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Chapter 10: Chemical Bonding

Remember to keep up with MasteringChemistry, Workshops, Mini-Reports and Labs

Chemical Bonding Theories:

How atoms are connected together and the three dimensional shapes of molecules are

very important. Many chemicals need to have the right shape to fit into the correct

receptor or react the expected way. Finding the correct “fit” will allow manmade

drugs to do a certain job. Artificial sweeteners have a shape that fits our receptors on

the tongue to fool our brain into believing we taste something sweet. Bonding

theories predict how and which atoms bond together to form molecules or ionic

compounds, and explain shapes of molecule which

determines many physical and chemical properties.

Worldwide Computer Game Foldit predicts Structures:

Foldit is a revolutionary computer game enabling anyone to

contribute to important scientific research.

Goals of Foldit from http://fold.it/portal : For protein structure prediction, the eventual goal is to have humans work on proteins that do not have a

known structure. This requires first attracting the attention of scientists and biotech companies and

convincing them that the process is effective. Another goal is to take folding strategies that human players

have come up with while playing the game, and automate these strategies to make protein-prediction

software more effective. These two goals are more or less independent and either or both may happen.

The more interesting goal for Foldit, perhaps, is not in protein prediction but protein design. Designing

new proteins may be more directly practical than protein prediction, as the problem you must solve as a

protein designer is basically an engineering problem (protein engineering), whether you are trying to

disable a virus or scrub carbon dioxide from the atmosphere.

In 2011, players of Foldit helped to decipher the crystal structure of the Mason-Pfizer monkey

virus (M-PMV) retroviral protease, an AIDS-causing monkey virus. Players produced an accurate

3D model of the enzyme in just ten days. The problem of how to configure the structure of the

enzyme had stumped scientists for 15 years.

January, 2012, Scientific America reported that the Foldit gamers achieved the first crowdsourced

redesign of a protein with more than 18-fold higher activity than the original. The protein is an

enzyme which catalyses the Diels-Alder reactions widely used in synthetic organic chemistry. A

science team including David Baker in the Center for Game Science at University of Washington

in Seattle computationally designed this enzyme from scratch but found the potency needing

improvement. The Foldit players reengineered the enzyme by adding 13 amino acids and

increased its activity by more than 18 fold.

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REVIEW

Ionic and Covalent Bonds:

Ionic bonds occur between a cation (metal) and an anion (nonmetal). Ions are held

together by electrostatic attraction, opposite charges ( +, − ) attracting each

other. These attractions are quite strong and increase with increasing charges

and decreasing ionic sizes. Ionic compounds have high melting points due to

a strong three dimensional network of attractions between ions.

Covalent bonds occur when electrons are shared between nonmetal atoms. The

length of a bond increases as the bond order decreases (triple < double < single).

The amount of energy that must be supplied to break a chemical covalent bond

in an isolated gas molecule is called the bond dissociation energy. The strength

of the bond energy increases with increasing bond order (single < double <

triple bonds). Covalent compounds generally have low melting points (below

300˚C). The strong molecular bonds do not break while melting. Covalent

compounds are made of discrete molecules held together by weak

intermolecular attractions.

Electronegativity:

Electronegativity is the ability of an atom to attract the shared electrons in a covalent

bond. Metallic elements attract weakly while nonmetals attract electrons more

strongly, especially as they become smaller and more nonmetallic. A way to

remember stronger to weaker electronegativities is the term FONClBrISCH.

Nonpolar covalent bonds occur when electrons share equally, polar covalent

bonds unequally share electrons, ionic compounds transfer electrons.

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Compounds:

Most compounds range somewhere between the two extremes of ionic and covalent

bonds. A large number of bonds share electrons unequally and are called polar

covalent bonds.

Bonding Theories:

The Lewis Theory, named after G. N. Lewis (1875-1946), predicts how atoms

connect while the VSEPR (valence shell electron pair repulsion) predicts shapes

Lewis dot structures:

A UC Berkeley chemistry professor in 1916, Gilbert Lewis, developed a method to

teach his beginning chemistry students how to understand chemical bonding that

represents valence electrons with dots for main group elements. These structures

have tremendous predictive power. Many advanced theories have evolved with

time, but the Lewis theory remains the simplest method for quick predictions.

Electron dot structures (Lewis Structures) show us a way to draw pictures of

compounds using all the valence electrons. Remember the valence electrons

are the outermost s and p electrons. Two s and six p electrons have a maximum

of eight. The structures show bonding electrons as single, double, and triple

lines (two electrons per line); nonbonding electrons as individual or pairs of

dots; and ions inside of square brackets with a charge on the outside.

