A group of elements in the periodic table: boron (B), aluminium (Al), gallium (Ga), indium (In), and thallium (Tl), which all have outer electronic configurations ns²np¹ with no partly filled inner levels. They are the first members of the p-block. The group differs from the alkali metals and alkaline-earth metals in displaying a considerable variation in properties as the group is descended. Formerly, they were classified in group III, which consisted of two subgroups: group IIIB (the main group) and group IIIA. Group IIIA consisted of scandium (Sc), yttrium (Yt), and lanthanum (La), which are generally considered with the lanthanoids, and actinium (Ac), which is classified with the actinoids. Scandium and yttrium now belong to group 3 (along with lutetium and lawrencium).

Boron has a small atomic radius and a relatively high ionization energy. In consequence its chemistry is largely covalent and it is generally classed as a metalloid. It forms a large number of volatile hydrides, some of which have the uncommon bonding characteristic of electron-deficient compounds. It also forms a weakly acidic oxide. In some ways, boron resembles silicon (see diagonal relationship).

As the group is descended, atomic radii increase and ionization energies are all lower than for boron. There is an increase in polar interactions and the formation of distinct M³⁺ ions. This increase in metallic character is clearly illustrated by the increasing basic character of the hydroxides: boron hydroxide is acidic, aluminium and gallium hydroxides are amphoteric, indium hydroxide is basic, and thallium forms only the oxide. As the elements of group 13 have a vacant p-orbital they display many electron-acceptor properties. For example, many boron compounds form adducts with donors such as ammonia and organic amines (acting as Lewis acids). A large number of complexes of the type [BF₄]⁻, [AlCl₄]⁻, [InCl₄]⁻, [TlI₄]⁻ are known and the heavier members can expand their coordination numbers to six as in [AlF₆]³⁻ and [TlCl₆]³⁻. This acceptor property is also seen in bridged dimers of the type Al₂Cl₆. Another feature of group 13 is the increasing stability of the monovalent state down the group. The electron configuration ns²np¹ suggests that only one electron could be lost or shared in forming compounds. In fact, for the lighter members of the group the energy required to promote an electron from the s-subshell to a vacant p-subshell is small. It is more than compensated for by the resulting energy gain in forming three bonds rather than one. This energy gain is less important for the heavier members of the group. Thus, aluminium forms compounds of the type AlCl in the gas phase at high temperatures. Gallium similarly forms such compounds and gallium(I) oxide (Ga₂O) can be isolated. Indium has a number of known indium(I) compounds (e.g. InCl, In₂O, In₃^I[In^IIICl₆]). Thallium has stable monovalent compounds. In aqueous solution, thallium(I) compounds are more stable than the corresponding thallium(III) compounds. See inert-pair effect.