged ion, a cation, forms when an atom loses one or more electrons. A negatively charged ion, an anion, forms when an atom gains one or more electrons.

Nonmetallic elements on the far right side of the periodic table (except the noble gases) have higher ionization energies and more negative electron affinities. It is energetically more favorable for them to gain electrons and form anions and less energetically favorable for them to form cations. For example, elements in groups VIIA, VIA, and some in VA can gain 1, 2, or 3 electrons, respectively, to achieve the electron configuration of a noble gas (a full octet, s²p⁶ in the outermost shell).

Metallic elements on the left side of the periodic table have lower ionization energies and less negative (or more positive) electron affinities. It is energetically more favorable for them to form cations. For examples, elements in groups IA, IIA, and IIIA can lose 1, 2, or 3 electrons, respectively, to achieve a full octet (plus additional filled d and f subshells).

To find the ground state electron configuration of a monoatomic ion, start with the electron configuration of the corresponding atom and remove (or add) an appropriate number of electrons from (or to) the valence orbital(s) of the atom. Here are some examples:

When transition elements and inner transition elements form cations, electron(s) in the outermost shell (largest n) are removed before any d or f electrons. For example, when Fe loses two electrons to form Fe²⁺, the two 4s electrons are lost:

This happens because electrons in the 3d subshell are very effective at screening the 4s electrons from the nucleus but much less effective at screening each other. Quantum mechanics calculates that the effective nuclear charge experienced by a 4s electron in Fe is 2.1, while the effective nuclear charge experienced by a 3d electron is 4.3. After 4s electrons have been removed, some 3d electrons can also ionize. Here are more examples:

Ions and atoms that have the same electron configuration are isoelectronic. For example, the isoelectronic Na⁺, Ne, and F⁻ all have ground state electron configuration of 1s²2s²2p⁶ (or [Ne]). For main-group elements, the most commonly formed ions are isoelectronic with a noble gas; that is, these ions have complete octets.