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Chapter Presentation

Transparencies

Bellringer

Standardized Test Prep Visual Concepts

Sample Problems

Resources



Table of Contents

Chapter 6 Covalent Compounds

Section 1 Covalent Bonds

Section 2 Drawing and Naming Molecules

Section 3 Molecular Shapes




Bellringer

• Make a list of the elements that form ionic bonds.

Note that most ionic bonds contain a metal and a

nonmetal.

Chapter 6



Objectives

• Explain the role and location of electrons in a

covalent bond.

• Describe the change in energy and stability that

takes place as a covalent bond forms.

• Distinguish between nonpolar and polar covalent

bonds based on electronegativity differences.


Chapter 6



Objectives, continued

• Compare the physical properties of substances that

have different bond types, and relate bond types to

electronegativity differences.


Chapter 6



Sharing Electrons

• When an ionic bond forms, electrons are rearranged

and are transferred from one atom to another to form

charged ions.

• In another kind of change involving electrons, the

neutral atoms share electrons.

Section 1 Covalent Bonds Chapter 6



Sharing Electrons, continued Forming Molecular Orbitals

• A covalent bond is a bond formed when atoms share

one or more pairs of electrons.

• The shared electrons move within a space called a

molecular orbital.

• A molecular orbital is the region of high probability

that is occupied by an individual electron as it travels

with a wavelike motion in the three-dimensional space

around one of two or more associated nuclei.




Formation of a Covalent Bond

Chapter 6



Visual Concepts

Chemical Bond

Chapter 6



Energy and Stability

Energy Is Released When Atoms Form a Covalent Bond




Energy and Stability, continued Potential Energy Determines Bond Length

• When two bonded hydrogen atoms are at their lowest

potential energy, the distance between them is 75 pm.

• The bond length is the distance between two bonded

atoms at their minimum potential energy.

• However, the two nuclei in a covalent bond vibrate back

and forth. The bond length is thus the average distance

between the two nuclei.




Visual Concepts

Bond Length

Chapter 6



Energy and Stability, continued Bonded Atoms Vibrate, and Bonds Vary in Strength

• The bond length is the average distance between two

nuclei in a covalent bond.

• At a bond length of 75 pm, the potential energy of H2 is

–436 kJ/mol.

• Thus 436 kJ of energy must be supplied to break the

bonds in 1 mol of H2 molecules.

• The energy required to break a bond between two

atoms is the bond energy.

• Bonds that have the higher bond energies

(stronger bonds) have the shorter bond lengths.




Visual Concepts

Bond Energy

Chapter 6



Electronegativity and Covalent Bonding

• In covalent bonds between two different atoms, the

atoms often have different attractions for shared

electrons.

• Electronegativity values are a useful tool to predict

what kind of bond will form.




Visual Concepts

Electronegativity

Chapter 6



Electronegativity and Covalent Bonding,

continued Atoms Share Electrons Equally or Unequally


• When the electronegativity values of two bonding

atoms are similar, bonding electrons are shared

equally.

• A covalent bond in which the bonding electrons in the

molecular orbital are shared equally is a nonpolar

covalent bond.

Chapter 6




continued Atoms Share Electrons Equally or Unequally,

continued


• When the electronegativity values of two bonding

atoms are different, bonding electrons are shared

unequally.

• A covalent bond in which the bonding electrons in the

molecular orbital are shared unequally is a polar

covalent bond.

Chapter 6



Predicting Bond Character from

Electronegativity Differences

Chapter 6




continued Polar Molecules Have Positive and Negative Ends


• In a polar covalent bond, the ends of the bond have

opposite partial charges.

• A molecule in which one end has a partial positive

charge and the other end has a partial negative charge

is called a dipole.

• In a polar covalent bond, the shared pair of electrons is

not transferred completely. Instead, it is more likely to be

found near the more electronegative atom.

Chapter 6




continued Polar Molecules Have Positive and Negative Ends,

continued


• The symbol is used to mean partial.

• + is used to show a partial positive charge

• – is used to show a partial negative charge charge

• example: H+F–

• Because the F atom has a partial negative charge, the

electron pair is more likely to be found nearer to the

fluorine atom

Chapter 6



Visual Concepts

Comparing Polar and Nonpolar Covalent

Bonds

Chapter 6



Polarity Is Related to Bond Strength

• In general, the greater the electronegativity difference,

the greater the polarity and the stronger the bond.




Electronegativity and Bond Types


• Differences in electronegativity values provide one

model that can tell you which type of bond two atoms

will form.

• Another general rule states:

• A covalent bond forms between two nonmetals.

• An ionic bond forms between a nonmetal and a metal.

Chapter 6



Properties of Substances Depend on Bond

Type


• The type of bond that forms (metallic, ionic, or

covalent) determines the properties of the substance.

