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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.


General Rules for Bond Type

Nonmetal

Nonmetal

Covalent

Bond

Metal

Ionic

Bond

Negative complex

ions


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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Electronegativity (EN) Values of the

Elements

Scale ranges from 0.7 to 4.0


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


Trends in Periodic table


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


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


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

3


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


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


Example of Nonpolar Covalent

Bonds� Diatomic elements e.g. H2

� Bonds between P and H = 2.1-2.1=0


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


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 -


Comparing Nonpolar and Polar

Covalent Bonds

4


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


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.


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


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


Electronegativity and Bond Types


Predicting Bond Types

5


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.


Nonpolar Molecules

A nonpolar molecule

� contains nonpolar bonds

Cl–Cl H–H

� or has a symmetrical arrangement of polar bonds.


Guide to Determination of

Polarity


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.


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.


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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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


General, Organic, and

Biological ChemistryFourth Edition

Karen Timberlake

5.9

Attractive Forces in

Compounds

Chapter 5Compounds and

Their Bonds

© 2013 Pearson Education, Inc.Lectures


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.


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.


Dipole-Dipole Attractions

In covalent compounds, polar molecules exert attractive forces between molecules called dipole-dipole

attractions.


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.

7


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.


Comparison of Bonding and

Attractive Forces


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.


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.


Melting Points of Selected

Substances


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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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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