Predicting Molecular Geometry

Start by drawing the Lewis structure of the molecule to visualize bonding and lone pairs. Then count the total number of electron groups on the central atom, where single, double, or triple bonds each count as one electron group. These steps establish the foundation for determining both electron-pair geometry and molecular structure.

Lone pairs occupy more space than bonding pairs, causing greater repulsion and reducing bond angles below their ideal values. In phosphorus trichloride, one lone pair on the central phosphorus atom reduces the bond angle to less than 109.5 degrees, changing the molecular geometry from tetrahedral to trigonal pyramidal despite the tetrahedral electron-pair geometry.

Carbon dioxide has two electron groups with no lone pairs on the central carbon atom, so its electron-pair geometry and molecular geometry are both linear with a 180-degree bond angle. Phosphorus trichloride has four electron groups including one lone pair, resulting in a trigonal pyramidal molecular geometry despite tetrahedral electron-pair geometry.

Tellurium tetrachloride has five electron groups around the central tellurium atom: four bonding pairs and one lone pair. The electron-pair geometry is trigonal bipyramidal, but the lone pair occupies an equatorial position to minimize repulsion, resulting in a seesaw-shaped molecular geometry.

In octahedral arrangements with two lone pairs, repulsion is minimized when the lone pairs occupy opposite sides of the central atom, perpendicular to the plane containing the bonding pairs. This arrangement in iodine tetrachloride anion produces a square planar molecular geometry with four bonding pairs in one plane.

Electron-pair geometry and molecular geometry are identical only when there are no lone pairs on the central atom. In carbon dioxide, both geometries are linear because all electron groups are bonding pairs with no lone pairs present to distort the arrangement.

VSEPR theory positions electron groups as far apart as possible to minimize repulsion. In trigonal bipyramidal arrangements, lone pairs occupy equatorial positions rather than axial positions. In octahedral geometries, two lone pairs are placed on opposite sides of the central atom to maximize distance and reduce repulsive forces.