• The difference in the strength of attraction between

the basic units of ionic and covalent substances

causes these types of substances to have different

properties.

Chapter 6



Properties of Substances with Metallic, Ionic,

and Covalent Bonds

Chapter 6



Section 2 Drawing and Naming

Molecules

Bellringer

• Classify the following compounds according to the

type of bonds they contain:

• NO

• CO

• HF

• NaCl

• HBr

• NaI

Chapter 6



Objectives

• Draw Lewis structures to show the arrangement of

valence electrons among atoms in molecules and

polyatomic ions.

• Explain the differences between single, double, and

triple covalent bonds.

• Draw resonance structures for simple molecules and

polyatomic ions, and recognize when they are

required.


Molecules Chapter 6



Objectives, continued

• Name binary inorganic covalent compounds by using

prefixes, roots, and suffixes.


Molecules Chapter 6



Lewis Electron-Dot Structures

• Valence electrons are the electrons in the outermost

energy level of an atom.

• A Lewis structure is a structural formula in which

valence electrons are represented by dots.

• In Lewis structures, dot pairs or dashes between two

atomic symbols represent pairs in covalent bonds.


Molecules Chapter 6



Visual Concepts

Valence Electrons

Chapter 6



Lewis Electron-Dot Structures, continued Lewis Structures Show Valence Electrons

• As you go from element to element across a period,

you add a dot to each side of the element’s symbol.


Molecules Chapter 6



Lewis Electron-Dot Structures, continued Lewis Structures Show Valence Electrons, continued


Molecules

• You do not begin to pair dots until all four sides of the

element’s symbol have a dot.

Chapter 6




• An element with an octet of valence electrons has a

stable configuration.

• The tendency of bonded atoms to have octets of

valence electrons is called the octet rule.


Molecules Chapter 6



Visual Concepts

The Octet Rule

Chapter 6




• When two chlorine atoms form a covalent bond, each

atom contributes one electron to a shared pair.


Molecules Chapter 6

• An unshared pair, or a lone pair, is a nonbonding

pair of electrons in the valence shell of an atom.




• A single bond is a covalent bond in which two atoms

share one pair of electrons

• The electrons can pair in any order. However, any

unpaired electrons are usually filled in to show how

they will form a covalent bond.


Molecules Chapter 6



Drawing Lewis Structures with Single Bonds

Sample Problem A

Draw a Lewis structure for CH3I.


Molecules Chapter 6




Sample Problem A Solution

Draw each atom’s Lewis structure, and count the total

number of valence electrons.


Molecules Chapter 6

number of dots: 14

Arrange the Lewis structure so that carbon is the central

atom.




Sample Problem A Solution, continued

Distribute one bonding pair of electrons between each of

the bonded atoms. Then, distribute the remaining electrons,

in pairs, around the remaining atoms to form an octet for

each atom.


Molecules Chapter 6

Change each pair of dots that represents a shared pair of

electrons to a long dash.



Drawing Lewis Structures for Polyatomic Ions

Sample Problem B

Draw a Lewis structure for the sulfate ion,


Molecules

2-

4SO .

Chapter 6




Sample Problem B Solution

Count electrons for all atoms. Add two additional electrons

to account for the 2− charge on the ion.


Molecules Chapter 6

number of dots: 30 + 2 = 32

Distribute the 32 dots so that there are 8 dots around each

atom.




Sample Problem B Solution, continued

Change each bonding pair to a long dash. Place

brackets around the ion and a 2 charge outside the

bracket to show that the charge is spread out over the

entire ion.


Molecules Chapter 6



Multiple Bonds

• For O2 to make an octet, each atom needs two more

electrons. The two atoms share four electrons.


Molecules Chapter 6

• A double bond is a covalent bond in which two

atoms share two pairs of electrons.



Multiple Bonds, continued

• For N2 to make an octet, each atom needs three more

electrons. The two atoms share six electrons.


Molecules Chapter 6

• A triple bond is a covalent bond in which two atoms

share three pairs of electrons.



Visual Concepts

Comparing Single, Double, and Triple Bonds

Chapter 6



Drawing Lewis Structures with Multiple Bonds

Sample Problem C

Draw a Lewis structure for formaldehyde, CH2O.


Molecules Chapter 6




Sample Problem C Solution

Draw each atom’s Lewis structure, and count the total dots.


Molecules Chapter 6

number of dots: 12

Arrange the atoms so that carbon is the central atom.

O

H C H




Sample Problem C Solution, continued

Distribute one pair of dots between each of the atoms

and the rest, in pairs, around the atoms. C does not

have an octet. To get an octet, move an unshared pair

from the O to between the O and the C.


