Boron atom in BF3 is sp2 hybridized. The orbital picture of boron in the excited state can be shown as:
Nitrogen atom in NH3 is sp3 hybridized. The orbital picture of nitrogen can be represented as:
After the reaction has occurred, an adduct F3BNH3 is formed as hybridization of ‘B’ changes to sp3 . However, the hybridization of ‘N’ remains intact.