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© 2013 Pearson Education, Inc. Chapter 5, Section 1 107
General, Organic, and
Biological ChemistryFourth Edition
Karen Timberlake
A closer look at
bonding type – the
concept of
electronegativity
Chapter 5Compounds and
Their Bonds
© 2013 Pearson Education, Inc.
© 2013 Pearson Education, Inc. Chapter 5, Section 1 108
General Rules for Bond Type
Nonmetal
Nonmetal
Covalent
Bond
Metal
Ionic
Bond
Negative complex
ions
© 2013 Pearson Education, Inc. Chapter 5, Section 1 109
Ionic or Covalent?
� So far, we have been employing the following
generalization for the type of bonding holding
atoms together in a substance
Generalization
� metal + nonmetal = ionic bond
� nonmetal + nonmetal = covalent bond
Reality
� Bonding type is on a continuum, from 100% ionic
to 100% covalent!
100% covalent 100% ionic© 2013 Pearson Education, Inc. Chapter 5, Section 1 110
Bonding Conundrum
� Arose in the 1930’s from American Chemist
Linus Pauling’s work on the nature of the
chemical bond.
� Pauling realized that there was some “in-
betweeness” with the bonding in many
substances
� Measured the ability of elements to attract
electrons to themselves when bonding
� Called this tendency Electronegativity
110
© 2013 Pearson Education, Inc. Chapter 5, Section 1 111 © 2013 Pearson Education, Inc. Chapter 5, Section 1 112
Determining Predominant Bond type
� Electronegativity is a measure of the tendency
for atoms of an element to attract electrons to
themselves in a chemical bond.
� Originated with American chemist Linus Pauling
(1901-1994), a 2x Nobel Prize winner who did
most of his work at Cal Poly Tech, but finished
his career at Stanford
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© 2013 Pearson Education, Inc. Chapter 5, Section 1 113
Electronegativity (EN) Values of the
Elements
Scale ranges from 0.7 to 4.0
© 2013 Pearson Education, Inc. Chapter 5, Section 1 114
Electronegativity
� is a measure of an atom’s ability to attract electrons to itself in a chemical bond
� increases from left to right, going across a period
on the periodic table.
� decreases going down a group on the periodic
table.
� is high for the nonmetals, with fluorine as the
highest.
� is low for the metals and transition metals.
Electronegativity
© 2013 Pearson Education, Inc. Chapter 5, Section 1 115
Trends in Periodic table
© 2013 Pearson Education, Inc. Chapter 5, Section 1 116
Periodic trends in electronegativity
� Atomic radius increases, valence e- further
away from nucleus, decreases pulling power
of nucleus
Electronegativity decreases down a group
because:
Electronegativity increases from left to right across
a period because:
� Atomic radius decreases, valence e- closer to
the nucleus, increases pulling power of nucleus
© 2013 Pearson Education, Inc. Chapter 5, Section 1 117
Bonding and Electronegativity
� According to Pauling, the difference in the electronegativity values of the two atoms involved in a chemical bond can
be used to predict the type of bond that forms.
� Pauling Classified chemical bonds into the following three types, based on differences in electronegativity
� Ionic
� Polar Covalent
� Nonpolar Covalent
© 2013 Pearson Education, Inc. Chapter 5, Section 1 118
Bond Character Summary
a) Nonpolar covalent bond �
electrons shared equally
b) Polar covalent bond � e- not
shared equally, more
electronegative atom has greater
share of e- cloud
c) Ionic bond – e- transferred from
one atom to other, creates a + and
– ion
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© 2013 Pearson Education, Inc. Chapter 5, Section 1 119
Electronegativity and Bond Type
� It is the difference in the electronegativies
(∆EN) of two bonded atoms that determines
the predominant bond type
∆EN ≤ 0.4 Nonpolar covalent bond
(e- shared equally by atoms)
∆EN between 0.5 and 1.7 polar covalent
bond
∆EN > 1.7 ionic bond (e- transferred) 119
© 2013 Pearson Education, Inc. Chapter 5, Section 1 120
A nonpolar covalent bond
� occurs between nonmetals.
� has an equal or almost equal sharing of electrons.
� has almost no electronegativity difference (0.0 to
0.4).
