Review the key concepts, formulae, and examples before starting your quiz.
πConcepts
The Group 13 elements (Boron family) include Boron (), Aluminum (), Gallium (), Indium (), and Thallium (). Their general valence shell electronic configuration is .
Atomic Radii Anomaly: The atomic radius of ( pm) is unexpectedly smaller than that of ( pm). This is due to the poor shielding effect of the intervening electrons in , which increases the effective nuclear charge.
Inert Pair Effect: The stability of the oxidation state increases down the group (), while the state becomes less stable. This is due to the reluctance of the electron pair to participate in bond formation.
Lewis Acid Character: Group 13 halides (like ) are electron-deficient ( electrons in the valence shell). They act as strong Lewis acids by accepting electron pairs to complete their octet.
Diborane () Structure: It is an electron-deficient molecule containing four terminal bonds (-centre--electron) and two bridging bonds (-centre--electron), also known as 'banana bonds'.
Anomalous properties of Boron: Due to its small size and absence of -orbitals, Boron differs from other members. It forms covalent compounds and has a maximum covalency of , whereas other members can expand their coordination number to using vacant -orbitals (e.g., ).
Amphoteric Nature: Aluminum oxide () and Aluminum hydroxide () are amphoteric, reacting with both acids and bases.
πFormulae
π‘Examples
Problem 1:
Explain why the Lewis acid strength of Boron halides follows the order: .
Solution:
The order is .
Explanation:
This is due to back-bonding. In , the small size of and allows effective overlap between the filled orbital of and the vacant orbital of . This reduces the electron deficiency of Boron. As the size of the halogen increases (), the -orbital size increases (), making back-bonding less effective, thus increasing Lewis acidity.
Problem 2:
What happens when Boric acid () is heated?
Solution:
Explanation:
On heating, Orthoboric acid () first loses water to form Metaboric acid (). Further heating yields Tetraboric acid (), and finally, at high temperatures, it decomposes into Boric anhydride ().
Problem 3:
Why is usually found as a dimer ?
Solution:
Explanation:
In , the Aluminum atom is electron-deficient (only electrons). To complete its octet, it forms a dimer where two chlorine atoms act as bridges by donating their lone pairs to the vacant -orbitals of the neighboring Aluminum atoms through coordinate covalent bonding.