5.10
Molecules and Covalent Bonding
Nitrous oxide, N2O, is a colourless, sweet-smelling gas that dentists
sometimes use to relax nervous patients (Figure 1(a)). You would not,
however, want to inhale nitrogen dioxide, NO2. This reddish-brown toxic
gas is produced in the atmosphere from pollutants emitted in automobile
exhaust. Nitrogen dioxide is a dangerous part of the smog that hangs over
large cities in the summer (Figure 1(b)). People with asthma and other
respiratory problems often find it more difficult to breathe on smoggy days.
Figure 1 (a) Inhaling nitrous oxide,
laughing gas, helps dental patients
relax. (b) Nitrogen dioxide is the gas
responsible for the reddish-brown colour
of smog over large cities during the
summer.
To find out more about the
Ontario Drive Clean Program,
GO TO NELSON SCIENCE
molecular compound a pure substance
formed from two or more non-metals
(a)
(b)
Figure 2 (a) Each nitrogen dioxide
particle is a molecule made up of one
nitrogen atom and two oxygen atoms.
(b) An ionic compound does not exist
as individual particles. Instead, millions
of ions are tightly held together in
a crystal.
(a)
(b)
Pollution control technology in modern cars helps reduce nitrogen
dioxide emissions. A catalytic converter, attached to the car’s exhaust system,
converts nitrogen dioxide into harmless nitrogen and oxygen. The Ontario
Drive Clean Program requires that most cars over five years old pass an
emissions test every two years. This helps reduce pollution in Ontario.
Nitrous oxide and nitrogen dioxide are both molecular compounds. As
the name implies, molecular compounds are made up of individual particles
called molecules (Figure 2(a)). (Ionic compounds, as Figure 2(b) shows,
consist of many ions in a crystal.) The chemical formula of a molecular
compound gives the exact numbers of atoms in each molecule. The elements
that make up molecular compounds are all non-metals.
Molecular compounds are all around you: in the air that you breathe and
the substances that you eat and drink (Figure 3). A soft drink, for example,
contains water molecules, H2O, and sugar molecules, C12H22O11, as well as
flavouring and colouring molecules. Carbonated soft drinks, like colas, also
contain a large quantity of dissolved carbon dioxide molecules, CO2. In
fact, the majority of all known compounds are molecular. Living organisms
make thousands of different kinds of molecular compounds. Sugars, fats,
and proteins are all molecular compounds. Some of them are very large,
containing thousands of atoms in a single molecule! Like nitrogen dioxide,
many molecular compounds affect our environment.
C05-F32B-UBOS10SB.ai
Will need photo to complete.
Figure 3 Most of the chemicals
that we encounter are molecular
compounds, including (a) sugar,
C05-F32A-UBOS10SB.ai
(b) water, and (c) acetylsalicylic acid,
or Aspirin.
(a)
(b)
(c)
Will need photo to complete.
206
Chapter 5 • Chemicals and Their Properties
NEL
Bonding in Molecules
In Section 5.4, you learned that ions form when metallic elements lose
electrons to non-metal elements. An electron transfer occurs because
• the metal’s hold on its outer electrons is weak
• the attraction of the non-metal for the metal’s electrons is strong
• a full outer electron orbit is very stable
Remember that non-metals all have almost-full outer electron orbits with
“spaces” available that attract other electrons. This allows non-metal atoms
to get relatively close to each other.
When two non-metals bond with each other, both nuclei form strong
attractions for the other’s electrons. However, neither atom attracts the
other’s electrons strongly enough to pull them away completely. What
results is a “tug of war” for electrons that neither atom ever wins. The net
effect is that the two atoms share each other’s electrons, resulting in a bond
that holds the atoms together. A chemical bond that results from atoms
sharing electrons is called a covalent bond. The bonded atoms form a
molecule. Molecules that consist of two atoms joined with a covalent bond
are called diatomic molecules.
Covalent bonds can form between two identical atoms or between atoms
of different elements. We will start by looking at two identical hydrogen
atoms. A hydrogen atom has one outer electron. To achieve a stable outer
orbit like that of the nearest noble gas (helium), hydrogen must acquire one
more electron. When two hydrogen atoms collide, the proton of one atom
attracts the electron of the other and vice versa (Figure 4). Since the atoms
are identical, they have the same ability to attract electrons. As a result,
both electrons are shared equally between the two atoms. This results in a
covalent bond between the atoms. The resulting hydrogen molecule has the
chemical formula H2. A solid line linking the atoms, H—H, represents a
covalent bond.
1p+
H
1p+
H
1p+
molecule a particle in which atoms are
joined by covalent bonds
diatomic molecule a molecule consisting
of only two atoms of either the same or
different elements
1p+
Figure 4 A covalent bond results
from the sharing of a pair of electrons
represented by a dash.
