Final answer:
The molecular geometry for a structure with 4 total electron regions and 3 bonded regions is trigonal pyramidal with an angle of about 107°.
Step-by-step explanation:
When you have a total of 4 regions of electron density and only 3 of them are bonded, the molecular geometry can be deduced by considering the presence of lone pairs. According to VSEPR theory, if there are four regions of electron density, the electron-pair geometry is tetrahedral.
However, if only three of those regions are occupied by bonded atoms and one is a lone pair, the molecular geometry is described by the positions of the nuclei only. Therefore, the molecular structure will be trigonal pyramidal, which can be visualized as a tetrahedron with one vertex missing due to the lone pair. The presence of the lone pair means that the bond angles are slightly less than the ideal tetrahedral angle of 109.5°, typically about 107°.