Unit 2: Chemical Bonding

Unit 2: Chemical Bonding
Chemistry2202

Outline Bohr diagrams Lewis Diagrams Types of Bonding Ionic bonding
Covalent bonding (Molecular) Metallic bonding Network covalent bonding

Types of Bonding (cont’d)
London Dispersion forces Dipole-Dipole forces Hydrogen Bonding VSEPR Theory (Shapes) Physical Properties

Bohr Diagrams (Review)
How do we draw a Bohr Diagram for The F atom? The F ion? Draw Bohr diagrams for the atom and the ion for the following: Al S C l Be

Lewis Diagrams LD provide a method for keeping track of electrons in atoms, ions, or molecules Also called Electron Dot diagrams the nucleus (P& N) and filled energy levels are represented by the element symbol

Lewis Diagrams dots are placed around the element symbol to represent valence electrons

F • • • • • • • Lewis Diagrams lone pair bonding electron lone pair
eg. Lewis Diagram for F lone pair • • bonding electron • F lone pair • • • • lone pair

Lewis Diagrams lone pair – a pair of electrons not available for bonding bonding electron – a single electron that may be shared with another atom

Lewis Diagrams eg. Lewis Diagram for C • • C • •

Lewis Diagrams eg. Lewis Diagram for P • • • P • •

Lewis Diagrams eg. Lewis Diagram for Na • Na

Li Be Al Si Mg N B O Lewis Diagrams
For each atom draw the Lewis diagram and state the number of lone pairs and number of bonding electrons Li Be Al Si Mg N B O

Lewis Diagrams for Compounds
draw the LD for each atom in the compound The atom with the most bonding electrons is the central atom Connect the other atoms using single bonds (1 pair of shared electrons) In some cases there may be double bonds or triple bonds

eg. Draw the LD for: PH3 CF4 Cl2O C2H6 C2H4 C2H2

eg. Draw the LD for: NH3 SiCl4 N2H4 HCN SI2 CO2 N2H2 CH2O POI CH3OH N2 H2 O2

A structural formula shows how the atoms are connected in a molecule. To draw a structural formula: replace the bonded pairs of electrons with short lines omit the lone pairs of electrons

Why is propane (C3H8) a gas at STP while kerosene (C10H22) a liquid?

Why is graphite soft enough to write with while diamond is the hardest substance known even though both substances are made of pure carbon?

Why can you tell if it is ‘real gold’ or just ‘fool’s gold’ (pyrite) by hitting it with a rock?

‘As Slow As Cold Molasses’
‘All Because of Bonding’

-30 ºC

Bonding Bonding between atoms, ions and molecules determines the physical and chemical properties of substances. Bonding can be divided into two categories: - Intramolecular forces - Intermolecular forces

Bonding Intramolecular forces are forces of attraction between atoms or ions. Intramolecular forces include: ionic bonding covalent bonding metallic bonding network covalent bonding

Bonding Intermolecular forces are forces of attraction between molecules. Intermolecular forces include: London Dispersion Forces Dipole-Dipole forces Hydrogen Bonding

Ionic and Covalent Bonding
ThoughtLab p. 161 Identify #’s 1 - 6

Ionic Bonding Occurs between cations and anions – usually metals and non-metals. An ionic bond is the force of attraction between positive and negative ions. Properties: conduct electricity as liquids and in solution hard crystalline solids high melting points and boiling points brittle

Ionic Bonding In an ionic crystal the ions pack tightly together.
The repeating 3-D distribution of cations and anions is called an ionic crystal lattice.

Ionic Bonding Each anion can be attracted to six or more cations at once. The same is true for the individual cations.

Ionic Bonding

Covalent Bonding Occurs between non-metals in molecular compounds.
Atoms share bonding electrons to become more stable (noble gas structure). A covalent bond is a simultaneous attraction by two atoms for a common pair of valence electrons.

Covalent Bonding Molecular compounds have low melting and boiling points. Exist as distinct molecules.

Covalent Bonding Molecular compounds do not conduct electric current in any form

Property Ionic Molecular
Type of elements Metals and nonmetals Non-Metals Force of Attraction Positive ions attract negative ions Atoms attract a shared electron pair Electron movement Electrons move from the metal to the nonmetal Electrons are shared between atoms State at room temperature Always solids Solids, liquids, or gas

Property Ionic Molecular
Solubility Soluble or low solubility Soluble or insoluble Conductivity in solid state None Conductivity in liquid state Conducts Conductivity in solution

Metallic Bonding (p. 171) metals tend to lose valence electrons.
valence electrons are loosely held and frequently lost from metal atoms. This results in metal ions surrounded by freely moving valence electrons. metallic bonding is the force of attraction between the positive metal ions and the mobile or delocalised valence electrons

Metallic Bonding

Metallic Bonding This theory of metallic bonding is called the ‘Sea of Electrons’ Model or ‘Free Electron’ Model

Metallic Bonding This theory accounts for properties of metals
electrical conductivity - electric current is the flow of electrons - metals are the only solids in which electrons are free to move solids Attractive forces between positive cations and negative electrons are very strong

Metallic Bonding malleability and ductility
metals can be hammered into thin sheets(malleable) or drawn into thin wires(ductile). metallic bonding is non-directional such that layers of metal atoms slide past each other under pressure.

