Lecture 32: Molecular Shape and Polarity

Comprehensive vocabulary flashcards reviewing molecular geometries, VSEPR theory, steric numbers, 3D representations, bond polarity continuum, and dipole moments from Lecture 32.

Steric Number (SN)

The total number of electron groups around a central atom, calculated as: $\text{SN} = (\text{number of atoms bonded to central atom}) + (\text{number of lone pairs on central atom})$.

Electron Geometry

The three-dimensional arrangement of all electron groups (bonding pairs and lone pairs) surrounding the central atom.

Molecular Geometry

The actual three-dimensional arrangement of the bonded atoms present in a molecule, determined by excluding lone pairs from the electron geometry.

Electron Group

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

Electron Group Repulsion Hierarchy

The relative strengths of electron pair repulsions in VSEPR theory, varying in the order: $\text{bp-bp} < \text{bp-lp} < \text{lp-lp}$ (bonding pair–bonding pair < bonding pair–lone pair < lone pair–lone pair).

Seesaw Molecular Geometry

The molecular shape resulting from a steric number of $5$ ($\text{SN} = 5$) with $4$ bonding groups and $1$ lone pair occupying an equatorial position (e.g., $\text{SF}_4$).

T-Shaped Molecular Geometry

The molecular shape resulting from a steric number of $5$ ($\text{SN} = 5$) with $3$ bonding groups and $2$ lone pairs placed in equatorial positions (e.g., $\text{ClF}_3$ or $\text{BrF}_3$), with bond angles $< 90^\circ$.

Linear Molecular Geometry (SN = 5)

The molecular geometry observed when a central atom has $5$ electron groups ($\text{SN} = 5$) consisting of $2$ bonding groups and $3$ lone pairs occupying the equatorial positions (e.g., $\text{XeF}_2$), yielding a $180^\circ$ bond angle.

Square Pyramidal Molecular Geometry

The molecular geometry resulting from an octahedral electron geometry ($\text{SN} = 6$) having $5$ bonding groups and $1$ lone pair (e.g., $\text{BrF}_5$), yielding bond angles $< 90^\circ$.

Square Planar Molecular Geometry

The molecular geometry resulting from an octahedral electron geometry ($\text{SN} = 6$) with $4$ bonding groups and $2$ lone pairs positioned opposite each other (e.g., $\text{XeF}_4$), yielding $90^\circ$ bond angles.

Equatorial Lone Pair Preference

In trigonal bipyramidal electron geometry ($\text{SN} = 5$), lone pairs preferentially occupy equatorial positions because this results in only two $90^\circ$ lone pair–bonding pair repulsions, whereas occupying an axial position produces three unfavorable $90^\circ$ repulsions.

3D Bond Symbols in 2D

Conventions used to depict 3D molecular geometries on paper: a straight line indicates a bond in the plane of the page, a hatched wedge indicates a bond going into the page, and a solid wedge indicates a bond coming out of the page.

Pure (Nonpolar) Covalent Bond

A bond where electrons are shared equally between atoms, typically defined by an electronegativity difference ($\Delta\text{EN}$) of $0.0$ to $0.4$.

Polar Covalent Bond

A bond where electrons are shared unequally between atoms, typically defined by an electronegativity difference ($\Delta\text{EN}$) of $0.4$ to $2.0$.

Ionic Bond

A chemical bond involving electron transfer between atoms, typically defined by an electronegativity difference ($\Delta\text{EN}$) of $2.0$ to $3.3$.

Continuum of Bond Types

The classification of chemical bonding based on electronegativity difference ($\Delta\text{EN}$): pure covalent ($0.0 - 0.4$), polar covalent ($0.4 - 2.0$), and ionic ($2.0 - 3.3$).

Molecular Dipole Moment Determination

The approach of determining overall molecular polarity by adding the vectors associated with the dipole moments of all individual polar bonds.

Debye (D)

A standard unit for expressing permanent dipole moments of molecules, where $1\,\text{Debye} = 1\,\text{D} \approx 3.34 \times 10^{-30}\,\text{C}\cdot\text{m}$.

Multi-Center Molecular Geometry Approach

The technique for predicting the three-dimensional shapes of larger molecules (such as methanol or glycine) by applying single-center VSEPR analysis to each interior atom separately.