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KEY CONCEPT
Chemical bonds hold
compounds together.
BEFORE, you learned
NOW, you will learn
• Elements combine to form
compounds
• Electrons are located in a cloud
around the nucleus
• Atoms can lose or gain
electrons to form ions
• How electrons are involved
in chemical bonding
• About the different types
of chemical bonds
• How chemical bonds affect
structure
VOCABULARY
THINK ABOUT
ionic bond p. 48
covalent bond p. 50
molecule p. 51
polar covalent bond p. 51
How do you keep things
together?
Think about the different ways the workers at
this construction site connect materials. They
may use nails, screws, or even glue, depending
on the materials they wish to keep together.
Why would they choose the method they do?
What factors do you consider when you join
two objects?
Chemical bonds between
atoms involve electrons.
MAIN IDEA AND DETAILS
Make a two-column
chart to organize
information on chemical
bonds.
Water is a compound of hydrogen and oxygen. The air you breathe,
however, contains oxygen gas, a small amount of hydrogen gas, as well
as some water vapor. How can hydrogen and oxygen be water sometimes and at other times not? The answer is by forming chemical bonds.
Chemical bonds are the “glue” that holds the atoms of elements
together in compounds. Chemical bonds are what make compounds
more than just mixtures of atoms.
Remember that an atom has a positively charged nucleus surrounded
by a cloud of electrons. Chemical bonds form when the electrons in
the electron clouds around two atoms interact. How the electron
clouds interact determines the kind of chemical bond that is formed.
Chemical bonds have a great effect on the chemical and physical properties of compounds. Chemical bonds also influence how different
substances interact. You’ll learn more about how substances interact in
a later chapter.
Chapter 2: Chemical Bonds and Compounds 47
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Atoms can transfer electrons.
Ions are formed when atoms gain or lose electrons. Gaining electrons
changes an atom into a negative ion. Losing electrons changes an atom
into a positive ion. Individual atoms do not form ions by themselves.
Instead, ions typically form in pairs when one atom transfers one or
more electrons to another atom.
Reminder
Remember that elements
in columns show similar
chemical properties.
1
2
Li+
Be2+
Na+ Mg2+
+
K
Ca
2+
An element’s location on the periodic table can give a clue as to
the type of ions the atoms of that element will form. The illustration
to the left shows the characteristic ions formed by several groups.
Notice that all metals lose electrons to form positive ions. Group 1
metals commonly lose only one electron to form ions with a single
positive charge. Group 2 metals commonly lose two
electrons to form ions with two positive charges. Other
16
17
metals, like the transition metals, also always form positive
O2–
F–
ions, but the number of electrons they may lose varies.
S2–
Cl –
Se2–
Br –
Rb+ Sr2+
I–
Cs+ Ba2+
Fr+
Nonmetals form ions by gaining electrons. Group 17
nonmetals, for example, gain one electron to form ions
with a 1– charge. The nonmetals in Group 16 gain two
electrons to form ions with a 2– charge. The noble gases do
not normally gain or lose electrons and so do not normally
form ions.
Ra2+
check your reading
What type of ions do metals form?
Ionic Bonds
What happens when an atom of an element from Group 1, like sodium,
meets an atom of an element from Group 17, like chlorine? Sodium is
likely to lose an electron to form a positive ion. Chlorine is likely to
gain an electron to form a negative ion. An electron, therefore, moves
from the sodium atom to the chlorine atom.
sodium atom (Na)
chlorine atom (Cl)
sodium ion (Na+) chloride ion (Cl–)
Remember that particles with opposite electrical charges attract
one another. When the ions are created, therefore, they are drawn
toward one another by electrical attraction. This force of attraction
between positive and negative ions is called an ionic bond.
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48 Unit: Chemical Interactions
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Electrical forces act in all directions. Each ion, therefore, attracts
all other nearby ions with the opposite charge. The next illustration
shows how this all-around attraction produces a network of sodium
and chloride ions known as a sodium chloride crystal.