Network Covalent Bonding (p. 199)
occurs in 3 compounds (memorize these) diamond – Cn carborundum – SiC quartz – SiO2 large molecules with covalent bonding in 3-d each atom is held in place in 3-d by a network of other atoms

Network Covalent bonding
Properties: the highest melting and boiling points the hardest substances brittle do not conduct electric current in any form

1. Network Covalent (Cn ,SiO2 , SiC)
MP & BP decreases Strongest 1. Network Covalent (Cn ,SiO2 , SiC) 2. Ionic bonding(metal & nonmetal) 3. Metallic bonding (metals) 4. Molecular (nonmetals) Weakest

Valence Shell Electron Pair Repulsion theory (VSEPR)
The shape of molecules is determined by the arrangement of valence electron pairs around the atoms in a compound. There are 5 shapes that can be determined by the # of bonds and # of lone pairs on the central atom.

1. Tetrahedral (4 bonds; 0 lone pairs)
2. Pyramidal (3 bonds; 1 lone pair)

3. V-shaped (2 bonds; 2 lone pairs)

4. Trigonal Planar (3 bonds; 0 lone pairs)

5. Linear (2 bonds; 0 lone pairs)

For each molecule below draw the Lewis diagram and the shape diagram.
HOCl H2Se H2O2 NBr3 C2F4 C2H6 CHCl3 CH3OH I2 SiH4 HSiHO C2H2 HCN PBr3

Electronegativity (EN - p. 174)
EN is a measure of the attraction that an atom has for shared electrons. A higher EN means a stronger attraction or electrostatic pull on valence electrons EN values increase as you move: from left to right in a period up in a group or family

Increases

Electronegativity & Covalent Bonds
polar covalent bond a bond between atoms with different EN the shared electron pair is attracted more strongly to the atom with the higher EN δ− δ+ Cl H

Electronegativity (p. 174)
polar covalent bond a covalent bond between atoms with different EN the shared electron pair is attracted more strongly to the atom with the higher EN nonpolar covalent bond bond dipole Complete: #’s 7 – 9 on p.178

Weakest Covalent (nonmetals) → London Dispersion (all molecules) → Dipole-Dipole (polar molecules) → H bonding (H-N, H-O, H-F)

p. 226 #13 Omit parts g), j) – o), q), u), & v)
- Answers on p. 815 for #13 - Incorrect answers c), d), & s)

Electronegativity and Ionic Bonds
Because the EN of metals is so low, metals lose electrons to form cations Nonmetals gain electrons to form anions because the EN of nonmetals is relatively high

Electronegativity and Ionic Bonds
When ions form, the resulting electrostatic force is an ionic bond

Electronegativity and Covalent Bonds
Atoms in covalent compounds can either have: the same EN eg. Cl2 , PH3, NCl3 different EN eg. HCl

Atoms that have the same EN attract the shared valence electrons to the same extent. Covalent bonds resulting from equal sharing of the bonding electron pairs are called Nonpolar Covalent Bonds

Atoms that have different EN attract the shared pair of valence electrons at different strengths The atom with the higher EN exerts a stronger attraction on the shared electron pair eg. H2O

Since the oxygen atom has a higher EN the bonding electrons will be pulled closer to the oxygen atom This results in slight positive and negative charges within the bond. These charges are referred to as “partial charges” and are denoted with the Greek letter delta (δ).

The region around the oxygen atom will be slightly negative, and around the hydrogens will be slightly positive

The symbol, δ+ represents a partial positive charge (less than +1) and δ− represents a partial negative charge (less than −1). Since the bond is polarized into a positive area and a negative area the bond has a “bond dipole”.

The arrow points to the atom with the higher EN. p.178

Covalent bonds resulting from unequal (electronegativities) sharing of bonding electron pairs are called Polar Covalent Bonds

Electronegativity Homework
#’s 7, 8, & p. 178 #’s 1, 2, & p. 180

Bond Energy (pp ) 1. Describe the forces of attraction and repulsion present in all bonds. 2. What is bond length? 3. Define bond energy. 4. Which type of bond has the most energy? 5. How can bond energy be used to predict whether a reaction is endothermic or exothermic?

