sp3d2

Meaning: sp³d² hybridization mixes one s, three p and two d orbitals into six equivalent hybrid orbitals aimed at the corners of an octahedron, 90° apart. The MCAT assigns it to a central atom with six electron domains (bonded atoms plus lone pairs). Because d orbitals are used, the atom must be in period 3 or lower, which is how expanded octets arise. Some modern treatments say d orbitals contribute little in such molecules, but the exam uses the hybridization model.

On the MCAT: Count domains, then name the hybridization and shape. Six bonded atoms and no lone pairs: octahedral, as in SF₆. One lone pair: square pyramidal, as in BrF₅. Two lone pairs opposite each other: square planar, as in XeF₄. All three are sp³d²; only the molecular geometry differs. A second-period atom such as N or O cannot do this, because it has no d orbitals in its valence shell.

Common confusion: XeF₄ looks flat, so students call it sp² or sp³. Hybridization follows electron domains, not the visible shape. Six domains means sp³d², and the number of hybrids (6) equals the number of orbitals mixed (1 + 3 + 2).