Examples:
Atoms Electronegativity Type of BondDifference
Nonpolar Covalent Bonds
© 2013 Pearson Education, Inc. Chapter 5, Section 1 121
Example of Nonpolar Covalent
Bonds� Diatomic elements e.g. H2
� Bonds between P and H = 2.1-2.1=0
© 2013 Pearson Education, Inc. Chapter 5, Section 1 122
A polar covalent bond
� occurs between nonmetal atoms that do not share electrons equally.
� has a moderate electronegativity difference
(0.5 to 1.7).Examples:
Atoms Electronegativity Type of Bond Difference
Polar Covalent Bonds
© 2013 Pearson Education, Inc. Chapter 5, Section 1 123
Result of differences in electronegativity:
• More electronegative element has greater
share of e- cloud, it is electron rich
•Less electronegative element is e- poor
•Creation of partial charges on the atoms in the
bond (bond dipoles) δδδδ+ and δδδδ -
© 2013 Pearson Education, Inc. Chapter 5, Section 1 124
Comparing Nonpolar and Polar
Covalent Bonds
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© 2013 Pearson Education, Inc. Chapter 5, Section 1 125
H Cl
Example: Bond between H and Cl
• Electronegativity values H= 2.1, Cl = 3.0
• e- cloud pulled towards Cl atom = Polar Covalent
Bond
electron richregion
electron poorregion
e- rich
e- poor
ClH
δ+ δ-
9.5
Direction e- cloud being pulled
2.1 3.0
© 2013 Pearson Education, Inc. Chapter 5, Section 1 126
Bond Polarity and Dipoles
Bonds become more polar as the difference in electronegativity values of bonding atoms increases.
Polar covalent bonds have a separation of charges called a dipole.
The positive and negative ends of the dipole are indicated by the lowercase Greek letter delta with a
positive or negative sign, δ+ and δ-, or an arrow that points from the positive to the negative charge.
© 2013 Pearson Education, Inc. Chapter 5, Section 1 127
Ionic Bonds
An ionic bond
� occurs between metal and nonmetal ions.
� is a result of electron transfer.
� has a large electronegativity difference (1.8 or more).
Examples:Atoms Electronegativity Type of Bond
Difference
© 2013 Pearson Education, Inc. Chapter 5, Section 1 128
Determining Predominant Bond Type
� Look at the electronegativity values of atoms
involved in bond
� Calculate electronegativity difference (∆EN)
� If the bond is polar covalent, draw arrow in
direction that e- cloud is pulled (more
electronegative atom)
� Also use lower case Greek symbol for delta, δ,
along with + or - sign
© 2013 Pearson Education, Inc. Chapter 5, Section 1 129
Electronegativity and Bond Types
© 2013 Pearson Education, Inc. Chapter 5, Section 1 130
Predicting Bond Types
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© 2013 Pearson Education, Inc. Chapter 5, Section 1 131
Polar Molecules
A polar molecule
� contains polar bonds.
� has a separation of positive and negative charge. called a dipole, indicated with δ+ and δ–.
� has dipoles that do not cancel.δ+ δ–
H–Cl NH3
dipole
Dipoles do not cancel.
© 2013 Pearson Education, Inc. Chapter 5, Section 1 132
Nonpolar Molecules
A nonpolar molecule
� contains nonpolar bonds
Cl–Cl H–H
� or has a symmetrical arrangement of polar bonds.
© 2013 Pearson Education, Inc. Chapter 5, Section 1 133
Guide to Determination of
Polarity
© 2013 Pearson Education, Inc. Chapter 5, Section 1 134
Molecular Polarity, H2O
Determine the polarity of the H2O molecule.
Step 1 Determine if the bonds are polar covalent ornonpolar covalent. From the electronegativitytable, O 3.5 and H 2.1 gives a difference of 1.4,
which makes the O — H bonds, polar covalent.
© 2013 Pearson Education, Inc. Chapter 5, Section 1 135
Molecular Polarity, H2O
Determine the polarity of the H2O molecule.
Step 2 If the bonds are polar covalent, draw theelectron-dot formula and determine if thedipoles cancel or not. The four electron
groups of oxygen are bonded to two H atoms.Thus, the H2O molecule has a net dipole, which
makes it a polar molecule.