Test Outline Bohr Diagrams (atoms & ions)
Lewis Diagrams (Electron Dot) Ion Formation Ionic Bonding, Structures & Properties Covalent Bonding, Structures & Properties

Test Outline Metallic Bonding Theory& Properties
Network Covalent Bonding & Properties Electronegativity Bond Dipoles & Polar Molecules VSEPR Theory LD, DD, & H-bonding Predicting properties (bp, mp, etc.)

Molecular Dipoles The vector sum of all the bond dipoles in a molecule is a Molecular Dipole A Polar Molecule has a molecular dipole that points toward the more electronegative end of the molecule. eg. H2O

Molecular Dipoles NonPolar Molecules DO NOT have molecular dipoles. This occurs when: - the bond dipoles cancel - there are no bond dipoles eg. CO2 PH3

Molecular Dipoles To determine whether a molecule is polar:
- draw the Lewis diagram and the shape diagram - draw the bond dipoles and determine whether they cancel

Intermolecular Forces

Strongest bonds; Highest mp and bp
1. Network Covalent (Cn SiO2 SiC) 2. Ionic bonding(metal & nonmetal) 3. Metallic bonding (metals) 4. Molecular (nonmetals) Weakest bonds; Lowest mp and bp - Intermolecular forces present

To compare mp and bp in covalent compounds you must use:
- London Dispersion forces (p. 204) (all molecules) - Dipole-Dipole forces (pp. 202, 203) (polar molecules) - Hydrogen Bonding (pp. 205, 206) (H bonded to N, O, or F)

Intermolecular Forces (p. 202)

Covalent compounds have low mp and bp because forces between molecules in covalent compounds are very weak. Intermolecular forces were studied extensively by the Dutch physicist Johannes van der Waals In his honor, two types of intermolecular force are called Van der Waals forces.

Intermolecular forces can be used to account for the physical properties of covalent compounds.

Van der Waals

1. London Dispersion Forces
LD forces exist in ALL molecular elements & compounds. The positive charges in one molecule attract the negative charges in a second molecule. The temporary dipoles caused by electron movement in one molecule attract the temporary dipoles of another molecule.

The strength of these forces depends on: the number of electrons more electrons produce stronger LD forces that result in higher mp and bp eg. CH4 is a gas at room temperature. C8H18 is a liquid at room temperature. C25H52 is a solid at room temperature. Account for the difference.

Two molecules that have the same number of electrons are isoelectronic eg. C2H6 and CH3F

b) shape of the molecule molecules that “fit together” better will experience stronger LD forces eg. Cl2 vaporizes at -35 ºC while C4H10 vaporizes at -1 ºC. Use bonding to account for the difference.

2. Dipole-dipole Forces - occur between polar molecules
the δ+ end of one polar molecule is attracted to the δ- end of another polar molecule (& vice-versa) eg. Which has the higher boiling point CH3F or C2H6 ?

p. 202

3. Hydrogen Bonds - a special type of dipole-dipole force (about 10 times stronger) - only occurs between molecules that contain a H atom which is directly bonded to F, O, or N ie. the molecule contains at least one H-F, H-O, or H-N covalent bond.

3. Hydrogen Bonds the hydrogen bond occurs between the H atom of one molecule and the N, O, or F of a second molecule. eg. Arrange these from highest to lowest boiling point C3H8 C2H5OH C2H5F

p. 206

NOTE: To compare covalent compounds you must use:
- London Dispersion forces (all molecules) - Dipole-Dipole forces (polar molecules) - Hydrogen Bonding (H bonded to N, O, or F)

Alchem worksheet pp. G32, 33

p. 210

1. Use intermolecular forces to explain the following: a) Ar boils at -186 °C and F2 boils at -188 °C . b) Kr boils at -152 °C and HBr boils at -67 °C. c) Cl2 boils at -35 °C and C2H5Cl boils at 13 °C . 2. Examine the graph on p. 210: a) Account for the increase in boiling point for the hydrogen compounds of the Group IV elements. b) Why is the trend different for the hydrogen compounds of the Group V, VI, and VII elements? c) Why are the boiling points of the Group IVA compounds consistently lower than the others.

3. Which substance in each pair has the higher boiling point
3. Which substance in each pair has the higher boiling point. Justify your answers. (a) SiC or KCl (b) RbBr or C6H12O6 (c) C3H8 or C2H5OH (d) C4H10 or C2H5Cl

2. Examine the graph on p. 210: a) Account for the increase in boiling point for the hydrogen compounds of the Group IV elements. b) Why is the trend different for the hydrogen compounds of the Group V, VI, and VII elements? c) Why are the boiling points of the Group IVA compounds consistently lower than the other compounds.

