It has 24 valence electrons but we can't draw a Lewis structure with all atoms haveing complete octets. An acceptable structure would be the on shown below. Note that the B has an incomplete octet which would imply, in this model, that it could easily react with something which could donate a pair of electrons. We could also draw 3 resonance structures for BF 3 by alternating the placement of the doubel bond between each F-B pair. This resonance structure would probably be a better representation due to the experimental bond lengths having some double bond character.
Some molecules contain atoms with expanded octets .
I 3 - 10 Be sure that you can draw the above and obtain the same number of electrons about the central atom. Do remember that these are models, and crude models at that. However they are useful for an introduction to molecular bonding and shape.