Molecular Geometry and Bonding Theories

Fundamental Properties of Models

Models are human inventions that explain how nature works but do not equal reality

Models are often wrong to some degree

Models tend to become more complicated as they are updated

Models are limited by their assumptions

We learn even when the model is shown to be wrong

The Localized Electron Bonding Model

The Localized Electron (LE) model assumes that a molecule is composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms

Electron pairs are assumed to be localized on a particular atom or between atoms

Electron pairs localized on an atom are lone pairs

Electron pairs localized between atoms are bonding pairs.

The LE model has 3 parts

· Description of valence electron arrangement using Lewis structures

· Prediction of the geometry of the molecule using valence shell electron pair repulsion (VSEPR) model

· Description of the type of atomic orbitals containing the electrons

Molecular Structure: The VSEPR Model

Molecular Geometry

Molecular Structure is the three dimensional arrangement of atoms in a molecule

Electronic Geometry

Electronic Structure is the three dimensional arrangement of valence orbitals (areas of electron density {Electron Domains}) in a molecule.

Electron Domains consist of each nonbonding electron pair and each bond (bonded atom).

Valence Shell Electron-Pair Repulsion (VSEPR) Model

Valence Shell Electron-Pair Repulsion (VSEPR) Model is a simple but useful method of estimating molecular structure of nonmetals

The postulate is that the structure around a given atom is determined by miminizing electron-pair repulsions.

VSEPR Model and Multiple Bonds

Double bonds count as one electron domain, one area of electron density

Triple bonds also count as one electron domain, one area of electron density

When resonance structures exist, any one of the resonance structures can be used to predict the molecular structure.

Linear Structure

A Linear Structure results when there are two areas of electron density around an atom
The bond angle is 180°

Occurs when there are two double bonds or one triple bond.

Trigonal Planar Structure

A Trigonal Planar Structure results when there are three areas of electron density around an atom
The bond angle is 120°

Occurs with 1 double bond.

Tetrahedral Arrangement

Tetrahedral Arrangement results when there are four areas of electron density around an atom
The bond angle is 109.5°

Occurs with only single bonds and complete octet.

Trigonal pyramid (only molecular geometry)

Three bonding pairs and a lone pair results in a Trigonal pyramid configuration

Bent or Angled (only molecular geometry)

Two bonding pairs and two lone pairs (tetrahedral electron geometry) results in a bent or angled configuration with bond angle of about 109°.

Two bonding pairs (1 double bond) and one lone pair (trigonal planar electron geometry) results in a bent or angled configuration with bond angle of about 120°.

Trigonal bipyramid (exceeding octet)

Trigonal bipyramid arrangement results when there are five areas of electron density around an atom

Octahedral Structure (exceeding octet)

Octahedral Structure arrangement results when there are six areas of electron density around an atom

Square Planar Structure (exceeding octet; molecular geometry)

Four bonding pairs and two lone pairs results in a Square Planar Structure

Total e- domains	Electron Geometry	Hybrid Orbitals	Bonding Domains	Nonbonding Domains	Molecular Geometry/ Bond Angle	Example	Comment
2	Linear	sp	2	0	Linear 180°	CO₂ HCN	Octet satisfied, Two double bonds or One triple bond
3	Trigonal planar	sp²	3	0	Trigonal Planar 120°	SO₃	Octet satisfied, One double bond
3	Trigonal planar	sp²	2	1	Bent 120°	SO₂	Octet satisfied, One double bond
4	Tetrahedral	sp³	4	0	Tetrahedral 109.5°	SO₄^2-	Octet satisfied, Only single bonds
			3	1	Pyramidal 107°	NH₃
			2	2	Bent 104.5°	H₂O
5	Trigonal bipyramidal	dsp³	5	0	Trigonal bipyramidal 90° & 120°	PCl₅	Octet exceeded, 10 electrons
			4	1	Seesaw 90° & 120°	SF₄
			3	2	T-shaped 90°	ClF₃
			2	3	Linear 180°	XeF₂
6	Octahedral	d²sp³	6	0	Octahedral 90°	SF₆	Octet exceeded, 12 electrons
			5	1	Square pyramidal 90°	BrF₅
			4	2	Square planar 90°	XeF₄

Molecules Containing No Single Central Atom

Larger molecules with no single central atom have complex geometries. We can describe the geometry around eacy Central stom

Summary of VSEPR Model

· Determine Lewis structure for molecule

· For molecules with resonance structures, use any of the structures

· Sum the lone pairs and bonds (electron domains) around the central atom (double or triple bonds count as one bond)

· Choose arrangement for number of electron domains

· Lone pairs require more space than bonding pairs

Polarity

The dipole moment (polarity) depends on both the polarities of the individual bonds and the geometry of the molecule.

Bond dipole is the dipole moment between two bonded atoms. A difference in electronegativity of greater than 0.4 is a polar bond.

If all the bonds in a molecule are nonpolar, then the molecule is nonpolar.

If some bonds are polar, then the molecule will be polar unless the molecule is highly symmetric allowing all the bond dipoles to cancel out leaving no net dipole across the molecule.

A molecule is symmetric (and nonpolar) if all the atoms attached to the central atom are identical and there are no nonbonding pairs on the central atom.