In the following reaction ${\mathrm{BF}}_3 \mathrm{and} {\mathrm{NH}}_3$ respectively are ${\mathrm{H}}_3\mathrm{N} : {+ \mathrm{BF}}_3\to {\mathrm{H}}_3\mathrm{N} : {\mathrm{BF}}_3$

 

Compounds whose central atom has an incomplete octet(BF₃) are Lewis acids, Molecules with one or more lone pairs on the central atom are Lewis bases(NH₃)