Dipole-Dipole Forces In the liquid state, polar molecules (dipoles) orient themselves so that oppositely charged ends of the molecules are near to one another.

Summary The types of bonding/forces ranked from strongest to weakest are: Strongest - Network Covalent - Ionic - Metallic Weakest - Covalent

NOTE: To compare covalent compounds you must use:
- London Dispersion forces (all molecules) - Dipole-Dipole forces (polar molecules) - Hydrogen Bonding (H bonded to N, O, or F)

p #’s 13 & 14

Dipole-Dipole Forces The electrostatic attractions between these oppositely charged ends of the polar molecules are called dipole-dipole forces.

Dipole-Dipole Forces Results of dipole-dipole attractions:
polar molecules will tend to attract one another more at room temperature than similarly sized non-polar molecules energy needed to separate polar molecules is therefore higher than for non-polar molecules of similar molar mass

Dipole-Dipole Forces Results of dipole-dipole attractions:
The melting points and boiling points of substances made of polar molecules are higher than for substances made of non-polar molecules.

Ion-Dipole Forces An ion-dipole force is the force of attraction between an ion and a polar molecule (a dipole).

Ion-Dipole Forces NaCl dissolves in water because the attractions between the Na+ and Cl- ions and the partial charges on the H2O molecules are strong enough to overcome the forces that bind the ions together.

Induced Intermolecular Forces

Induction of electric charge occurs when a charge on one object causes a change in the distribution of charge on a nearby object. (for example, the balloon)

There are two types of charge -induced dipole forces: 1. An ion-induced dipole force results when an ion in close proximity to a non-polar molecule distorts the electron density of the non-polar molecule

The molecule then becomes momentarily polarized, and the two species are attracted to each other. (ie. hemoglobin) 2. In a dipole-induced dipole force the charge on a polar molecule is responsible for inducing the charge on the non-polar molecule.

Dispersion (London) Forces
Bond vibrations, which are part of the normal condition of a non-polar molecule, cause momentary, uneven distribution of charge; a non-polar becomes slightly polar for an instant, and continues to do so in a random but constant basis.

At the instant that one non-polar molecule is in a slightly polar condition, it is capable of inducing a dipole in a nearby molecule This force of attraction is called a dispersion force.

Two factors affecting the magnitude of dispersion forces are: The number of electrons in the molecule: Vibrations within larger molecules that have more electrons than smaller molecules can easily cause an uneven distribution of charge.

The dispersion forces between these larger molecules are thus stronger, which has the effect of raising the boiling point for larger molecules. The shape of the molecule: A molecule with a spherical shape has a smaller surface area than a straight chain molecule that has the same number of electrons

Therefore, the substance with molecules that have a more spherical shape will have weaker dispersion forces and a lower boiling point. London dispersion forces are responsible for the formation and stabilization of the biological membranes surrounding every living cell.

Hydrogen Bonding In order to form a hydrogen bond, a hydrogen atom must be bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine.

Hydrogen Bonding These bonds are very polar, and since hydrogen has no other electrons, the positive proton, H+, is exposed and can become strongly attracted to the negative end of another dipole nearby

… Hydrogen Bonding H H H H O O δ− H δ−
A hydrogen bond is an electrostatic attraction between the nucleus of a hydrogen atom, bonded to fluorine, oxygen, or nitrogen and the negative end of a dipole nearby. H δ+ H δ+ H … H O δ+ δ+ O δ− H δ−

Hydrogen Bonding In biological systems, these polar bonds are often parts of much larger molecules (ie. N H bonds and C O bonds found in biological molecules)

Hydrogen Bonding in Water
Hydrogen bonds between the hydrogen atoms in one water molecule and the oxygen atom in another account for many unique properties of water. H δ+ H δ+ H … H O δ+ δ+ O δ− H δ−

In liquid water, each water molecule is hydrogen bonded to at least four other water molecules. The large number of bonds between water molecules makes the net attractive force quite strong

the strong attractive forces are responsible for the relatively high boiling point of water. The water molecules are farther apart in ice then they are in liquid water making ice less dense than liquid water.

Hydrogen bonds force water molecules into the special hexagonal, crystalline structure of ice when the temperature is below 4 degrees celcius.

Assignment # 4

Electronegativity Electronegativity is a result of the space between the nucleus and the electrons As the number of protons in the nucleus increases, the attractive force on the electrons increases, pulling them closer to the nucleus

Unit 2: Chemical Bonding

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