H–H
Fluorine is another example of a diatomic molecule. Fluorine has seven
outer electrons—one electron short of a stable electron arrangement. When
two fluorine atoms share a pair of electrons (one from each atom) to form
a covalent bond, they form a relatively stable fluorine molecule with the
chemical formula F2.
There are other kinds of diatomic molecules. Some are made up of atoms
that share two pairs of electrons. Oxygen is an example of this. There is
a double covalent bond joining the two atoms: O==O. Other diatomic
molecules are made up of two different elements (such as hydrogen fluoride,
HF, shown in Figure 5).
Other molecules are made up of three or more atoms. A water molecule,
for example, consists of one oxygen atom and two hydrogen atoms: H2O.
NEL
covalent bond a bond that results from
the sharing of outer electrons between
non-metal atoms
1p+
9p+
H–F
Figure 5 A hydrogen atom and a fluorine
atom form a hydrogen fluoride molecule.
5.10 Molecules and Covalent Bonding
207
Table 1 lists common elements that exist as diatomic molecules. This list will
be useful when you write chemical equations in Chapter 6.
LEARNING TIP
Who Is Hofbrincl?
Hofbrincl, more correctly written
as HOFBrINCl, is a name made up
using the chemical symbols of the
diatomic elements. It might help you
remember this list of elements.
T RY THIS
Table 1 Common Diatomic Elements
Name of element
State at room
temperature
Chemical symbol
Formula of molecule
hydrogen
H
H2
gas
oxygen
O
O2
gas
fluorine
F
F2
gas
bromine
Br
Br2
liquid
iodine
I
I2
solid
nitrogen
N
N2
gas
chlorine
Cl
Cl2
gas
MOLECULAR MODELS
SKILLS: Observing, Communicating
3. Connect one black and four white spheres. Sketch your
model. C
Making models of molecules might help you understand how some
elements form compounds. Each sphere, representing an atom,
has a number of connection sites. This number represents the
number of bonds that the atom can make with another atom. Each
different colour represents a different element: white = hydrogen;
red = oxygen; green = a halogen (e.g., chlorine); black = carbon.
4. Build a model of a molecule with one oxygen atom and
two hydrogen atoms. Sketch your model. C
5. Build a molecule of hydrogen chloride. Sketch your
model. C
Equipment and Materials: molecular model kit
6. Build any other molecule using the molecular model kit.
Sketch your model. C
1. Select two white spheres and connect them together to
represent a hydrogen molecule. Sketch your model. C
2. Select two red spheres and connect them together to
represent a molecule of oxygen. Sketch your model.
A. Beside each sketch, write the chemical formula and, if
possible, the name of the molecule. K/U
C
Naming Molecular Compounds
Unfortunately, the naming of molecular compounds is not as
straightforward as the naming of ionic compounds. Many molecular
compounds have been known for centuries and have common names
that are still in use today (Figure 6). Some of these common names are
given in Table 2.
(a)
Table 2 Common Names of Some Molecular Compounds
(b)
Common name
(c)
Figure 6 Molecular models of (a) water,
(b) ammonia, and (c) nitric oxide
208
Chemical
formula
Use/Occurrence
water
H2O
the most commonly available molecular compound on Earth;
the “universal solvent”
ammonia
NH3
used in window cleaners and in the production of fertilizers
nitric oxide
NO
an air pollutant produced in the automobile engine when
gasoline is burned
hydrogen sulfide
H2S
an invisible gas with a distinctive “rotten eggs” odour
Chapter 5 • Chemicals and Their Properties
NEL
Chemists have established a system for naming molecular compounds
that involves using prefixes to specify the number of atoms. The prefix
is attached to the name of the element to which it refers (Table 3). For
example, the name “dinitrogen pentoxide” tells us that there are two
nitrogen atoms (di means two) and five oxygen atoms (penta means five) in
the compound. The chemical formula for this compound, therefore, is N2O5.
The prefix mono is used only for the second element in the compound, so
CO2 is carbon dioxide. Similarly, the name “carbon monoxide” states that
there is one carbon and one oxygen atom in the molecule: CO.
LEARNING TIP
Using Prefixes
Note that mono is used only for the
second element in the compound.
Further, the second “o” in mono is
dropped when used with oxide to
become “monoxide” rather than
“monooxide.”
Table 3 Prefixes Used for Molecular Compounds
Prefix
Number of atoms
Sample molecular compound
mon(o)-
1
carbon monoxide, CO
di-
2
carbon dioxide, CO2
tri-
3
sulfur trioxide, SO3
tetra-
4
carbon tetrachloride, CCl4
penta-
5
phosphorus pentafluoride, PF5
When you are asked to write the name of a compound, first check the
formula to see if it includes a metal. If the first element is a metal, the
substance is an ionic compound and should be named accordingly (with
no prefixes). If the compound consists only of non-metals, it is a molecular
compound, and you should follow these steps to name it.
