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Houston Community College System
Chemistry 1405
Chapter 3
Molecules and Chemical Bonds
By
Mounia Elamrani
Blei / Odian’s General, Organic, and Biochemistry
Objectives
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Ionic and covalent bonds
Octet rule and the formation of ions
Formulas of binary ionic compounds
Naming ionic and covalent compounds
Lewis structures
Bond polarity
VSEPR theory and molecular shape
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
The Octet Rule and Chemical Bonding
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Noble gases are very stable elements and virtually
non-reactive
Noble gases have an octet (8) of electrons in their
valence shell
Other Elements try to achieve an octet by forming
chemical bonds and compounds
There are two types of chemical bonds:
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Ionic Bonds
Covalent Bonds
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
3.1 Ionic vs. Covalent Bonds
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Ionic bond:
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Covalent bond:
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forms between a positive ion and a negative ion
There is a complete transfer of electrons from one element
to the other
Strong electrostatic forces hold the ions together
Mostly formed between metals and nonmetals
Forms between elements sharing electrons
Mostly formed between nonmetals
Electron affinity and ionization energy help
determine the type of bond formed
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
3.1 Electron Affinity and Ionization Energy
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Electron Affinity (EA) measures the ability of an
atom to attract an electron:
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The higher the EA, the stronger the attraction
Elements with high EA tend to gain electrons, forming
negative ions
EA increases up in a group and to the left of a period
Ionization Energy (IE) measures the ability of an
atom to loose an electron from the outer shell
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The lower the IE, the easier it is to loose the electron
Elements with low IE tend to loose electrons, forming
positive ions
IE increases up in a group and to the left of a period
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
3.2 Lewis Symbols and Formulas of Ionic
Compounds
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Lewis Symbols represent the valence electrons as
dots around the chemical symbol of the element
Lewis Symbols are used to depict how elements can
transfer or share electrons to form chemical bonds
Ionic compounds result when metals react with
nonmetals
We can write the formula of the compound formed
using simple rules:
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Determine the charges on each ions
Balance positive and negative charges
Write a formula with the metal first then the nonmetal
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Formation of Ions from Metals
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Metals will lose electrons to nonmetals to form ionic
compounds
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Li gives 1 e- to F  Li+ and F-
Group 1A metals 
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Mg gives 1e- each to two F  Mg2+ and two F-
Group 2A metals 
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ion 1+
ion 2+
Al gives 1e- each to three F  Al3+ and three F-
Group 3A metals 
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
ion 3+
Ions from Nonmetal Ions
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In ionic compounds, nonmetals in 5A, 6A, and 7A
gain electrons from metals
Nonmetal add electrons to achieve the octet
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F, Cl, Br… gain 1e-  F- , Cl-,…
Group 7A metals 
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ion 1-
O, S… gain 2e-  O2-, S2-…
Group 6A metals 
ion 2-
Group 5A metals 
ion 3-
N, P gain 3e-  N3-, P3-
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Learning Check
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A. Why does Ca form a Ca2+ ion and Na form a
Na+ ion?
B. Why does F form a F- ion and O form a O2- ion?
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Formation of Ionic Compounds
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Attraction between + ions and - ions
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Electrons go from metals to nonmetals
electron transfer
metal
+
nonmetal
ion+ + ion–
Electrons lost = Electrons gain
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Formulas of Ionic Compounds
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Formulas of ionic compounds are determined from the charges
on the ions
atoms
Na 
+

ions

 –
F :  Na+ + : F :  NaF

sodium fluorine
Charge balance:
Mounia Elamrani

sodium
1+
fluoride
1-
formula
= 0
Chapter 3 – Molecules and Chemical Bonds
Writing a Formula
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Write the formula for the ionic compound that will
form between Ba2+ and Cl.
Solution:
1. Balance charge with + and – ions
2. Write the positive ion of metal first, then the
negative ion
Ba2+
Cl
Cl
3. Write the number of ions needed as subscripts
BaCl2
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Learning Check
Write the correct formula for the compounds
containing the following ions:
A. Na+, S21) NaS
2) Na2S
3) NaS2
B. Al3+, Cl1) AlCl3
2) AlCl
3) Al3Cl
C. Mg2+, N31) MgN
2) Mg2N3
3) Mg3N2
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
3.3 Naming Binary Ionic Compounds
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Binary compounds contain 2 different elements
Name the metal first, then the nonmetal as -ide.
Examples:
NaCl
ZnI2
Al2O3
Mounia Elamrani
sodium chloride
zinc iodide
aluminum oxide
Chapter 3 – Molecules and Chemical Bonds
Learning Check

