Answer: an empty orbital and incomplete octet.
- A a lone pair available for donation
- B a very high electronegativity value
- C an empty orbital and incomplete octet
- D a natural ionic bonding character
Correct answer: C. an empty orbital and incomplete octet
Explanation: Boron in BF₃ has only six electrons and a vacant 2p orbital, so it accepts an electron pair - the definition of a 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.