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Chemical Bonds:

The Formation of

Compounds From AtomsChapter 11

Hein and Arena

Eugene Passer

Chemistry Department

Bronx Community College

© John Wiley and Sons, Inc.

Version 1.1

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Metals and Nonmetals Reviewed

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Chemical Properties

of Metals

• metals tend to lose

electrons and form

positive ions called

cations.

• nonmetals tend to

gain electrons and

form negative ions

called anions.

Chemical Properties

of Nonmetals

When metals react with nonmetals, electrons

are usually transferred from the metal to the

nonmetal; each obtains a FULL OCTET.

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Metalloids have properties that

are intermediate between metals

and nonmetals

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Metals are found to the left of the metalloidsNonmetals are found to the right of the metalloids.

11.1

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Periodic Trends in

Atomic Properties

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Atomic Radius

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Atomic radii

increase down a

group.

11.2

n = 1

n = 2

n = 3

n = 4

n = 5

n = 6

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Radii of atoms tend to decrease

from left to right across a period.

For

representative

elements within

the same period,

n remains

constant as

electrons are

added.

This increase in

positive nuclear

charge pulls all

electrons closer

to the nucleus.

11.2

Each time an

electron is

added, a proton

is also added to

the nucleus.

n=1

n=2

n=3

n=4

n=5

n=6

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Ionization Energy

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The ionization energy of an atom is the

energy required to remove an electron from

an atom.

Na + ionization energy → Na+ + e-

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Periodic relationship of the first ionization energy for

representative elements in the first four periods.11.3

Ionization energies of Group A elements decrease

from top to bottom in a group.

IA

IIA IIIA

IVA

VAVIA

VIIA

Noble

Gases

ns2np3

ns2np6

ns2

ns2np1

ns2np4ns1

ns2np2

ns2np5

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As each succeeding electron is removed from

an atom ever higher energies are required.

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Lewis Structures

of Atoms

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The Lewis structure of an atom uses dots to

show the valence electrons of atoms.

The number of dots equals the number of s

and p electrons in the atom’s valence shells.

BPaired

electrons

Unpaired

electron

Symbol of

the element

2s22p1

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11.4

Lewis Structures of the first 20 elements.

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The Ionic Bond: Transfer of

Electrons From One Atom

to Another

(Metal – Nonmetal)

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An ionic bond results from the complete

transfer of an electron(s) from the metal

atom to the nonmetal atom involved in a

bond.

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The Formation ofSodium Chloride

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The 3s electron of sodium transfers to the 3p orbital of

chlorine.

Lewis representation of sodium chloride formation.

A sodium ion (Na+) and a chloride ion (Cl-) are formed.

The force holding Na+ and Cl- together is an ionic bond.

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Relative Size ofSodium Ion to Chloride Ion

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The sodium ion is smaller than the sodium atom

because it lost an electron.

Na [Ne] 3s1 Na+ [Ne]

The chloride ion is larger that the chlorine atom

because it gained an electron.

Cl [Ne] 3s2 3p5 Cl- [Ar]

11.6

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Predicting Formulas of

Ionic Compounds

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Because of similar electronic configurations,

the elements of a group, or family, generally

form ionic compounds with the same atomic

ratios.

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The Covalent Bond:

Sharing Electrons

(Nonmetal – Nonmetal)

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A covalent bond results when the

bonding electron pair is shared between

the two nonmetal atoms involved in a

bond.

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Covalent bonding in the hydrogen molecule

involving s atomic orbitals

11.8

:. .

+

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11.9

Covalent bonding in the chlorine molecule

involving p atomic orbitals

:. .

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hydrogen chlorine iodine nitrogen

Covalent bonding with equal sharing of

electrons occurs in diatomic molecules

formed from one element.

A dash may replace a pair of dots.

N N

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Electronegativity and Bond

Polarity

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Electronegativity is the relative ability of a

covalently bonded atom to attract the

bonding pair of electrons to itself.

The greater the electronegativity of the atom,

the greater the attraction it has for the

bonding electrons.

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• If the two atoms that constitute a

covalent bond are identical then there

is equal sharing of electrons.

• This is called nonpolar covalent

bonding.

• Ionic bonding and nonpolar covalent

bonding represent two extremes.

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• If the two atoms that constitute a

covalent bond are not identical then

there is unequal sharing of electrons.

• This is called polar covalent bonding.

• One atom assumes a partial positive

charge and the other atom assumes a

partial negative charge.

