UPSEE Solved Model Paper 5

According to Lewis electronic theory, acids are electron pair acceptors i.e., an acid is a species that accepts an electron pairs and will have vacant orbitals. A base is an electron pair donor. i.e., a Lewis base is a species that donates an electron pair and will have lonepair electrons. In $BF_3$, The number of valence electrons can be calculated as B = 3 electrons F = 7 × 3 = 21 electrons Therefore, total valence electrons = 21 + 3 = 24 electrons. The number of octet electrons can be given as B = 6 electrons F = 8 × 3 = 24 electrons Therefore, total valence electrons = 24 + 6 = 30 electrons. Number of bonding electrons = Number of octet electrons – Number of valence electrons = 30 – 24 = 6 There are three covalent bonds in $BF_3$ as seen in the diagram. Because it takes six electrons to form the structure, there are 18 non bonding electrons. Each fluorine atom needs six non-bonding electrons to satisfy its octet. Thus of all non-bonding electrons are consumed by three fluorine atoms. As a result, we run out of electrons while central Boron has only six valence electrons. Thus with 6 electrons in the valence shell it can accept a pair of electrons easily from electron rich species thus act as Lewis acid.