CHEMISTRY COMMUNITY

When writing out the molecular formula BF3 + :NH3 --> NH3BF3, why does one draw the two dots next to NH3 reactant?

The two dots represent 2 unpaired electrons on the nitrogen. Remember that there must be 8 valence electrons, in which 6 (2 per bond) are with the H, and 2 are with the N. This gives the nitrogen in :NH₃ a formal charge of 0. (5-3-2=0)