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Chapter 8

Bonding: General Concepts

Shaun Williams, PhD

Questions to Consider

Types of Chemical Bonds

A Chemical Bond

The Interaction of Two Hydrogen Atoms

As two hydrogen atoms come close together, the electron cloud from each atom stretches out towards the other hydrogen atom. Eventually the electron clouds merge.

The Interaction of Two Hydrogen Atoms - Plot

A plot of the energy of a system of two interacting hydrogen atoms. As the atoms come together, the energy drops, slowly at first and then ever faster until it reaches a minimum. Then the energy rises at very small distances between the hydrogen atoms due to nuclear-nuclear repulsion.

Key Ideas in Bonding

Polar Covalent Bond

The Effect of an Electric Field on Hydrogen Fluoride Molecules

\(\delta^-\) or \(\delta^+\) indicates a positive or negative fractional charge.

In the hydrogen fluoride molecule, the hydrogen atom is partially positive and the fluorine atom is partially negative. If the molecule is placed between metal plates, one charges postive and the other negative, the molecules will align since opposite charges attract.

Electronegativity

The Pauling Electronegativity Values

The trends in electronegativity values is that electronegativities decrease down the periodic table and they increase from left to right.

The Relationship Between Electronegativity and Bond Type

When there is no electronegativity difference then the bond is entirely covalent. At intermediate differences in electronegativity we have a polar covalent bond. At large differences, we have ionic bonds.

Exercise 1

Arrange the following bonds from most to least polar:

  1. N-F, O-F, C-F
  2. C-F, N-O, Si-F
  3. Cl-Cl, B-Cl, S-Cl

Exercise 1 - Answer

Arrange the following bonds from most to least polar:

  1. N-F, O-F, C-F → C-F, N-F, O-F
  2. C-F, N-O, Si-F → Si-F, C-F, N-O
  3. Cl-Cl, B-Cl, S-Cl → B-Cl, S-Cl, Cl-Cl

Bond Polarity and Dipole Moments

Dipole Moment

Dipole Moment in Water

In water, the hydrogen atoms are partially positive and the oxygen is partially negative. The water molecule will points its hydrogen end towards a negatively charged plate and its oxygen end towards a positively charged place. The electrons have a higher probability of being at the oxygen end rather than the hydrogen end.

Dipole Moment in Carbon Dioxide - No Net Dipole Moment (Dipoles Cancel)

In carbon dioxide, the carbon atom is partially positive and the oxygen atoms are partially negative. Due to the geometry of the molecule, the pull of electrons towards the oxygen atoms cancel out so there is not net dipole moment.

Ions: Electron Configurations and Sizes

Stable Compounds

Electron Configurations in Stable Compounds

Isoelectronic Series

Periodic Table Allows Us to Predict Many Properties

Energy Effects in Binary Ionic Compounds

Lattice Energy

Formation of an Ionic Solid

Comparing Energy Changes

A graphical representation of all the information provided on the previous slide.

Partial Ionic Character of Covalent Bonds

The relationship between the ionic character of a covalent bond and the electronegativity difference of the bonded atoms

The differences between the electronegativities in various compounds and their percent ionic character. For the most part, nonmetal-nonmetal compounds have less than 50% ionic character while metal-nonmetal compounds have greater than 50% ionic character.

Operational Definition of Ionic Compound

The Covalent Chemical Bond: A Model

Models

Fundamental Properties of Models

  1. A model does not equal reality.
  2. Models are oversimplifications, and are therefore often wrong.
  3. Models become more complicated and are modified as they age.
  4. We must understand the underlying assumptions in a model so that we don't misuse it.
  5. When a model is wrong, we often learn much more than when it is right.

Covalent Bond Energies and Chemical Reactions

Bond Energies

$$ \Delta H = \sum n \times D_\text{bonds broken} - \sum n \times D_\text{bonds formed} $$ \( D \) represents the bond energy per mole of bonds (always has a positive sign).

