Chapter 8
◦ These toy models are made
from circular pieces joined
together in units by sticks.
Atoms can also be arranged
in different ways to make a
variety of products.
 In nature, matter takes many
forms. The noble gases,
including helium and neon, are
monatomic. That means they
exist as single atoms.
◦ Some compounds are so different from ionic
compounds that attractions between ions fail to
explain their bonding.
 The atoms held together by sharing electrons are
joined by a covalent bond.
◦ A molecule is a neutral group of atoms joined
together by covalent bonds. Air contains oxygen
molecules.
 A diatomic molecule is a molecule consisting of two
atoms. An oxygen molecule is a diatomic molecule.
 A compound composed of molecules is called a
molecular compound. Water and carbon monoxide are
molecular compounds.
◦ Molecular compounds tend to have relatively lower
melting and boiling points than ionic compounds.
 Unlike ionic compounds, many molecular compounds
are liquids or gases at room temperature.
 Unlike ionic compounds, molecular compounds usually
contain only nonmetal atoms.
 Ethane, a component of natural gas, is also a
molecular compound.
◦ A molecular formula is the chemical formula of a
molecular compound.
 A molecular formula shows how many atoms of each
element a molecule contains.
 Formulas of Some Molecular Compounds
◦ The colors in this map
indicate the concentrations
of ozone in various parts of
Earth’s atmosphere. Oxygen
atoms can join in pairs to
form the oxygen you
breathe and can also join in
groups of three oxygen
atoms to form ozone.
8.
2
◦ In covalent bonds, electron sharing usually occurs
so that atoms attain the electron configurations of
noble gases.
 Two atoms held together by sharing a pair of electrons
are joined by a single covalent bond.
8.
2
◦ An electron dot structure such as H:H represents
the shared pair of electrons of the covalent bond by
two dots.
 A structural formula represents the covalent bonds by
dashes and shows the arrangement of covalently bonded
atoms.
◦ The halogens form single covalent bonds in their
diatomic molecules. Fluorine is one example.
8.
2
◦ A pair of valence electrons that is not shared
between atoms is called an unshared pair, also
known as a lone pair or a nonbonding pair.
◦ The hydrogen and oxygen atoms attain noble-gas
configurations by sharing electrons.
◦ The ammonia molecule has one unshared pair of
electrons.
◦ Methane has no unshared pairs of electrons.
for Conceptual Problem 8.1
8.
2
◦ Atoms form double or triple covalent bonds if they
can attain a noble gas structure by sharing two
pairs or three pairs of electrons.
8.
2
◦ A bond that involves two shared pairs of electrons
is a double covalent bond.
◦ A bond formed by sharing three pairs of electrons
is a triple covalent bond.
8.
2
 Carbon dioxide gas is soluble in water and is used to
carbonate many beverages. A carbon dioxide molecule
has two carbon-oxygen double bonds.
◦ Carbon dioxide is an example of a triatomic
molecule.
8.
2

In a coordinate covalent bond, the shared
electron pair comes from just one of the
bonding atoms.
 A polyatomic ion, such as NH4+, is a tightly bound
group of atoms that has a positive or negative charge
and behaves as a unit.
 Most plants need nitrogen that is already combined in
a compound to grow.
for Conceptual Problem 8.2
◦ The energy required to break the bond between two
covalently bonded atoms is known as the bond
dissociation energy.
 A large bond dissociation energy corresponds to a strong
covalent bond.

This car is being painted by a process called
electrostatic spray painting. The negatively
charged droplets are attracted to the auto
body. You will learn how attractive and
repulsive forces influence the shapes of
molecules.
8.
3
◦ The valence-shell electron-pair repulsion theory, or
VSEPR theory, explains the three-dimensional shape
of methane.
8.
3
 The hydrogens in a methane molecule are at the four
corners of a geometric solid. All of the H—C—H angles
are 109.5°, the tetrahedral angle.
8.
3
◦ According to VSEPR theory, the repulsion
between electron pairs causes molecular shapes
to adjust so that the valence-electron pairs stay
as far apart as possible.
8.
3
 The measured H—N—H bond angle is only 107°.
8.
3
◦ The measured bond angle in water is about 105°.
◦ The carbon dioxide molecule is linear.
8.
3
 Nine Possible Molecular Shapes