+
–
+ –
–
+
–
+
–
+
–
+
–
+
–
+
–
+
+
–
+
+ –
– +
–
+
–
+
–
Notice how each positive ion is
surrounded by six negative ions, and
each negative ion is surrounded by six
positive ions. This regular arrangement gives the sodium chloride
crystal its characteristic cubic
shape. You can see this distinctive crystal shape when you look
at table salt crystals through a
magnifying glass.
Ionic bonds form between
all nearby ions of opposite
charge. These interactions make ionic compounds very stable and their crystals very strong. Although sodium chloride
crystals have a cubic shape, other ionic compounds form crystals
with different regular patterns. The shape of the crystals of an ionic
compound depends, in part, on the ratio of positive and negative ions
and the sizes of the ions.
Names of Ionic Compounds
The name of an ionic compound is based on the names of the
ions it is made of. The name for a positive ion is the same as the name
of the atom from which it is formed. The name of a negative ion is
formed by dropping the last part of the name of the atom and adding
the suffix -ide. To name an ionic compound, the name of the positive
ion is placed first, followed by the name of the negative ion. For example, the chemical name for table salt is sodium chloride. Sodium is the
positive sodium ion and chloride is the negative ion formed from
chlorine.
The cubic shape of
sodium chloride crystals
is a result of how the ions
form crystals.
Therefore, to name the compound with the chemical formula BaI2
•
•
•
First, take the name of the positive metal element: barium.
Second, take the name of the negative, nonmetal element, iodine,
and give it the ending -ide: iodide.
Third, combine the two names: barium iodide.
Similarly, the name for KBr is potassium bromide, and the name
for MgF2 is magnesium fluoride.
Chapter 2: Chemical Bonds and Compounds 49
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Atoms can share electrons.
In general, an ionic bond forms between atoms that lose electrons easily
to form positive ions, such as metals, and atoms that gain electrons
easily to form negative ions, such as nonmetals. Another way in which
atoms can bond together is by sharing electrons. Nonmetal atoms
usually form bonds with each other by sharing electrons.
VOCABULARY
Make a description wheel
for covalent bond and
other vocabulary words.
Covalent Bonds
A pair of shared electrons between two atoms is called a covalent bond.
In forming a covalent bond, neither atom gains or loses an electron, so
no ions are formed. The shared electrons are attracted to both positively
charged nuclei. The illustrations below show a covalent bond between
two iodine atoms. In the first illustration, notice how the electron
clouds overlap. A covalent bond is also often represented as a line
between the two atoms, as in the second illustration.
Iodine (I2)
I
electron cloud model
Reading Tip
To help yourself remember
that a covalent bond
involves a sharing of electrons, remember that the
prefix co- means “partner.”
I
ball-and-stick model
The number of covalent bonds that an atom can form depends on
the number of electrons that it has available for sharing. For example,
atoms of the halogen group and hydrogen can contribute only one
electron to a covalent bond. These atoms, therefore, can form only
one covalent bond. Atoms of group 16 elements can form two covalent bonds. Atoms of the elements of Group 15 can form three bonds.
Carbon and silicon in Group 14 can form four bonds. For example, in
methane (CH4), carbon forms four covalent bonds with four hydrogen atoms, as shown below.
Methane (CH4)
H
C
H
H
H
ball-and-stick model
space-filling model
We don’t always show the lines representing the covalent bonds
between the atoms. The space-filling model still shows the general
shape of the bonded atoms, but occupies far less space on the page.
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Each carbon-hydrogen bond in methane is a single bond because
one pair of electrons is shared between the atoms. Sometimes atoms
may share more than one pair of electrons with another atom. For
example, the carbon atom in carbon dioxide (CO2) forms double
bonds with each of the oxygen atoms. A double bond consists of four
(two pairs of) shared electrons. Two nitrogen atoms form a triple
bond, meaning that they share six (three pairs of) electrons.
Carbon Dioxide (CO2)
O
C
O
Nitrogen (N2)
N
reading tip
Remember that each line in
the model stands for a
covalent bond—one shared
pair of electrons.
N
A group of atoms held together by covalent bonds is called a
molecule. A molecule can contain from two to many thousand
atoms. Most molecules contain the atoms of two or more elements.