© 2013 Pearson Education, Inc. Chapter 5, Section 1 136
Learning Check
Determine the shape of each of the following molecules and whether they are polar or nonpolar. Explain.
1. PBr3
2. HBr
3. Br2
4. SiBr4
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© 2013 Pearson Education, Inc. Chapter 5, Section 1 137
Solution
Determine the shape of each of the following molecules and whether they are polar or nonpolar. Explain.
1. PBr3 pyramidal; polar; dipoles don’t cancel
2. HBr linear; polar; one polar bond (dipole)
3. Br2 linear; nonpolar; nonpolar bond
4. SiBr4 tetrahedral; nonpolar; dipoles cancel
© 2013 Pearson Education, Inc. Chapter 5, Section 1 138
General, Organic, and
Biological ChemistryFourth Edition
Karen Timberlake
5.9
Attractive Forces in
Compounds
Chapter 5Compounds and
Their Bonds
© 2013 Pearson Education, Inc.Lectures
© 2013 Pearson Education, Inc. Chapter 5, Section 1 139
Ionic Compounds
In ionic compounds, ionic bonds
� require large amounts of energy to break.
� hold positive and negative ions together.
� explain their high melting points.
© 2013 Pearson Education, Inc. Chapter 5, Section 1 140
Covalent Compounds
In covalent compounds, the attractive forces betweensolid and liquid molecules
� are weaker than ionic bonds.
� require less energy to break.
� explain why their melting points are lower than ionic compounds.
These attractive forces include
� dipole-dipole attractions,
� dispersion forces, and
� hydrogen bonding.
© 2013 Pearson Education, Inc. Chapter 5, Section 1 141
Dipole-Dipole Attractions
In covalent compounds, polar molecules exert attractive forces between molecules called dipole-dipole
attractions.
© 2013 Pearson Education, Inc. Chapter 5, Section 1 142
Dipole-Dipole Attractions,
Hydrogen Bonds
In covalent compounds, some polar molecules form strong dipole attractions called hydrogen bonds, which
occur between the partially positive hydrogen atom of one molecule and a lone pair of electrons on a nitrogen,
oxygen, or fluorine atom in another molecule.
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© 2013 Pearson Education, Inc. Chapter 5, Section 1 143
Dispersion Forces
Dispersion forces are
� weak attractions between nonpolar molecules.
� caused by temporary dipoles that develop when electrons are not distributed equally.
Nonpolar molecules form attractions when they form temporarydipoles.
© 2013 Pearson Education, Inc. Chapter 5, Section 1 144
Comparison of Bonding and
Attractive Forces
© 2013 Pearson Education, Inc. Chapter 5, Section 1 145
Melting Points and Attractive
Forces
The stronger the attractive force between ions or molecules, the higher the melting points.
Ionic compounds, have the strongest attractive force and, therefore the highest melting points.
Covalent molecules have less attractive forces than ionic compounds and, therefore lower melting points.
© 2013 Pearson Education, Inc. Chapter 5, Section 1 146
Melting Points and Attractive
Forces
The attractive forces between covalent molecules varyin magnitude; the stronger the attractive force, the
higher its melting point.
� Hydrogen bonds are the strongest type of dipole–
dipole attractions, requiring the most energy to break, followed by dipole–dipole forces.
� Dispersion forces are the weakest, requiring even less energy to break them, and therefore have lower melting points than hydrogen bonds and dipole–
dipole forces.
© 2013 Pearson Education, Inc. Chapter 5, Section 1 147
Melting Points of Selected
Substances
© 2013 Pearson Education, Inc. Chapter 5, Section 1 148
Learning Check
Identify the main type of attractive forces for each of the following compounds: ionic bonds, dipole–dipole,
hydrogen bonds or dispersion.
1. NCl3
2. H2O
3. Br2
4. KCl
5. NH3
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© 2013 Pearson Education, Inc. Chapter 5, Section 1 149
Solution
Identify the main type of attractive forces for each of the following compounds: ionic bonds, dipole–dipole,
hydrogen bonds or dispersion.
1. NCl3 dipole–dipole
2. H2O hydrogen bonds
3. Br2 dispersion
4. KCl ionic bonds
5. NH3 hydrogen bonds
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