Snow covers approximately 23 percent of
Earth’s surface. Each individual snowflake is
formed from as many as 100 snow crystals.
The polar bonds in water molecules influence
the distinctive geometry of snowflakes.
8.
4
 The bonding pairs of electrons in covalent bonds are
pulled by the nuclei.
8.
4
◦ When the atoms in a bond pull equally (as occurs
when identical atoms are bonded), the bonding
electrons are shared equally, and the bond is a
nonpolar covalent bond.
◦ The bond in a molecule of hydrogen is nonpolar.
◦ The bond between carbon atoms in ethane is
nonpolar.
8.
4
◦ A polar covalent bond, known also as a polar bond,
is a covalent bond between atoms in which the
electrons are shared unequally.
 The more electronegative atom attracts electrons more
strongly and gains a partial negative charge (δ-). The less
electronegative atom has a partial positive charge (δ+).
8.
4
 The chlorine atom attracts the electron cloud more
than the hydrogen atom does.
◦ The carbon to hydrogen bonds in ethane are polar.


Pg 239
Which type of bond (nonpolar covalent, polar
covalent or ionic) will form between each of
the following pairs of atoms?
a.
b.
c.
d.
N and H
F and F
Ca and Cl
Al and Cl

Pg. 239
Identify the following bond types:
a.
b.
c.
d.
e.
f.
H and Br
K and Cl
C and O
Cl and F
Li and O
Br and Br
◦ In a polar molecule, one end of the molecule is
slightly negative and the other end is slightly
positive.
 A molecule that has two poles is called a dipolar
molecule, or dipole.
◦ When polar molecules are placed between
oppositely charged plates, they tend to become
oriented with respect to the positive and negative
plates.
 This is referred to as the “dipole moment”.
 A hydrogen chloride molecule is a dipole.

Other polar molecules:
◦
◦
◦
◦
H2O
NH3
HF
CH3Cl

Molecules containing all nonpolar bonds, and
molecules with polar bonds oriented
symmetrically in space are nonpolar.

When nonpolar molecules are placed between
oppositely charged plates, they do not become
oriented with respect to the positive and negative
plates.

Nonpolar molecules do not exhibit a dipole moment.

Nonpolar molecules with nonpolar bonds:
◦
◦
◦
◦
◦
◦
◦
H2
N2
Cl2
O2
F2
I2
Br2

Nonpolar molecules with polar bonds:
◦
◦
◦
◦
CH4
CO2
C2H2
C2H4
8.
4

Intermolecular forces of attraction (IMFs) are
attractive forces that exist between
molecules.
◦ Intermolecular forces of attraction (IMFs) are weaker
than either ionic or covalent bonds.
 These attractions are responsible for determining
whether a molecular compound is a gas, a liquid, or a
solid at a given temperature.
◦ Dipole interactions
occur when polar
molecules are attracted
to one another.
8.
4
◦ Dispersion forces, the weakest of all molecular
interactions, are caused by the shifting motion of
electrons which create temporary dipoles.
 The strength of dispersion forces generally increases
as the number of electrons in a molecule increases.
 Ex: the halogens
◦ Hydrogen “bonds” are attractive forces in which a
hydrogen atom covalently bonded to a very
electronegative atom (N,O, or F) is also weakly
bonded to another molecule’s unshared electron
pair.
 Ex: H2O, NH3, HF
 H-bonds play a significant role in the shape of DNA and
protein molecules.
8.
4
 Hydrogen Bonding in Water
◦ Hydrogen bonding in water.
 The relatively strong attractive forces between water
molecules cause the water to form small drops on a
waxy surface. Without H-bonds, water would be a gas
at room temperature.
8.
4
◦ Network solids (network crystals) are solids in
which all of the atoms are covalently bonded to
each other (no molecules).
 Melting a network solid would require breaking
covalent bonds throughout the solid.
 Network solids do not melt until the temperature
reaches 1000°C or higher, or they decompose without
melting at all.
8.
4
 Diamond is an example of a network solid. Diamond
does not melt. It vaporizes to a gas at 3500°C or
above.
8.
4
 Silicon Carbide is a network solid. It has a melting
point of about 2700°C.