How does a trigonal pyramid differ from a tetrahedron so far as molecular geometry is concerned? bond angles in the tetrahedral molecule are 120°, while those for trigonal pyramid are 109.5° a trigonal pyramid is a tetrahedron with two vacant vertex. a trigonal pyramid is a tetrahedron with one vacant vertex. bond angles in the tetrahedral molecule are 109.5°, while those for trigonal pyramid are 120° a tetrahedron has a square base, while the base of a trigonal pyramid is a triangle.