The Localized Electron Bonding Model

Localized Electron Model

Localized Electron Model - What is it?

  1. Description of valence electron arrangement (Lewis structure).
  2. Prediction of geometry (VSEPR model).
  3. Description of atomic orbital types used by atoms to share electrons or hold lone pairs.

Lewis Structures

Lewis Structure

Duet Rule

In the hydrogen molecules, the electron from each atom is shared, as a pair, by both atoms.

Octet Rule

The fluorine molecule consists of two fluorine atoms sharing electrons.

Types of Covalent Bonds

Steps for Writing Lewis Structures

  1. Sum the valence electrons from all the atoms.
  2. Use a pair of electrons to form a bond between each pair of bound atoms.
  3. Atoms usually have noble gas configurations. Arrange the remaining electrons to satisfy the octet rule (or duet rule for hydrogen).

Step 1: Sum the valence electrons from all the atoms. (Use the periodic table.)

$$ \text{Example: }\chem{H_2O} $$ $$ 2\left( 1\,e^- \right) + 6 e^- = 8\,e^-\,\text{total} $$

Step 2: Use a pair of electrons to form a bond between each pair of bound atoms.

$$ \text{Example: }\chem{H_2O} $$ Hydrogen atom single bond to oxygen single bond to the other hydrogen atom.

Step 3: Atoms usually have noble gas configurations. Arrange the remaining electrons to satisfy the octet rule (or duet rule for hydrogen).

$$ \text{Example: }\chem{H_2O,\, PBr_3, \,and\, HCN} $$ Hydrogen atom single bond to oxygen single bond to the other hydrogen atom. Two lone pair of electrons on the oxygen. Hydrogen atom single bond to carbon atom triple bond to nitrogen atom and a lone pair of electrons on the nitrogen atom. A phosphorous atom with a lone pair of electrons singly bonded to three bromine atoms. Each bromine atom has three lone electron pairs.

Concept Check 1

Draw a Lewis structure for each of the following molecules:

Exceptions to the Octet Rule

Too Few Electrons

Expanded Octets

Concept Check 2

Draw a Lewis structure for each of the following molecules:

Let's Review

Resonance

Concept Check 3

Draw a Lewis structure for each of the following molecules:

Formal Charge

Rules Governing Formal Charge

Concept Check 4

Consider the Lewis structure for \(\chem{POCl_3}\). Assign the formal charge for each atom in the molecule. A phosphorous atom singly bonded to one oxygen atom with three lone electron pairs and three chlorine atoms each with three lone electron pairs.

Molecular Structure: The VSEPR Model

VSEPR Model

Steps to Apply the VSEPR Model

  1. Draw the Lewis structure for the molecule.
  2. Count the electron pairs and arrange them in the way that minimizes repulsion (put the pairs as far apart as possible.
  3. Determine the positions of the atoms from the way electron pairs are shared (how electrons are shared between the central atom and surrounding atoms).
  4. Determine the name of the molecular structure from positions of the atoms.

Arrangements of Electrons Around an Atom Yielding Minimum Repulsion

A listing of the parent shapes. Two electron pairs create a linear molecule. Three electron pairs create a trigonal planar. Four electron pairs create a tetrahedral molecule. Five electron pairs create a trigonal bipyramidal molecule. Six electron pairs create an oxtahedral molecule.

Structures of Molecules That Have Four Electron Pairs Around the Central Atom

A central atom bonded to four atoms with no lone pairs has a tetrahedral shape. A central atom bonded to three atoms with one lone pair has a trigonal pyramidal shape. A central atom bonded to two atoms with two lone pairs is v-shaped or bent.

Structures of Molecules with Five Electron Pairs Around the Central Atom

A central atom bonded to five atoms with no lone pairs has a trigonal bipyramidal shape. A central atom bonded to four atoms with one lone pair has a see-saw shape. A central atom bonded to three atoms with two lone pairs is T-shaped. A central atom bonded to two atoms with three lone pairs has a linear shape.

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