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 BF3, 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 BF3 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.