What angle is characteristic of a trigonal bipyramidal arrangement?

• A. 120 degrees
• B. 90 degrees
• C. 109.5 degrees
• D. 104.5 degrees

Answer: (B)

In trigonal bipyramidal geometry, three bonds lie in a plane with 120° between them, and two bonds sit above and below that plane. Because the axial bonds are perpendicular to the equatorial plane, each axial bond forms a 90° angle with every equatorial bond. This 90° interaction between axial and equatorial positions is the defining angle of this arrangement. The equatorial bonds themselves are 120° apart, and the axial bonds are 180° apart. Angles like 109.5° or 104.5° come from other geometries or distortions, not the ideal trigonal bipyramidal case.