Group 13 Elements: The Boron Family — Core Principles

Core Principles

Group 13 elements, the Boron family (B, Al, Ga, In, Tl), are p-block elements with a general electronic configuration of $ns^2np^1$. They typically exhibit a +3 oxidation state. However, due to the inert pair effect, the stability of the +1 oxidation state increases down the group, becoming predominant for Thallium.

Boron is a non-metal, showing anomalous behavior due to its small size, high ionization enthalpy, and absence of d-orbitals, leading to covalent, electron-deficient compounds like diborane ($B_2H_6$) with unique 3c-2e 'banana bonds'.

Boric acid ($H_3BO_3$) is a weak monobasic Lewis acid. Aluminium is a metal but amphoteric, reacting with both acids and bases. Trends in atomic radii and ionization enthalpy are irregular due to the poor shielding of d and f electrons in heavier elements.

Key compounds include borax, boric acid, diborane, and aluminium oxide/chloride. Lewis acid character is prominent, with $BF_3$ being the weakest among boron trihalides due to back-bonding.

Important Differences

vs Aluminium

Aspect	This Topic	Aluminium
Nature	Non-metal	Metal
Physical State	Hard, black solid (amorphous) or crystalline	Soft, silvery-white solid
Oxidation State	Predominantly +3, but often electron-deficient	Exclusively +3 in stable compounds
Covalency	Maximum covalency of 4 (due to absence of d-orbitals)	Can exhibit covalency of 4 or 6 (due to presence of vacant d-orbitals)
Oxides	Acidic ($B_2O_3$)	Amphoteric ($Al_2O_3$)
Hydroxides	Weak Lewis acid ($B(OH)_3$)	Amphoteric ($Al(OH)_3$)
Halides	Monomeric, strong Lewis acids ($BF_3$)	Dimeric ($Al_2Cl_6$) in vapor, polymeric in solid, strong Lewis acids

Boron, the first member of Group 13, stands in stark contrast to Aluminium, its immediate successor. Boron is a non-metal, forming predominantly covalent and electron-deficient compounds, with a maximum covalency of four due to the absence of d-orbitals. Its oxide is acidic, and boric acid acts as a Lewis acid. Aluminium, on the other hand, is a typical metal, though amphoteric, and can expand its covalency to six by utilizing its vacant d-orbitals. These fundamental differences highlight the anomalous behavior of Boron within its own group, a common trend for the first element of many p-block groups.