VSEPR Molecular Geometry Reference Cheat Sheet

Key Facts

• VSEPR stands for valence shell electron pair repulsion, and it predicts shapes by placing electron domains as far apart as possible.
• The steric number is $SN = X + E$, where $X$ is the number of bonded atoms and $E$ is the number of lone pairs on the central atom.
• For $SN = 2$, the electron geometry is linear, the common AXE form is $AX_2$, and the ideal bond angle is $180^\circ$.
• For $SN = 3$, the electron geometry is trigonal planar, $AX_3$ has bond angles of $120^\circ$, and $AX_2E_1$ is bent with angles less than $120^\circ$.
• For $SN = 4$, the electron geometry is tetrahedral, $AX_4$ has bond angles of $109.5^\circ$, $AX_3E_1$ is trigonal pyramidal, and $AX_2E_2$ is bent.
• For $SN = 5$, the electron geometry is trigonal bipyramidal, with ideal angles of $90^\circ$, $120^\circ$, and $180^\circ$.
• For $SN = 6$, the electron geometry is octahedral, with ideal angles of $90^\circ$ and $180^\circ$.
• Lone pairs repel more strongly than bonding pairs, so the repulsion order is $E-E > E-X > X-X$ and lone pairs usually compress bond angles.

Vocabulary

VSEPR theory

A model that predicts molecular shape by arranging electron domains around a central atom to minimize repulsion.

Electron domain

A region of electron density around a central atom, including a single bond, double bond, triple bond, or lone pair.

Steric number

The total number of bonded atoms and lone pairs around the central atom, calculated as $SN = X + E$.

AXE notation

A shorthand system where $A$ is the central atom, $X$ is the number of bonded atoms, and $E$ is the number of lone pairs.

Electron geometry

The arrangement of all electron domains around the central atom, including both bonds and lone pairs.

Molecular geometry

The shape formed by the atoms only, after lone pairs are considered but not shown as vertices of the shape.