Answer: Back-bonding into boron's empty 2p-orbital is strongest in BF3 and weakest in BBr3.
- A Fluorine is the least electronegative halogen and forms the weakest B–X bond
- B BBr3 has the smallest halogen atoms
- C Back-bonding into boron's empty 2p-orbital is strongest in BF3 and weakest in BBr3
- D The B–Br bond is the most ionic
Correct answer: C. Back-bonding into boron's empty 2p-orbital is strongest in BF3 and weakest in BBr3
Explanation: p-p back-bonding from halogen to boron is most effective for the small, well-matched F (2p), partially satisfying boron's electron deficiency; it weakens down to Br, so BBr3 is the strongest Lewis acid.
Diamond's rigid 3D tetrahedral network makes it the hardest natural material, while graphite's flat hexagonal sheets are held together only by weak forces, letting them slide past each other (used as a lubricant and in pencils).
Concept context
Covers the Boron family (Group 13: B, Al) and the Carbon family (Group 14: C, Si, Ge, Sn, Pb). Key topics include Lewis acid behaviour of BF₃, borax bead test, silicates, silicones, allotropes of carbon, and trends in properties down the group.