molecular geometry

Meaning: Molecular geometry is the three-dimensional arrangement of atoms around a central atom, predicted by VSEPR theory: electron domains (bonded atoms and lone pairs) repel and spread as far apart as possible. Count domains for the electron geometry, then name the shape using only the atoms. Lone pairs repel more than bonding pairs, so they compress bond angles. Examples: 2 domains, linear (CO₂); 3, trigonal planar (BF₃) or bent (SO₂); 4, tetrahedral (CH₄), trigonal pyramidal (NH₃) or bent (H₂O); 5, trigonal bipyramidal (PCl₅); 6, octahedral (SF₆) or square planar (XeF₄).

On the MCAT: Given a formula or Lewis structure, you name the shape, rank bond angles (CH₄ > NH₃ > H₂O), give the hybridization (sp, sp², sp³ for 2, 3, 4 domains), or decide polarity. Polar bonds give a polar molecule unless symmetry cancels the dipoles: CO₂ and CCl₄ are nonpolar, while H₂O and NH₃ are polar.

Common confusion: Electron geometry counts lone pairs; molecular geometry names only the positions of atoms. H₂O has tetrahedral electron geometry but bent molecular geometry. To settle it, draw the Lewis structure, count lone pairs on the central atom, then describe just the atoms.