sp3d

Meaning: One s, three p and one d orbital mix to form five sp3d hybrid orbitals on a central atom with five electron domains. The electron geometry is trigonal bipyramidal: three equatorial positions 120° apart and two axial positions at 90° to the equatorial plane (180° from each other). In this model only atoms in period 3 or lower have d orbitals available, so sp3d accounts for expanded octets. Some modern accounts say d orbitals contribute little and describe the bonding with delocalized multicenter bonds, but the MCAT uses the sp3d label.

Equation: steric number = (atoms bonded to the central atom) + (lone pairs on the central atom); a steric number of 5 means sp3d. The steric number is a unitless count.

On the MCAT: You are given a formula or Lewis structure and asked for hybridization, shape or bond angles. Count domains, then place lone pairs. Five domains give PCl₅ (trigonal bipyramidal, 0 lone pairs), SF₄ (seesaw, 1), ClF₃ (T-shaped, 2) and XeF₂ or I₃⁻ (linear, 3). Lone pairs go in equatorial positions because that minimizes 90° repulsions.

Common confusion: Hybridization follows from electron domains, including lone pairs, but molecular shape describes only the atoms. So sp3d does not always mean trigonal bipyramidal shape: XeF₂ is sp3d and linear. Also, second-period atoms such as N and O lack d orbitals and do not form sp3d, so NCl₅ does not exist while PCl₅ does.