Group 13 Boron

A Lewis acid is a species that can accept a pair of electrons, and this ability depends on having an empty orbital available for bonding. In boron trifluoride, boron forms three covalent bonds with fluorine using its three valence electrons, but this leaves boron with only six electrons in its valence shell, an incomplete octet, and a vacant 2p-orbital. This empty orbital can accept a lone pair from a donor such as ammonia, making BF_3 a strong Lewis acid. The option of a lone pair on boron is false, since boron has no lone pair in BF_3. The option of a negative charge is incorrect, as boron is electron deficient. The option of an expanded octet is impossible for boron, which lacks d-orbitals and cannot exceed eight electrons. This electron deficiency is a defining NCERT feature of Group 13 chemistry. Understanding group 13 boron in this way ties directly into the wider study of p-block elements, where the same reasoning recurs across many problems. This concept also bridges to Chemical Bonding and Molecular Structure and Classification of Elements and Periodicity in Properties, so mastering it strengthens performance on linked questions from those topics as well. Plausibility check: BF_3 readily forms the adduct F_3B-NH_3 by accepting the nitrogen lone pair, confirming its electron-pair-accepting Lewis acidity.