SAMPLE PROBLEM 1
WRITING TIP
Naming Molecular Compounds
Name the molecular compound with chemical formula PCl3.
Step 1 Write the names of both elements in the same order as in the formula. Replace
the ending of the second element with “ide.”
phosphorus
chlorine ide
Concluding Your Summary
Write a closing sentence that connects
the main idea and key points. For
example, “The use of prefixes helps
students remember the number of
atoms in a molecular compound.”
Step 2 Add prefixes. Remember that the prefix “mono” is never used for the first
element.
phosphorus tri chloride
The compound with chemical formula PCl3 is called phosphorus trichloride.
sulfur dioxide
Practice
Name the compound with chemical formula N2O.
SO2
Writing Chemical Formulas of Molecular
Compounds
Given its name, writing the formula of a molecular compound is relatively
simple. The prefixes in the name become the subscripts in the formula. For
example, the molecular compound called sulfur dioxide has the chemical
formula SO2 (Figure 7).
NEL
Figure 7 In the chemical name, the
prefix specifying the number of atoms
comes before the element’s name. In
the chemical formula, however, the
number of atoms is specified by a
subscript after the element’s chemical
symbol.
5.10 Molecules and Covalent Bonding
209
Molecular Compounds from Fossil Fuels
Figure 8 Even something as ordinary
as a backpack could contain hundreds
of different molecular compounds, most
of them made from fossil fuels.
Natural Gas
Processing
Most compounds are molecular. Living things make a huge variety of different
molecular compounds. Another source of thousands of different molecules is
fossil fuels. Coal, oil, and natural gas are the most common fossil fuels. These
substances take millions of years to form from the partially decayed remains
of ancient plants and animals. Fossil fuels are called a non-renewable resource
because they are not formed as quickly as we are using them.
Fossil fuels have become very important to our way of life. When we burn
fossil fuels, the energy stored within them heats our homes, powers our
vehicles, and can be harnessed to generate electricity. Without the energy
released from fossil fuels, our lives would be very different. But there is
more to fossil fuels than just their energy. Compounds extracted from fossil
fuels are processed into petrochemicals. We use these compounds to make
important consumer products and industrial chemicals, including plastics,
pharmaceuticals, and synthetic fabrics (Figures 8 and 9).
Try to imagine life without the products from petrochemicals. Half of your
clothing is now gone! Personal products like toiletries, cosmetics, and their
containers no longer exist. The paint on the walls and the synthetic carpet on
the floor—gone. Do you need medication regularly? Too bad! Many drugs are
made using petrochemicals. And what about communication? Computers,
phones, and all portable electronic devices cannot function without their
plastic cases and the insulation covering their electrical parts.
fuels and other
products (helium,
natural gas)
gases
Petrochemical
Plant
methane
ethane
propane
butane
agriculture
fertilizers
pesticides
herbicides
detergents
synthetic fibres
inorganic
chemicals
sulfuric acid
ammonia
nitric acid
organic chemicals
other products
Oil Refinery
gases and liquids
fuels
gasoline
kerosene
jet fuel
heating oils
lubricants and
other products
paints
varnishes
solvents
adhesives
explosives
plastics
rubber
greases
waxes
solvents
asphalt
sulfur
Figure 9 Crude oil and natural gas are the raw materials for a vast range of chemical products.
210
Chapter 5 • Chemicals and Their Properties
NEL
Spills and Leaks of Molecular Compounds
DID YOU KNOW?
Petrodollars
The world’s oil and natural gas deposits are concentrated in a few places,
Annual sales from the global
far from the places where they are sold and used. Most of Canada’s oil is
petrochemical industry are estimated
in the west and north of the country, while most Canadian consumers
to be over $1 trillion. With a global
live in the south and east. As a result, huge quantities of oil and natural
population of almost 7 billion, that is
gas are carried across North America and around the world. Oil is
about $150 for every person in the
world.
transported by rail, ship, or pipeline. Natural gas is
transported by pipeline or in specially designed
tankers. Accidents are inevitable. Loaded oil tankers
travel across oceans and through the Great Lakes.
Spills from oil tankers can have a devastating
effect on local ecosystems, contaminating water
and shorelines and killing birds and other aquatic
organisms (Figure 10). A lot of research is done
on the best way to deal with the leaked chemicals.
Accidental spills and poor waste disposal
methods leak toxins into the ground, where
they pollute groundwater. One such toxin is a
molecular compound called trichloroethene.