Elements: Count valence electrons by the main group vertical column.

H-1, Be-2, B-3, C-4, N-5, O-6, F-7, Ne-8

Four regions have a maximum of two dots each = 8 dots maximum.

Atomic Lewis structures

For the atom oxygen

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Molecules and Compounds:

Lines represent shared electrons, one line for each two electrons. It is possible

to have single bonds (−) sharing 2 electrons, double bonds (=) sharing 4

electrons, or triple bonds (≡) sharing 6 electrons. Remaining electrons are

nonbonding dots. Ions are inside square brackets [X]charge.

Lewis Dot Structure Guidelines: Two-dimensional

Count all valence electrons

For ions:

1) add electrons for negative charges

2) subtract electrons for positive charges.

Two-Dimensional drawing, four sides (up, down, right, left)

Octet and Duet rules are generally followed

Many formulas tend to be symmetrical, when a formula has many of the

same element, they are often in terminal positions.

Generally O will not bond to O except in oxygen (O2), ozone (O3), and

peroxide or superoxides (O2-2, O2

-1).

Octet deficient atoms include: H-2, Be-4, B-6

Odd number of electrons will not have a complete octet (single dot)

Extended octets may occur with P or larger nonmetals (10 or 12 electrons).

Extended octets are mentioned, but not studied in Introductory Chemistry

courses. Examples: PCl5, SF6

Start with skeleton structure.

1) Choose an appropriate center atom, the one that desires multiple

bonds such as C or N.

2) Bonded H will be a terminal atom bonded with a single line and no

dots to fulfill the duet rule.

3) First connect all atoms with single bonds or ion brackets.

4) Work outside towards the center adding dots to fulfill octet/duet rule.

5) If you need more electrons then move nonbonding electrons to create

double or triple bonds.

6) Ions are inside square brackets with its charge outside the brackets

7) The best structure will minimize formal charges and may have

resonances. Structural isomers may occur.

8) (not for introductory chemistry) If you have too many electrons, the

center has an extended octet and gets the extra electrons.

Coordinate covalent bonds occur when both electrons of a bond come from

just one of the atoms as the bond between N and B in the molecule H3NBH3,

where the two nonbonding electrons on the nitrogen in NH3 share with

boron to increase the 6 shared electrons in BH3 to an octet

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Single bonds can twist and rotate, but double and triple bonds cannot rotate.

This fact causes molecules with a C=C double bond to have structural

isomers when the other two groups attached to each carbon are different.

Bond polarity (ionic, polar covalent, nonpolar covalent)

Resonance occurs by moving electrons without moving atom locations.

Structural isomers occur by moving atoms to create new connections

making a new substance

Formal Charge = group number – (lines + dots) around each atom

Lowest formal charges are preferred. All else being equal the more

electronegative atom prefers to have the negative formal charge

Count the number of electron regions: (a) any bond (single, double, triple)

counts as just one region, (b) a lone pair of nonbonding electrons or a single

dot for an odd number of electrons count as one electron region.

Mirror images have different structural isomers if all four groups attached

to the central carbon atom are different.

Lewis structures of ionic compounds show electrons transferred and do not

have bonds between the ions.

Practice Lewis Structures: H2, BH3, O2, CN-1, CO2, HCN, NO, SO2, C2H4, CHO, C2H2Br2, C3H8O,

F-1, NH4+1, NH4F, NaClO3, BaS, C6H14, SO3, MgO, BeH2

Count valence electrons, draw skeleton structure, solve for the best Lewis Structure

following the guidelines. Identify ionic vs. covalent compounds; ionic, polar and nonpolar

bonding; dipole arrows; resonances; structural isomers; formal charges; number of electron

regions.

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Valence Shell Electron Pair Repulsion (VSEPR) Theory: Three-dimensional

Use all the information that has been gained in the Lewis Dot Structure and convert

it to a three dimensional model to predict electronic and molecular shapes, angles,

and polarity of the molecule.

VSEPR Guidelines:

Use all the information from a Lewis Dot Structure

Three-Dimensional

Identify the Electronic and Molecular Shapes

Bonds angles

Polarity of substance (ionic, ion, nonpolar, polar molecule)

Lone pair (nonbonding) electrons take up more space

Electronic and Molecular Geometry:

Count the electron regions. Electron regions will give an electronic shape

while the number of bonded versus nonbonded regions will give the molecular

shape.