Molecules Chapter 6

Change each bonding pair to a long dash. Two pairs

of dots represent a double bond.



Resonance Structures

• Some molecules, such as ozone, O3, cannot be

represented by a single Lewis structure.

• When a molecule has two or more possible Lewis

structures, the two structures are called resonance

structures.


Molecules Chapter 6



Visual Concepts

Atomic Resonance

Chapter 6



Multiple Bonds, continued Naming Covalent Compounds

• The first element named is usually the first one written

in the formula. It is usually the less-electronegative

element.

• The second element named has the ending -ide.

• Unlike the names for ionic compounds, the names for

covalent compounds must often distinguish between

two different molecules made of the same elements.


Molecules Chapter 6



Naming Covalent Compounds, continued

• This system of prefixes is used to show the number of

atoms of each element in the molecule.


Molecules Chapter 6



Naming Covalent Compounds, continued

• Prefixes can be used to show the numbers of each

type of atom in diphosphorus pentasulfide.


Molecules Chapter 6



Visual Concepts

Naming Compounds Using Numerical Prefixes

Chapter 6




Bellringer

• Write a short paragraph telling what you think the

“valence shell electron pair repulsion theory” might

have to do with molecular shape.

Chapter 6



Objectives

• Predict the shape of a molecule using VSEPR

theory.

• Associate the polarity of molecules with the shapes

of molecules, and relate the polarity and shape of

molecules to the properties of a substance.

Section 3 Molecular Shapes Chapter 6



Determining Molecular Shapes

• The three-dimensional shape of a molecule is

important in determining the molecule’s physical and

chemical properties.


A Lewis Structure Can Help Predict Molecular Shape

• You can predict the shape of a molecule by

examining the Lewis structure of the molecule.

Chapter 6



Determining Molecular Shapes, continued A Lewis Structure Can Help Predict Molecular Shape,

continued

• The valence shell electron pair repulsion (VSEPR)

theory is a theory that predicts some molecular

shapes based on the idea that pairs of valence

electrons surrounding an atom repel each other.




Determining Molecular Shapes, continued Electron Pairs Can Determine Molecular Shape

• According to the VSEPR theory, the shape of a

molecule is determined by the valence electrons

surrounding the central atom.

• Electron pairs are negative, so they repel each other.

• Therefore, the shared pairs that form different bonds

repel each other and remain as far apart as possible.




Determining Molecular Shapes, continued Electron Pairs Can Determine Molecular Shape,

continued

• For CO2, the two double bonds around the central

carbon atom repel each other and remain far apart.


• For BF3, the three single bonds around the central

fluorine atom will be at a maximum distance apart.



Determining Molecular Shapes, continued Electron Pairs Can Determine Molecular Shape,

continued

• The four shared pairs of electrons in CH4 are farthest

apart when each pair is positioned at the corners of a

tetrahedron.




Visual Concepts

VSEPR and Lone Electron Pairs

Chapter 6 Chapter 6



Visual Concepts

VSEPR and Basic Molecular Shapes

Chapter 6



Predicting Molecular Shapes

Sample Problem D

Determine the shape of H2O.





Sample Problem D Solution

Draw the Lewis structure for H2O.


Count the number of shared and unshared pairs of

electrons around the central atom.

H2O has two shared pairs and two unshared pairs.




Sample Problem D Solution, continued

Find the shape that allows the shared and unshared

pairs of electrons to be as far apart as possible.

The water molecule will have a bent shape.




Molecular Shape Affects a Substance’s

Properties Shape Affects Polarity


• One property that shape determines is the polarity of

a molecule.

• The polarity of a molecule that has more than two

atoms depends on the polarity of each bond and the

way the bonds are arranged in space.

Chapter 6



Molecular Shape Affects a Substance’s

Properties, continued Shape Affects Polarity, continued


• If two dipoles are arranged in opposite directions,

they will cancel each other.

• If two dipoles are arranged at an angle, they will not

cancel each other.

• Compare the molecules of nonpolar carbon dioxide,

CO2, which has a linear shape, and polar water, H2O,

which has a bent shape.

Chapter 6



Molecular Shape

Affects Polarity

Chapter 6



1. Which of these combinations is likely to have a polar

covalent bond?

A. two atoms of similar size

B. two atoms of very different size

C. two atoms with different electronegativities

D. two atoms with the same number of electrons

Standardized Test Preparation

Understanding Concepts

Chapter 6



1. Which of these combinations is likely to have a polar

covalent bond?

A. two atoms of similar size

B. two atoms of very different size

C. two atoms with different electronegativities

D. two atoms with the same number of electrons


Standardized Test Preparation Chapter 6



2. According to VSEPR theory, which of these is caused

by repulsion between electron pairs surrounding an

atom?