Complete the names of the following binary
compounds:
Na3N
sodium
________________
KBr
potassium ________________
Al2O3
aluminum ________________
MgS
_____________________________
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Transition Metals
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Many form 2 or more positive ions:
Transition Metals
Chromium
Cr2+, Cr3+
Manganese
Mn2+, Mn3+
Iron
Fe2+, Fe3+
Cobalt
Co2+, Co3+
Nickel
Ni2+
Copper
Cu+, Cu2+
Post-transition Metals
Tin
Sn2+, Sn4+
Lead
Pb2+, Pb4+
Bismuth
Bi3+
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Zinc
Silver
Cadmium
Gold
Mercury
Chapter 3 – Molecules and Chemical Bonds
Zn2+
Ag+
Cd2+
Au+, Au3+
Hg22+, Hg2+
Names of Variable Ions
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Use a roman number after the name of a metal that
forms two or more ions:
FeCl2
CuCl
SnF4
PbCl2
Fe2S3
Mounia Elamrani
(Fe2+)
(Cu+ )
(Sn4+)
(Pb2+)
(Fe3+)
iron (II) chloride
copper (I) chloride
tin (IV) fluoride
lead (II) chloride
iron (III) sulfide
Chapter 3 – Molecules and Chemical Bonds
Learning Check
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Complete the names of the following binary
compounds with variable metal ions:
FeBr2
Cu2O
SnCl4
Fe2O3
CuS
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iron (_____) bromide
copper (_____) oxide
_____(___ ) _______________
_____(____)_______________
_____(____)_______________
Chapter 3 – Molecules and Chemical Bonds
Learning Check
Name the following compounds:
A.
CaO
1) calcium oxide
2) calcium(I) oxide
3) calcium (II) oxide
B.
SnCl4
1) tin tetrachloride 2) tin(II) chloride
3) tin(IV) chloride
C. Co2O3
1) cobalt oxide
3) cobalt trioxide
Mounia Elamrani
2) cobalt (III) oxide
Chapter 3 – Molecules and Chemical Bonds
3.4 Polyatomic Ions
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Ions consisting of 2 or more atoms combined, with a
total charge that is not zero
Consider these ions as one unit, similar to the
monoatomic ions
NH4+
ammonium
OH-
hydroxide
NO3-
nitrate
NO2-
nitrite
CO32-
______________
HCO3-
hydrogen carbonate (bicarbonate)
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
3.4 More Polyatomic Ions
Sulfur
SO42- sulfate
SO32- sulfite
HSO4- hydrogen sulfate
HSO3- hydrogen sulfite
Phosphorous
PO43-
phosphate
HPO42-
_______________________________
H2PO4-
dihydrogen phosphate
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PO33- ____________
Chapter 3 – Molecules and Chemical Bonds
Learning Check
Match each set with the correct name:
A.
Na2CO3
1) magnesium sulfite
MgSO3
2) magnesium sulfate
MgSO4
3) sodium carbonate
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B.
Ca(HCO3)2
CaCO3
Ca3(PO4)2
Mounia Elamrani
1) calcium carbonate
2) calcium phosphate
3) calcium bicarbonate
Chapter 3 – Molecules and Chemical Bonds
Learning Check
A. aluminum nitrate
1) AlNO3
2) Al(NO)3
B. copper(II) nitrate
1) CuNO3
2) Cu(NO3)2
3) Al(NO3)3
3) Cu2(NO3)
C. Iron (III) hydroxide
1) FeOH
2) Fe3OH
3) Fe(OH)3
D. Tin(IV) hydroxide
1) Sn(OH)4 2) Sn(OH)2
3) Sn4(OH)
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
3.6 Covalent Bonds and There Nomenclature
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Covalent bonds form between two nonmetals
Nonmetals have high electronegativity values
Electrons are shared
single bond shares one pair electrons
double bond shares two pairs electrons
triple bond shares three pairs electrons
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Learning Check
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Indicate whether a bond between the following
would be
1) Ionic
2) covalent
____
____
____
____
____
Mounia Elamrani
A.
B.
C.
D.
E.
sodium and oxygen
nitrogen and oxygen
phosphorus and chlorine
calcium and sulfur
chlorine and bromine
Chapter 3 – Molecules and Chemical Bonds
Covalent Bonds
Two nonmetal atoms form a covalent bond because they
have less energy after they bonded
H +
H
Hydrogen atoms
Mounia Elamrani
H : H = HH = H2
Hydrogen molecule
Chapter 3 – Molecules and Chemical Bonds
Diatomic Molecules
Gases that exist as diatomic molecules
are H2, F2, N2, O2, Cl2, Br2, I2
octets