Extended Octets

# Electron

regions

2 3 4 5 6

Electronic

geometry

Linear

180˚

trigonal

planar

120˚

tetrahedral

109.5˚

trigonal

bipyramidal

90˚, 120˚, 180˚

octahedral

90˚, 180˚

molecular

geometry

Linear Trig planar,

bent

Tetrahedral,

Trigonal

pyramidal,

bent

trig.bipyramidal,

see saw,

T-shaped,

linear

octahedral,

square

pyramidal,

square

planar

CO2

CH2O

NH3 H2O

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Fill in the following tables: VSEPR drawings

#of electron

regions

number of

bonded

atoms

electronic

geometry

name

molecular

geometry

name

bond angles Rough

3-D

Sketch

an example

molecule or ion

any

1

linear

linear

(180)

H−−H

H2

CO

HF

N2

CN-1

CO2

3

120

3

bent

or angular

4

109.5

trigonal

pyramidal

H2O

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Molecular Shapes, Handedness and Drugs:

The shapes of molecules can dramatically change its characteristics. Mirror images

have different biological properties due to the specific shapes of receptor sites in the

body. For a molecule to exhibit handedness it needs 4 different groups attached to a

carbon.

Electronegativity and Polarity:

Why don’t Oil and Water Mix

If you combine oil and water in a container, they

separate into distinct regions.

Water is a Polar molecule and Oil is Nonpolar. Like

dissolves in like.

Polar Covalent Bonds:

• Covalent bonds that have an uneven sharing of the electrons that creates a

dipole moment are called polar covalent bonds.

• The magnitude of the dipole moment, and the polarity of the bond, depend

on the electronegativity difference between the two elements in the bond

and the length of the bond.

• For a fixed bond length, the greater the electronegativity difference, the

greater the dipole moment and the more polar the bond

Nonpolar Covalent Bonds:

• Nonpolar covalent bonds have identical electronegativities and even sharing

of the electrons.

• The Cl2 molecule is nonpolar.

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Ionic Bonds:

• Ionic bonds have larger differences in electronegativities and will transfer

electrons.

• NaCl forms an ionic bond

The degree of bond polarity is a continuous function. The guidelines given here are

approximate and it is often better to use the position of atoms on a periodic table as a

guide. When both atoms are nonmetals they generally form a type of covalent bond,

while a metal and nonmetal will generally form ionic bonds.

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Polar Bonds and Polar Molecules:

• Does the presence of one or more polar bonds in a molecule always result

in a polar molecule? The answer is no.

• A polar molecule is one with polar bonds that add together. They do not

cancel each other to form a net dipole moment. One must look at the 3-D

structure from VSEPR to determine polarity.

• When a diatomic molecule contains a polar bond, then the molecule is

polar.

• For molecules with more than two atoms, it is more difficult to tell polar

molecules from nonpolar ones because two or more polar bonds may

cancel one another.

• Consider CO2: the C=O bonds are polar, but the molecule overall is

nonpolar.

• Consider H2O: the H-O bonds are polar, the

molecule overall is polar.

• Water has two dipole moments that do not

cancel due to the angles in the formula caused

by the two lone pairs of dots on the O (not

shown)

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Predicting Polarity of Molecules:

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PRACTICE: Complete the following table for the indicated substances.

Electronegativities: Na = 0.9, H = 2.1, C = 2.5, Br = 2.8, N = 3.0, O = 3.5, F = 4.0

substance NO3-1 C2H4O2 OF2 NaBrO3

Draw the best

Lewis

structure(s),

resonances, and

structural isomers

if any with octet

3 resonances Structural isomers,

draw 2

Follows octet rule Ionic compound

name electronic

geometry around

central atom

name molecular

geometry around

central atom

a)show 3-D

sketch with

atoms & bonds in

it

b) Indicate polar

bonds with dipole

arrows toward

the more

electronegative

give all bond

angles

is it an ionic

compound, polar

or nonpolar

molecule or an

ion?

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How Soap Works:

• After eating a greasy meal, your hands are coated with grease and oil. If you try to

wash them with only water, they remain greasy. However, if you add a little soap,

the grease washes away. Why?

• Water molecules are polar, and the molecules that compose grease and oil are

nonpolar. As a result, water and grease repel each other.

• One end of a soap molecule is polar, while the other end is nonpolar.

• The polar head of a soap molecule strongly attracts water molecules, while the

nonpolar tail strongly attracts grease and oil molecules.

• Soap allows water and grease to mix, removing the grease from your hands and

washing it down the drain.