Molecular Geometry and Orbital Hybridization (5B) - MCAT Chemical and Physical Foundations of Biological Systems

VSEPR (Valence Shell Electron Pair Repulsion) theory. VSEPR theory predicts molecular geometry by minimizing repulsion between electron domains around the central atom.

All count as one electron domain each. In VSEPR, any bond, regardless of being single, double, or triple, is treated as a single electron domain due to shared electron density.

Linear. With 2 electron domains, repulsion is minimized when they are opposite each other, forming a linear arrangement.

$180^\circ$. In linear geometry, the two domains are positioned at opposite ends, resulting in a $180^\circ$ bond angle to minimize repulsion.

Trigonal planar. Three electron domains arrange in a plane equidistant from each other to minimize repulsion, forming trigonal planar geometry.

$120^\circ$. Trigonal planar geometry spaces three domains equally in a plane, yielding ideal bond angles of $120^\circ$.

Tetrahedral. Four electron domains minimize repulsion by arranging at the vertices of a tetrahedron around the central atom.

$109.5^\circ$. Tetrahedral geometry positions four domains to maximize separation, resulting in bond angles of $109.5^\circ$.

Trigonal bipyramidal. Five electron domains arrange with three in a plane and two axial to minimize repulsion, forming a trigonal bipyramid.

$90^\circ$, $120^\circ$, and $180^\circ$. Trigonal bipyramidal geometry has equatorial angles of $120^\circ$, axial-equatorial at $90^\circ$, and axial-axial at $180^\circ$ for minimal repulsion.

Octahedral. Six electron domains minimize repulsion by occupying positions at the vertices of an octahedron around the central atom.

$90^\circ$ and $180^\circ$. Octahedral geometry arranges domains at $90^\circ$ for adjacent and $180^\circ$ for opposite positions to minimize repulsion.

Linear. AX$_2$ has two bonding domains and no lone pairs, so molecular geometry matches the linear electron-domain geometry.

Trigonal planar. AX$_3$ features three bonding domains without lone pairs, aligning with trigonal planar electron-domain geometry.

Tetrahedral. AX$_4$ has four bonding domains and no lone pairs, resulting in tetrahedral molecular geometry.

Trigonal pyramidal. In AX$_3$E, the lone pair occupies one tetrahedral position, distorting the three bonds into a trigonal pyramidal shape.

Bent (angular). AX$_2$E$_2$ has two lone pairs in tetrahedral positions, bending the two bonds into an angular shape with angle less than $109.5^\circ$.

Trigonal bipyramidal. AX$_5$ with five bonding domains and no lone pairs adopts the trigonal bipyramidal electron-domain geometry.

Seesaw. In AX$_4$E, the lone pair prefers an equatorial position in trigonal bipyramidal geometry, resulting in a seesaw shape.

T-shaped. AX$_3$E$_2$ places two lone pairs equatorially in trigonal bipyramidal geometry, leaving a T-shaped arrangement of bonds.

Linear. In AX$_2$E$_3$, three equatorial lone pairs in trigonal bipyramidal geometry force the two bonds into axial positions, forming linear geometry.

Octahedral. AX$_6$ with six bonding domains and no lone pairs matches the octahedral electron-domain geometry.

Square pyramidal. In AX$_5$E, the lone pair occupies one octahedral position, distorting the five bonds into a square pyramidal shape.

$sp$, $sp^2$, and $sp^3$. Hybridization corresponds to electron domains: $sp$ for linear (2), $sp^2$ for trigonal planar (3), $sp^3$ for tetrahedral (4).