Final answer:
A trigonal pyramidal molecule like NH3 has three bonds and one lone pair around the central atom, with bond angles less than 109.5° due to LP-BP repulsions.
Step-by-step explanation:
A trigonal pyramidal molecule is a molecular geometry resulting from three bonds and one lone pair surrounding a central atom, resembling a tetrahedron with one vertex missing. The molecular formula for a common trigonal pyramidal molecule is NH3 (ammonia). The bond angles in a trigonal pyramidal structure are typically less than 109.5° due to the repulsion between the lone pair and the bonding pairs (LP-BP repulsions).