Final answer:
Ammonia (NH3) is the molecule with only one lone pair in its Lewis model. It has a trigonal pyramidal shape due to this lone pair and the three single bonds to hydrogen atoms.
Step-by-step explanation:
The molecule that has only one lone (non-bonding) pair in its Lewis model is ammonia, NH3. In the Lewis structure of ammonia, the central nitrogen atom is bonded to three hydrogen atoms and has one lone pair of electrons. This arrangement gives ammonia a trigonal pyramidal shape. In contrast, water (H2O) has two lone pairs, methane (CH4) has no lone pairs, and boron trifluoride (BF3) also has no lone pairs, but instead has an incomplete octet which makes it very reactive.