F. breaking of a chemical bond

G. formation of a sea of electrons

H. formation of a covalent chemical bond

I. separation of electron pairs as much as possible


Chapter 6 Standardized Test Preparation



3. How many electrons are shared in a double covalent

bond?

A. 2

B. 4

C. 6

D. 8





4. How can the difference in number of valence

electrons between nitrogen and carbon account for

the fact that the boiling point of ammonia, NH3, is

130°C higher than that of methane, CH4.





4. How can the difference in number of valence

electrons between nitrogen and carbon account for

the fact that the boiling point of ammonia, NH3, is

130°C higher than that of methane, CH4.

Answer: Ammonia is a polar molecule because nitrogen

has a pair of electrons that are not involved in a

covalent bond, while methane is a nonpolar molecule.

The attraction between polar ammonia molecules

causes the higher boiling point.





5. Why don’t scientists need VESPR theory to predict

the shape of HCl?





5. Why don’t scientists need VESPR theory to predict

the shape of HCl?

Answer: Because HCl has two atoms, the shape can be

only linear.





6. What are the attractive and repulsive forces involved

in a covalent bond and how do their total strengths

compare?





6. What are the attractive and repulsive forces involved

in a covalent bond and how do their total strengths

compare?

Answer: Attractive forces exist between each electron

and each nucleus. Repulsive forces exist between

electrons and between nuclei. In a covalent bond,

total attractive and repulsive forces are balanced.





Read the passage below. Then answer the questions.

Although water is a polar molecule, pure water

does not carry an electric current. It is a good solvent

for many ionic compounds, and solutions of ionic

compounds in water do carry electric currents. The

charged particles in solution move freely, carrying

electric charges. Even a dilute solution of ions in water

becomes a good conductor. Without ions in solution,

there is very little electrical conductivity.

Reading Skills




7. Why is a solution of sugar in water not a good electrical conductor?

F. Sugar does not form ions in solution.

G. The ionic bonds of sugar molecules are too strong to carry a current.

H. Not enough sugar dissolves for the solution to become a conductor.

I. A solution of sugar in water is not very conductive because it is mostly water, which is not very conductive.

Reading Skills




Reading Skills


7. Why is a solution of sugar in water not a good electrical conductor?

F. Sugar does not form ions in solution.

G. The ionic bonds of sugar molecules are too strong to carry a current.

H. Not enough sugar dissolves for the solution to become a conductor.

I. A solution of sugar in water is not very conductive because it is mostly water, which is not very conductive.



Reading Skills


8. Why do molten ionic compounds generally conduct electric current well, while molten covalent compounds generally do not?

A. Ionic compounds are more soluble in water.

B. Ionic compounds have more electrons than compounds.

C. When they melt, ionic compounds separate into charged particles.

D. Most ionic compounds contain a metal atom which carries the electric current.



8. Why do molten ionic compounds generally conduct electric current well, while molten covalent compounds generally do not?

A. Ionic compounds are more soluble in water.

B. Ionic compounds have more electrons than compounds.

C. When they melt, ionic compounds separate into charged particles.

D. Most ionic compounds contain a metal atom which carries the electric current.

Reading Skills




9. If water is not a good conductor of electric current,

why is it dangerous to handle an electrical appliance

when your hands are wet or when you are standing

on wet ground?

Reading Skills




9. If water is not a good conductor of electric current,

why is it dangerous to handle an electrical appliance

when your hands are wet or when you are standing

on wet ground?

Answer: Because even a small amount of ionic

compounds dissolved in water makes it a good

conductor. The salts in your body or on the ground

are enough to cause the water to carry a current.

Reading Skills




Use the diagram below to answer question 10.

Interpreting Graphics




10. The diagram above best represents which type of

chemical bond?

F. ionic

G. metallic

H. nonpolar covalent

I. polar covalent





The table below shows the connection between

electronegativity and bond strength (kilojoules per mole).

Use it to answer questions 11 through 13.





11.Which of these molecules has the smallest partial

positive charge on the hydrogen end of the molecule?

A. HF

B. HCl

C. HBr

D. HI





12. How does the polarity of the bond between a halogen and

hydrogen relate to the number of electrons of the halogen atom?

F. Polarity is not related to the number of electrons of the

halogen atom. G. Polarity decreases as the number of unpaired halogen

electrons increases. H. Polarity decreases as the total number of halogen atom

electrons increases. I. Polarity decreases as the number of valence electrons of the

halogen atom increases.





13. Based on the information in this table, how does the

electronegativity difference in a covalent bond relate

to the strength of the bond?





13. Based on the information in this table, how does the

electronegativity difference in a covalent bond relate

to the strength of the bond?

Answer: A stronger bond is indicated by greater bond

energy, so the strength of the bond increases as

electronegativity increases.



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