in group 13, the only metalloid, boron, is at the very top of the group. explain why the rest of the group has metallic behavior but boron does not.

The reason boron, the only metalloid in group 13, does not exhibit metallic behavior like the rest of the elements in the group is due to its small atomic size and high charge density. Boron has a much smaller atomic radius compared to other group 13 elements, which results in stronger attraction of its electrons and a tendency to form covalent bonds rather than metallic bonds. This makes boron behave more like a nonmetal or metalloid, lacking the delocalized electrons necessary for metallic conductivity and malleability. In contrast, the other group 13 elements—aluminum, gallium, indium, and thallium—have larger atomic sizes and lower charge densities. They lose electrons more easily and form metallic bonds with delocalized electrons, leading to the characteristic metallic properties of conductivity, malleability, and luster. As one moves down the group, metallic character increases due to decreasing ionization energy and increasing atomic size. Additionally, boron has a very high ionization energy and forms complex covalent compounds, while the other elements form more metallic lattices. This fundamental difference in bonding type accounts for boron's metalloid behavior at the top of group 13, while the rest are metals exhibiting metallic behavior.