Electronegativity of boron is the highest in the group and it will form covalent bonds in all his combinations.
The rest of the group will form bonds with intermediate nature between covalent and ions bods in their respective compounds, with thallium (Tl) behaving most close to a metal.
Moreover boron have a very high melting points (around 2200 °C) while in the boron cristal the chemical bonds are directed in space, similar with carbon suggesting his nature as a non-metal.
Other elements form the group Al, Ga, In, Tl have lower melting points 660, 30, 157 and 304 °C, respectively. Also in the elemental state, they have metallic characteristics: metalic luster, ductility, high electrical and thermal conductivity.
