chapter 10 -- the shapes of Molecules

axial group

An atom (or group) that lies above or below the trigonal plane of a trigonal bipyramidal molecule, or a similar structural feature in a molecule.

bond angle

The angle formed by the bonds joining the nuclei of two surrounding atoms to the nucleus of the central atom, which is at the vertex

dipole moment (μ)

A measure of molecular polarity; the magnitude of the partial charges on the ends of a molecule (in coulombs) times the distance between them (in meters).

electron deficient

Referring to a bonded atom, such as Be or B, that has fewer than eight valence electrons

electron-pair delocalization

(also delocalization) The process by which electron density is spread over several atoms rather than remaining between two

equatorial group

An atom (or group) that lies in the trigonal plane of a trigonal bipyramidal molecule, or a similar structural feature in a molecule

expanded valence shell

A valence level that can accommodate more than eight electrons by using available d orbitals; occurs only with central nonmetal atoms from Period 3 or higher.

formal charge

The hypothetical charge on an atom in a molecule or an ion, equal to the number of valence electrons minus the sum of all the unshared and half the shared valence electrons

free radical

A molecular or atomic species with one or more unpaired electrons, which typically make it very reactive

Lewis structure

(also Lewis formula) A structural formula consisting of electron-dot symbols, with lines as bonding pairs and dots as lone pairs

linear arrangement

The geometric arrangement obtained when two electron groups maximize their separation around a central atom.

linear shape

A molecular shape formed by three atoms lying in a straight line, with a bond angle of 180° (shape class AX2 or AX2E3)

molecular polarity

The overall distribution of electronic charge in a molecule, determined by its shape and bond polarities

molecular shape

The three-dimensional arrangement of the atomic nuclei in a molecule.

octahedral arrangement

The geometric arrangement obtained when six electron groups maximize their space around a central atom; when all six groups are bonding groups, the molecular shape is octahedral (AX6; ideal bond angle = 90°)

resonance hybrid

The weighted average of the resonance structures for a species.

resonance structure

(also resonance form) One of two or more Lewis structures for a species that cannot be adequately depicted by a single structure. Resonance structures differ only in the position of bonding and lone electron pairs.

seesaw shape

A molecular shape caused by the presence of one equatorial lone pair in a trigonal bipyramidal arrangement (AX4E).

square planar shape

A molecular shape (AX4E2) caused by the presence of two lone pairs at opposite vertices in an octahedral electron-group arrangement.

square pyramidal shape

A molecular shape (AX5E) caused by the presence of one lone pair in an octahedral electron-group arrangement.

T-shaped

A molecular shape caused by the presence of two equatorial lone pairs in a trigonal bipyramidal arrangement (AX3E2).

tetrahedral arrangement

The geometric arrangement formed when four electron groups maximize their separation around a central atom; when all four groups are bonding groups, the molecular shape is tetrahedral (AX4; ideal bond angle 109.5°).

trigonal bipyramidal arrangement

The geometric arrangement formed when five electron groups maximize their separation around a central atom. When all five groups are bonding groups, the molecular shape is trigonal bipyramidal (AX5; ideal bond angles, axial-center-equatorial 90° and equatorial-center-equatorial 120°).

trigonal planar arrangement

The geometric arrangement formed when three electron groups maximize their separation around a central atom. When all three groups are bonding groups, the molecular shape is trigonal planar (AX3; ideal bond angle 120°).

trigonal pyramidal shape

A molecular shape (AX3E) caused by the presence of one lone pair in a tetrahedral arrangement.

valence-shell electron-pair repulsion (VSEPR) theory

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