Chapter 10 Chemical Bonding II

Structure Determines Properties! properties of molecular substances depend on the structure of the molecule the structure includes many factors, including: the skeletal arrangement of the atoms the kind of bonding between the atoms ionic, polar covalent, or covalent the shape of the molecule bonding theory should allow you to predict the shapes of molecules

Molecular Geometry Molecules are 3-dimensional objects We often describe the shape of a molecule with terms that relate to geometric figures These geometric figures have characteristic “corners” that indicate the positions of the surrounding atoms around a central atom in the center of the geometric figure The geometric figures also have characteristic angles that we call bond angles

Using Lewis Theory to Predict Molecular Shapes Lewis theory predicts there are regions of electrons in an atom based on placing shared pairs of valence electrons between bonding nuclei and unshared valence electrons located on single nuclei this idea can then be extended to predict the shapes of molecules by realizing these regions are all negatively charged and should repel

VSEPR Theory electron groups around the central atom will be most stable when they are as far apart as possible – we call this valence shell electron pair repulsion theory since electrons are negatively charged, they should be most stable when they are separated as much as possible the resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule

Electron Groups the Lewis structure predicts the arrangement of valence electrons around the central atom(s) each lone pair of electrons constitutes one electron group on a central atom each bond constitutes one electron group on a central atom regardless of whether it is single, double, or triple there are 3 electron groups on N 1 lone pair 1 single bond 1 double bond

Molecular Geometries there are 5 basic arrangements of electron groups around a central atom based on a maximum of 6 bonding electron groups though there may be more than 6 on very large atoms, it is very rare each of these 5 basic arrangements results in 5 different basic molecular shapes in order for the molecular shape and bond angles to be a “perfect” geometric figure, all the electron groups must be bonds and all the bonds must be equivalent for molecules that exhibit resonance, it doesn’t matter which resonance form you use – the molecular geometry will be the same

Parent electronic structure

Trigonal Bipyramidal Geometry when there are 5 electron groups around the central atom, they will occupy positions in the shape of a two tetrahedral that are base-to-base with the central atom in the center of the shared bases this results in the molecule taking a trigonal bipyramidal geometry the positions above and below the central atom are called the axial positions the positions in the same base plane as the central atom are called the equatorial positions the bond angle between equatorial positions is 120° the bond angle between axial and equatorial positions is 90°

Octahedral Geometry when there are 6 electron groups around the central atom, they will occupy positions in the shape of two square-base pyramids that are base-to-base with the central atom in the center of the shared bases this results in the molecule taking an octahedral geometry it is called octahedral because the geometric figure has 8 sides all positions are equivalent the bond angle is 90°

Examples How many electron groups (charge clouds) are around the central atom in the following? SO2 NH4+ PCl5

The Effect of Lone Pairs lone pair groups “occupy more space” on the central atom because their electron density is exclusively on the central atom rather than shared like bonding electron groups relative sizes of repulsive force interactions is: Lone Pair – Lone Pair > Lone Pair – Bonding Pair > Bonding Pair – Bonding Pair this effects the bond angles, making them smaller than expected

Effect of Lone Pairs The bonding electrons are shared by two atoms, so some of the negative charge is removed from the central atom. The nonbonding electrons are localized on the central atom, so area of negative charge takes more space.

Derivative of Trigonal Geometry when there are 3 electron groups around the central atom, and 1 of them is a lone pair, the resulting shape of the molecule is called a trigonal planar - bent shape the bond angle is < 120°

Tetrahedral-Bent Shape

Bond Angle Distortion from Lone Pairs

Replacing Atoms with Lone Pairs in the Trigonal Bipyramid System

T-Shape

Linear Shape

Predicting the Shapes Around Central Atoms Total # of e- groups on central atom “Parent” electronic geometry # Bonded atoms # Lone pairs Idealized molecular shape Idealized bond angles 2 Linear 180o 3 Trigonal Planar 120 o 1 Bent 4 Tetrahedral 109.5 o Trigonal Pyramidal 5 Trigonal Bipyramidal 90 o, 120 o, 180 o Seesaw T-shaped 90 o, 180 o 180 o 6 Octahedral Square Pyramidal Square Planar

Real bond angles vs. Idealized bond angles VSEPR predicts the idealized bond angle(s) by assuming that all electron groups take up the same amount of space. Since lone pairs are attracted to only one nucleus, they expand into space further than bonding pairs, which are attracted to two nuclei. As a result, real molecules that has lone pairs on the central atom often have bond angles that are slightly different than the idealized prediction Central atom without lone pairs has the same real bond angle as the idealized angle.   The exceptions to this are square planar shapes and linear (derived from trigonal bipyramidal electronic structure) shapes where the lone pairs offset one another, thus causing no deviation from ideality.

Example Lewis structure Shape Idealized bond angle Real bond angle  

Multiple Central Atoms many molecules have larger structures with many interior atoms we can think of them as having multiple central atoms when this occurs, we describe the shape around each central atom in sequence

Example: Give the approximate molecular shape at the numbered “central” atoms

Representing 3-Dimensional Shapes on a 2-Dimensional Surface one of the problems with drawing molecules is trying to show their dimensionality by convention, the central atom is put in the plane of the paper put as many other atoms as possible in the same plane and indicate with a straight line for atoms in front of the plane, use a solid wedge for atoms behind the plane, use a hashed wedge

Polarity of Molecules in order for a molecule to be polar it must have polar bonds electronegativity difference - theory bond dipole moments - measured have an unsymmetrical shape vector addition polarity affects the intermolecular forces of attraction therefore boiling points and solubilities like dissolves like nonbonding pairs affect molecular polarity, strong pull in its direction

Molecule Polarity The H-Cl bond is polar. The bonding electrons are pulled toward the Cl end of the molecule. The net result is a polar molecule.

Molecule Polarity The O-C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule.

Molecule Polarity The H-O bond is polar. The both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule. Tro, Chemistry: A Molecular Approach

Factors Affecting Dipole Moments Lone-pair electrons on oxygen and nitrogen project out into space away from positively charged nuclei giving rise to a considerable charge separation and contributing to the dipole moment

Molecular Polarity Affects Solubility in Water polar molecules are attracted to other polar molecules since water is a polar molecule, other polar molecules dissolve well in water and ionic compounds as well some molecules have both polar and nonpolar parts

A Soap Molecule Sodium Stearate

Example - Decide Whether the Following Are Polar EN O = 3.5 N = 3.0 Cl = 3.0 S = 2.5

Problems with Lewis Theory Lewis theory gives good first approximations of the bond angles in molecules, but usually cannot be used to get the actual angle Lewis theory cannot write one correct structure for many molecules where resonance is important Lewis theory often does not predict the correct magnetic behavior of molecules e.g., O2 is paramagnetic, though the Lewis structure predicts it is diamagnetic