VSEPR Theory and Molecular Geometry

Comprehensive vocabulary flashcards covering Valence Shell Electron Pair Repulsion (VSEPR) theory, electron-domain geometries, molecular shapes, bond angles, hybridization, and notation.

Valence Shell Electron Pair Repulsion (VSEPR) Theory

A model stating that electron groups around a central atom repel each other and arrange themselves as far apart as possible.

Valence

The outermost electrons of an atom.

Shell

The region around the atom where the valence electrons are located.

Electron Pair

A pair of electrons occupying a region around the central atom.

Repulsion

The electrostatic force pushing electron groups apart because electrons share the same negative charge.

Electron Group

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

Four Steps of VSEPR Analysis

The systematic procedure used to predict molecular shape: Step 1: Count the electron groups; Step 2: Determine the electron geometry; Step 3: Determine the molecular geometry; Step 4: Classify the polarity.

$AX_mE_n$ Notation

A formula representing molecular geometry where $A$ represents the central atom, $X$ represents a bonded atom, $E$ represents a nonbonding valence electron group (usually a lone pair), and $m$ and $n$ are integers.

Linear Electron-Domain Geometry

An ideal symmetrical geometry containing $2$ electron groups with an ideal bond angle of $180^\circ$ and $sp$ hybridization (e.g., $BeCl_2$, $CO_2$).

Trigonal Planar Electron-Domain Geometry

An ideal symmetrical geometry containing $3$ electron groups pointing toward the corners of an equilateral triangle in a single plane, with an ideal bond angle of $120^\circ$ and $sp^2$ hybridization (e.g., $BF_3$).

Tetrahedral Electron-Domain Geometry

An ideal symmetrical geometry containing $4$ electron groups directed toward the vertices of a regular tetrahedron, with an ideal bond angle of $109.5^\circ$ and $sp^3$ hybridization (e.g., $CH_4$).

Trigonal Bipyramidal Electron-Domain Geometry

An electron geometry containing $5$ electron groups across two distinct spatial environments (equatorial at $120^\circ$ and axial at $90^\circ$), with $sp^3d$ hybridization (e.g., $PCl_5$).

Octahedral Electron-Domain Geometry

A highly symmetrical geometry containing $6$ equivalent electron groups pointing toward the corners of a regular octahedron ($4$ equatorial and $2$ axial), with adjacent bond angles of $90^\circ$, opposite bond angles of $180^\circ$, and $sp^3d^2$ hybridization (e.g., $SF_6$).

Equatorial Position

Positions in a trigonal bipyramidal geometry where three groups lie within a single central triangular plane, separated by angles of $120^\circ$.

Axial Position

Positions in trigonal bipyramidal or octahedral geometries situated perpendicular to and lying above and below the central plane ($90^\circ$ relative to equatorial groups).

Bent Molecular Geometry ($AX_2E$)

A molecular geometry with a trigonal planar parent electron geometry containing $2$ bonding pairs and $1$ lone pair, resulting in a bond angle of $< 120^\circ$ (approx. $119^\circ$; e.g., $SO_2$, $O_3$, $NO_2^-$).

Trigonal Pyramidal Molecular Geometry ($AX_3E$)

A molecular geometry with a tetrahedral parent electron geometry containing $3$ bonding pairs and $1$ lone pair, producing a bond angle of $< 109.5^\circ$ (approx. $107^\circ$ in $NH_3$; e.g., $NH_3$, $H_3O^+$, $PCl_3$).

Bent / Angular Molecular Geometry ($AX_2E_2$)

A molecular geometry with a tetrahedral parent electron geometry containing $2$ bonding pairs and $2$ lone pairs, resulting in significant repulsion that compresses the bond angle to $< 109.5^\circ$ (approx. $104.5^\circ$ in $H_2O$; e.g., $H_2O$, $H_2S$, $OF_2$).

Sawhorse / Seesaw Molecular Geometry ($AX_4E$)

A molecular geometry with a trigonal bipyramidal parent electron geometry containing $4$ bonding pairs and $1$ lone pair placed in an equatorial position, with equatorial bond angles of $< 120^\circ$ and axial bond angles of $< 90^\circ$ (e.g., $SF_4$).

T-Shaped Molecular Geometry ($AX_3E_2$)

A molecular geometry with a trigonal bipyramidal parent electron geometry containing $3$ bonding pairs and $2$ lone pairs occupying equatorial positions, resulting in a bond angle of $< 90^\circ$ (approx. $87.5^\circ$; e.g., $ClF_3$, $BrF_3$).

Linear Molecular Geometry ($AX_2E_3$)

A molecular geometry with a trigonal bipyramidal parent electron geometry containing $2$ bonding pairs and $3$ lone pairs in equatorial positions $120^\circ$ apart, leaving the bonded atoms in an axial line with an exact bond angle of $180^\circ$ (e.g., $XeF_2$, $I_3^-$).

Square Pyramidal Molecular Geometry ($AX_5E$)

A molecular geometry with an octahedral parent electron geometry containing $5$ bonding pairs and $1$ lone pair occupying an axial site, resulting in a bond angle of $< 90^\circ$ (approx. $84.8^\circ$; e.g., $BrF_5$, $IF_5$).

Square Planar Molecular Geometry ($AX_4E_2$)

A molecular geometry with an octahedral parent electron geometry containing $4$ bonding pairs and $2$ lone pairs positioned $180^\circ$ opposite each other at axial sites, resulting in an exact bond angle of $90^\circ$ (e.g., $XeF_4$, $PtCl_4^{2-}$).

T-Shaped Molecular Geometry ($AX_3E_3$)

A molecular geometry derived from an octahedral parent electron geometry containing $3$ bonding pairs and $3$ lone pairs.

Linear Molecular Geometry ($AX_2E_4$)

A molecular geometry derived from an octahedral parent electron geometry containing $2$ bonding pairs and $4$ lone pairs.