Concept:
According to Lewis theory:
• A Lewis acid is a species that can accept an electron pair.
Analysis of given species:
H2O
Water has lone pairs of electrons and can donate an electron pair.
Hence, H2O is a Lewis base, not a Lewis acid.
BF3
Boron in BF3 has an incomplete octet and can accept an electron pair.
Hence, BF3 is a Lewis acid.
H+
H+ has no electrons and can accept an electron pair.
Hence, H+ is a Lewis acid.
NH4+
NH4+ does not have a lone pair to donate, but it can accept an electron pair under suitable conditions.
Hence, NH4+ is considered a Lewis acid.
Final Answer:
Lewis acids among the given species are:
BF3, H+ and NH4+