Trichloroethene is widely used in industry as a
degreaser for cleaning metal and glass. Spills of
this compound are a serious problem in Canada
because they can contaminate the groundwater.
Nearly 9 million Canadians rely on groundwater
for their drinking water. Canadian researchers are
working on innovative ways to clean groundwater.
Dr. Elizabeth Edwards, a professor of chemical
Figure 10 Spilled oil is a frequent environmental problem.
engineering at the University of Toronto, has
found that some pollutants are food for microbes.
University professors are often
Dr. Edwards has successfully used microbes to remove trichloroethene
involved in solving real-world
from soil. As the microbes “dine,” they convert the pollutant into ethene,
problems. To find out more about Dr.
Edwards’ research,
a relatively harmless gas. These microbes are now being used to remove
GO TO NELSON SCIENCE
trichloroethene from polluted sites around the world.
RESEARCH THIS
SLICKS FROM SHIPS
SKILLS: Researching, Identifying Alternatives, Analyzing the Issue, Defending a Decision, Evaluating
Accidental spills of oil can be devastating for the environment.
Environmental engineers develop strategies to clean up oil spills,
using their knowledge of oil’s chemical and physical properties.
GO TO NELSON SCIENCE
Recently, biological agents have also been successfully
used to clean up oil spills. Naturally occurring micro-organisms
such as algae and bacteria help break down the spilled oil. This
process is very slow but can be sped up by adding fertilizer to
the contaminated area.
NEL
SKILLS HANDBOOK
4.A., 4.C.
1. Research the role of chemical methods, physical methods,
and biological agents in cleaning an oil spill.
GO TO NELSON SCIENCE
A. Analyze the advantages and disadvantages of each
clean-up strategy. Which strategies are the most
economically viable? Which strategies are the most
environmentally friendly? T/I A
B. Which method can be used to clean up the majority of
a spill on calm water? T/I
5.10 Molecules and Covalent Bonding
211
IN
SUMMARY
• Molecular compounds both occur naturally and
are produced synthetically. Some are beneficial;
others are not.
• Molecular compounds are made up of molecules.
A molecule is a group of two or more atoms
joined by covalent bonds.
• A covalent bond forms when two non-metallic
atoms share electrons.
• A variety of strategies are used to reduce the
environmental damage resulting from spilled
chemicals.
CHECK
• Fossil fuels provide valuable energy and
petrochemicals. Many important industrial
chemicals and consumer products are made
from petrochemicals.
YOUR LEARNING
1. (a) Name these compounds: NI3, CCl4, OF 2, P2O5, and N2O3.
(b) Describe how each compound name indicates the ratio of
elements. K/U
2. Write the chemical formula for each of the following
molecular compounds: K/U
(a) carbon monoxide
(d) nitrogen tribromide
(b) sulfur tetrafluoride
(e) carbon disulfide
(c) dinitrogen tetroxide
3. For each of the following compounds, classify the elements
as metal or non-metal, classify the compound as ionic or
molecular, and name the compound. K/U
(a) SO2
(e) KClO3
(b) PbO2
(f) SnO2
(c) AlCl3
(g) FePO4
(d) N2O
(h) N2O4
4. (a) How many electrons do atoms of hydrogen and oxygen
have in their outer orbits?
(b) How many electrons will these elements gain before they
become stable?
(c) Sketch a diagram to show how hydrogen and oxygen
could bond to form a stable molecule. K/U
5. Explain, with diagrams, why the term “molecule” is
appropriate for hydrogen chloride but not for
sodium chloride. K/U
6. Contrast the way in which the elements in ionic and
molecular compounds achieve stability. K/U
212
• The name of a molecular compound includes
prefixes to indicate how many atoms of each
element are present (for example, dinitrogen
pentoxide). If there is only one atom of the first
element, the prefix “mono” is omitted. The name
of the last element ends in “ide.”
Chapter 5 • Chemicals and Their Properties
Ontario Science 10 SB
7. (a) Why are fossil fuels a non-renewable resource?
(b) What are the two main benefits that we get from fossil
fuels?
(c) What are two disadvantages of our dependence on fossil
fuels? K/U A
8. Explain why chlorine occurs as diatomic molecules in nature,
rather than individual atoms. K/U
9. Hydrogen peroxide, H2O2, is a molecular compound used
to disinfect cuts (Figure 11). Why is the formula of this
compound not written as HO? A
Figure 11
10. How can molecular compounds be distinguished from ionic
compounds
(a) by looking at their chemical formulas?
(b) by testing them in the lab?
K/U
11. What effect could C05-F48-UBOS10SB.ai
a disruption in the supply of oil have on the
cost of goods you purchase? Why? A
NEL