This essential provides a concise and comprehensive overview of the elements of the third main group of the periodic table, also known as the boron group or earth metals. All elements in this group, except for ununtrium, which was first produced about ten years ago, were discovered in the 19th century. Although aluminum is the most abundant metal, it remained hidden in its elemental form for a long time. In contrast, gallium, indium, and thallium are very rare. The elements in this group change their character significantly, ranging from a metalloid (boron) to the ubiquitous aluminum, to low-melting heavy metals (gallium, indium, and thallium), and to a possibly volatile metal that occasionally appears anionic (ununtrium).