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Why do Group 13 elements commonly have a +3 oxidation state?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — boron group element Modern version of the periodic table of the elements (printable).

Elements in the boron group (Group 13) typically exhibit a +3 oxidation state because they possess three electrons in their outermost valence shell.1

The maximum oxidation number of +3 corresponds to the loss of these three valence electrons. While these elements can form ions with three positive charges, the M³⁺ ionic state is not universal for the entire group; it is often energetically unfavorable for heavier elements like gallium, indium, and thallium due to high ionization energies that are not always balanced by crystal energies.1

Instead of forming simple ions, these elements more commonly achieve a +3 oxidation state through covalent bonding. This is typically accomplished by promoting one electron from the outer s orbital to an p orbital, which allows for the formation of hybrid orbitals (such as sp²). As one moves down the group, there is an increasing tendency toward the +1 oxidation state, which becomes the more stable state for thallium.1