BF₃ is an electron-deficient species as the octet of boron is incomplete (1s² 2s² 2p¹). According to Lewis concept e- deficient species are called lewis acid so BF₃ will act as a lewis acid while NH₃ (N=1s² 2s² 2p³) has a lone pair so it will act as a lewis base and it will donate its lone pair to the empty p-orbital of Boron through a coordinate bond to form an adduct.