– This charge difference is a result of the

unequal attractions of shared electron

pair.

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:H Cl

+ -

Shared electron pair.

:

The shared electron pair

is closer to chlorine than

to hydrogen.

Partial positive charge

on hydrogen.

Partial negative charge

on chlorine.

Chlorine has a greater attraction for the

shared electron pair than hydrogen.

Polar Covalent Bonding in HCl

The chlorine atom is more electronegative than

hydrogen.

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Electronegativity decreases down a group for

representative elements.

Electronegativity generally increases left to right

across a period.

1

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The polarity of a bond is determined by the

difference in electronegativity values of the

atoms forming the bond.

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A electronegativity difference > 1.6-1.9,

the bond will be more ionic than

covalent.

A electronegativity difference > 2, the

bond is strongly ionic.

A electronegativity difference < 1.5, the

bond is strongly covalent.

General Guideline to Bond Polarity

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H H

Hydrogen Molecule

If the electronegativities are the same, the bond

is nonpolar covalent and the electrons are shared

equally.The molecule is

nonpolar covalent.

Electronegativity

2.1

Electronegativity

2.1

11.10

Electronegativity

Difference = 0.0

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If the electronegativities are not the same, the

bond is polar covalent and the electrons are

shared unequally.

H Cl

Hydrogen Chloride Molecule

Electronegativity

2.1

Electronegativity

3.0

The molecule is

polar covalent.

+ -

Electronegativity

Difference = 0.9

11.10

42Sodium Chloride

Na+ Cl-

If the electronegativities are very different, the

bond is ionic and the electrons are transferred to

the more electronegative atom.

Electronegativity

0.9

Electronegativity

3.0

The bond is ionic.No molecule exists.

Electronegativity

Difference = 2.1

11.10

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An arrow can be used to indicate a dipole.

The arrow points to the negative end of the

dipole.

H Cl H Br H

O

H

Molecules of HCl, HBr and H2O are polar .

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A molecule containing different kinds of

atoms may or may not be polar depending

on its shape.

The carbon dioxide molecule is nonpolar

because its carbon-oxygen dipoles cancel

each other by acting in opposite directions.

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Lewis Structures of

Compounds

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Step 1. Count the total number of valence

electrons in the molecule.

Step 2. Draw a symmetric skeletal structure

of the molecule with the most

electropositive element as central

atom.

Step 3. Give a full octet to each atom in

skeletal structure (except H and B).

Writing Lewis Structures: Step by Step.

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Step 4. If the electron count in Step 1 and

Step 3 are equal; you have drawn the

correct structure (e.g. CH4).

Step 5. If the electron count in Step 1 is less

than Step 3, use multiple bonds (e.g.

CO2).

Step 6. If the electron count in Step 1 is

greater than Step 3, add additional

electron pair(s) to the central atom

(e.g. ICl3).

Writing Lewis Structures, Cont.

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Rule 1. Carbon is usually the central atom,

It can bond to itself. It can form

4-bonds, 2 double bonds or a triple

bond.(e.g. O=C=O and C=O).

Rule 2. Oxygen does not bond to itself,

except in peroxide, and forms 2-

bonds or a double bond

(e.g. CH3-O-H).

Rule 3 Hydrogen forms only 1-bond.

Writing Lewis Structures, Cont.

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Write a Lewis structure for CO2.

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Write a Lewis structure for ICl3.

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Write a Lewis structure for NO3-.

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Molecular Shape

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11.12

105° 180° 120° 109.5°

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The Valence Shell

Electron Pair Repulsion

(VSEPR) Model

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The VSEPR model is based on the idea

that electron pairs will repel each other

electrically and will seek to minimize

this repulsion.

VSEPR: the electron pairs are arranged as

far apart as possible around a central

atom.

Non-bonding electron pairs are more

delocalized than bonding electron pairs.

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Its electrons are arranged 180o apart for

maximum separation.

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• Its electrons are arranged 120o apart for

maximum separation.

Trigonal planer geometry

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• However, since the molecule is 3-dimensional

the molecular structure is tetrahedral with a

bond angle of 109.5o.

Tetrahedral geometry

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Ball and stick models of methane, CH4, and carbon

tetrachloride, CCl4.11.13

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The NH3 molecule

is pyramidal.

NH3 has one

unbonded pair

of electrons.

~109°

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The H2O molecule

is bent.

H2O has two

unbonded pair

of electrons.

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