N

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
+

N




N:::N
triple bond
Chapter 3 – Molecules and Chemical Bonds
Learning Check
Use the name of the element to name the following
diatomic molecules.
H2
hydrogen
N2
nitrogen
Cl2
_______________
O2
_______________
I2
_______________
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Covalent Bonds in NH3
Bonding pairs
H

H
:
N
:
H

Lone pair of electrons
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Naming Binary Covalent Compounds
Two nonmetals
 Name each element
 End the last element in -ide
 Add prefixes to show more than 1 atom
Prefixes
mono
1
tetra
di
2
penta
tri
3
hexa
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
4
5
6
Learning Check
Fill in the blanks to complete the following names of covalent
compounds.
CO
carbon ______oxide
CO2
carbon _______________
PCl3
phosphorus _______chloride
CCl4
carbon ________chloride
N2 O
_____nitrogen _____oxide
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Learning Check
A. P2O5
1) phosphorus oxide
2) phosphorus pentoxide
3) diphosphorus pentoxide
B.
Cl2O7
1) dichlorine heptoxide
2) dichlorine oxide
3) chlorine heptoxide
C.
Cl2
1) chlorine
2) dichlorine
3) dichloride
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Electronegativity
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Electronegativity is defined as the ability of an
atom in a molecule to attract electrons to
itself.
Electronegativity of an atom is related to its
IE and EA.
Electronegativity is a scale from 0.7 (Cs) to
4.0 (F)
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Bond Polarity
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In a covalent bond, electrons are shared.
Sharing of electrons to form a covalent bond does
not imply equal sharing of those electrons.
There are some covalent bonds in which the
electrons are located closer to one atom than the
other.
Unequal sharing of electrons results in polar
covalent bonds.
If the difference in electronegativity is large enough,
an ionic bond is formed.
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Bond Polarity
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Nonpolar covalent bond
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Polar covalent bond
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Electrons are shared between atoms with the same
electronegativity values.
Difference = 0
Examples:
N2
Br2
Electrons are shared between different nonmetal atoms
Examples:
O-Cl
O-S
N-Cl
Ionic bond
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Electrons are transferred between metal and nonmetal
atoms
Examples:
NaCl
KF
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Bond Polarity
a) The electrons are shared
equally between the two H
atoms in H2, which gives a
nonpolar covalent bond.
b) The electrons of the bond in
HCl are pulled towards the
Cl end of the molecule
giving a polar covalent
bond.
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Learning Check
Identify the type of bond between the following
atoms
A. K-N
1) nonpolar
2) polar
3) ionic
B. N-O
1) nonpolar
2) polar
3) ionic
C. Cl-Cl
1) nonpolar 2) polar
3) ionic
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Drawing Lewis Structures
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add the valence electrons from all atoms. If ions, then add
negative charge and subtract positive charge
Identify the central atom
Place the central atom in the center of the molecule and add
all other atoms around it.
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Place one bond (two electrons) between each pair of atoms.
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Complete the octet for all atoms surrounding the central atom.
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Complete the octets for the central atom. Use double bonds if
necessary.
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Learning Check
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Draw the Lewis Structures of the following
compounds:
PCl3
SCl2
CO2
NH4+
IF4-
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
3.10 3-D Molecular Structures, VSEPR
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Lewis structures do not indicate the shapes of
molecules; they simply show the # and types of
bonds between atoms.
The shape of a molecule is determined by its bond
angles.
In CCl4, experimentally all Cl-C-Cl bond angles are
109.5°.
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
3.10 VSEPR
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In order to predict molecular shape, we assume the
valence electrons repel each other. Therefore, the
molecule adopts whichever 3D geometry minimizing
this repulsion.
We call this process Valence Shell Electron Pair
Repulsion (VSEPR) theory.
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
3.10 VSEPR
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To determine the electron pair geometry:
1. draw the Lewis structure.
2. count the total number of electron domains around the
central atom and arrange them in the way (geometry) that
minimizes e--e- repulsion.
3. multiple bonds (double or triple bond), and lone pairs count
as one electron domain when predicting geometry
# electron domain
=
# of atoms bonded to central atom
+
# of nonbonding pairs on the central atom.
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
3.10 Molecular Geometry (no lone pairs)
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
3.10 Molecular Geometry (with lone pairs)
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NH3 has: 3 bonds + 1 lone pair = 4 electron domains
4 electron domains = Tetrahedral geometry
Tetrahedral geometry with one missing bond =
Trigonal Pyramidal geometry
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
3.10 Molecular Geometry (with lone pairs)
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds
Learning Check
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Draw the 3-D structure of the following compounds:
CCl4
NI3
OF2
Mounia Elamrani
Chapter 3 – Molecules and Chemical Bonds