Chapter 10.2: Valence-Shell Electron-Pair Repulsion (VSEPR) Theory

Flashcards from Chapter 10.2 of Chemistry: The Molecular Nature of Matter and Change.

Valence-shell electron pair repulsion theory (VSEPR theory)

A model explaining that the shapes of molecules and ions result from minimizing electron-pair repulsions around a central atom

• Only groups around a central atom affect shape

• Bonds, lone pairs, or lone electrons can all count as one group

• Ultimately derived from the Lewis structure

Electron group arrangement

The arrangement of bonding and nonbonding groups (unshared electrons) around a central atom; includes different shapes, such as:

• Linear (180°)

• Trigonal planar (120°)

• Tetrahedral (109.5°)

• Trigonal bipyramidal (90° and 120°)

• Octahedral (90°)

Molecular shape

The three-dimensional arrangement of the atoms connected by bonding electron groups only

• Only the same as the electron group arrangement when nonbonding groups are absent

• Presence of nonbonding groups results in differing types of this, requiring AXE designation

AXE designation

A way to notate the arrangement of atoms and unshared electrons in a molecule in the form AX_mE_n to determine the correct electron group arrangement and molecular shape

• A is the central atom

• X is a surrounding atom

• E is an unshared electron group (either a lone pair or lone electron)

Bond angle

The angle formed by two different atom bonds joined to a central atom

• Often deviate from the ideal number due to different bond types, atoms, or unshared groups

Linear arrangement

An electron group arrangement where two groups point in opposite directions, resulting in a molecule with a similarly named shape and 180° angles

• Applies to class AX₂ only

Trigonal planar arrangement

An electron group arrangement where three groups point to the corners of an equilateral triangle, varying with the presence of a lone pair:

• AX₃ gives a similarly named shape with the ideal 120° angles, such as in BF₃

• AX₂E gives a bent shape with <120° angles due to the presence of a lone pair

• Nonidentical bond types may also result in distortions, as in CH₂O

Perspective drawing

A drawing made to depict a three-dimensional structure using solid and dashed wedges for bonds out of the plane of the page

• Solid wedges lie above the page

• Dashed wedges lie below the page

Tetrahedral arrangement

An electron group arrangement where four groups lie at the corners of a tetrahedron, varying with the presence of a lone pair

• AX₄ gives a similarly named shape with the ideal 109.5° angles, such as in CH₄

• AX₃E gives a trigonal pyramidal shape with <109.5° angles as the lone pair repulses others

• AX₂E₂ gives a bent shape with <<109.5° angles with even stronger repulsion

Trigonal bipyramidal arrangement

An electron group arrangement where five groups form two trigonal pyramids, with three equatorial groups separated by 120° angles and two axial groups separated by 90° angles

• Lone pairs occupy equatorial positions when possible to minimize repulsion

• AX₅ gives a similarly named shape with the ideal angles

• AX₄E gives a seesaw shape, resulting in <120° and <90° angles

• AX₃E₂ gives a T-shaped shape, resulting in <<120° and <<90° angles

• AX₂E₃ gives a linear shape, resulting in the ideal angles once more

Octahedral arrangement

An electron group arrangement where six groups form an octahedron, separated by 90° angles on all sides

• AX₆ gives a similarly named shape with the ideal angles

• AX₅E gives a square pyramidal shape, with one lone pair reducing the bond angles to <90°

• AX₄E gives a square planar shape, with another oppositely-facing lone pair restoring the 90° angles