Boron is a non-metal, whereas aluminium (Al) is a metal. This is because of ?

Boron and aluminium both belong to Group 13 of the periodic table, yet they show a clear difference in their physical and chemical behavior. Boron is a non-metal, while aluminium is a metal. This difference arises mainly due to boron’s small atomic size and high nuclear charge compared to aluminium.

Boron has a much smaller atomic radius, which means its valence electrons are held tightly by the nucleus. The strong attraction between the nucleus and the outer electrons restricts their movement, so boron cannot easily lose electrons to form positive ions. This limited electron mobility gives boron non-metallic properties such as low electrical conductivity, high ionization energy, and covalent bonding in its compounds.

In contrast, aluminium has a larger atomic size and lower effective nuclear charge on its outermost electrons. This allows aluminium atoms to lose electrons more easily, forming metallic bonds and exhibiting typical metallic properties like malleability, ductility, and good electrical conductivity.

Boron forms covalent compounds such as boron trichloride (BCl₃) and boron hydrides (boranes), while aluminium forms ionic and metallic compounds like aluminium chloride (AlCl₃) and metallic aluminium. The bonding behavior difference also supports boron’s classification as a non-metal and aluminium’s as a metal.

Thus, both the small atomic size and high nuclear charge of boron are responsible for its non-metallic character, while aluminium’s lower nuclear attraction and larger size account for its metallic nature. This gradual change in character from non-metal to metal down the group is a common trend observed in many groups of the periodic table.