For example, water (H2O), ammonia (NH3), and methane (CH4)
are all compounds made up of molecules. However, some molecules
contain atoms of only one element. The following elements exist as
two-atom molecules: H2, N2, O2, F2, Cl2, Br2, and I2.
check your reading
What is a molecule?
Polar Covalent Bonds
In an iodine molecule, both atoms are exactly the same. The shared
electrons therefore are attracted equally to both nuclei. If the two
atoms involved in a covalent bond are very different, however, the
electrons have a stronger attraction to one nucleus than to the other
and spend more time near that nucleus. A covalent bond in which the
electrons are shared unequally is called a polar covalent bond. The
word polar refers to anything that has two extremes, like a magnet
with its two opposite poles.
Reading Tip
To remind yourself that
polar covalent bonds have
opposite partial charges,
remember that Earth has
both a North Pole and
a South Pole.
Water (H2O)
O
H
H
ball-and-stick model
space-filling model
In a water molecule (H2O), the oxygen atom attracts electrons
far more strongly than the hydrogen atoms do. The oxygen nucleus
has eight protons, and the hydrogen nucleus has only one proton.
The oxygen atom pulls the shared electrons more strongly toward it.
In a water molecule, therefore, the oxygen side has a slightly negative
charge, and the hydrogen side has a slightly positive charge.
VISUALIZATION
CLASSZONE.COM
Examine how electrons
move in a polar covalent
molecule.
Chapter 2: Chemical Bonds and Compounds 51
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Comparing Bonds
In Salar de Uyuni, Bolivia, salt is mined in
great quantities from salt water. The salt is
harvested as the water evaporates into the
air, leaving the salt behind. All types of
chemical bonds are involved.
air
salt
–
+
– +
+
–
– +
+
–
+
–
+
–
+
– –+
+
–
+
–
– +
– +
+
–
–
+
water
Covalent Bonds (air)
Polar Covalent Bonds (water)
Sodium Chloride (NaCl)
Nitrogen (N2) and Oxygen (O2)
Water (H2O)
A complete transfer of electrons
produces the ionic bonds that
hold sodium chloride (table salt)
crystals together.
Some molecules in air contain
multiple covalent bonds. Nitrogen
has triple bonds. Oxygen has
double bonds.
The covalent bonds in water are
very polar because oxygen attracts
electrons far more strongly than
hydrogen does.
Ionic Bonds (salt)
–
N
+
N
O
O
Atoms of which element are shown both in
the air and in the water?
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52 Unit: Chemical Interactions
O
H
H
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Chemical bonds give all materials
their structures.
The substances around you have many different properties. The
structure of the crystals and molecules that make up these substances
are responsible for many of these properties. For example, crystals
bend rays of light, metals shine, and medications attack certain diseases in the body because their atoms are arranged in specific ways.
Ionic Compounds
Most ionic compounds have a regular crystal structure. Remember
how the size, shape, and ratio of the sodium ions and chloride ions
give the sodium chloride crystal its shape. Other ionic compounds,
such as calcium chloride, have different but equally regular structures
that depend upon the ratio and sizes of the ions. One consequence
of such rigid structures is that, when enough force is applied to the
crystal, it shatters rather than bends.
Crystals
SKILL FOCUS
How does a crystal grow?
Observing
PROCEDURE
1
Add a small amount of the crystal-growing substance to a beaker of hot tap
water. Stir until it mixes completely with the water. Keep adding the substance
and stirring until no more will dissolve.
2 Pour the mixture into another beaker.
3 Tie one end of the string to the paper clip and the other end to a pencil.
Lower the paper clip into the solution and lay the pencil across the top of
the beaker. The paper clip should hang at about the middle of the beaker.
4 Use a hand lens to observe the paper clip several times a week for three weeks.
WHAT DO YOU THINK?
MATERIALS
• crystal-growing
substance
• 2 glass beakers
• hot tap water
• stirring stick
• cotton string
• paper clip
• pencil
• hand lens
TIME
30 minutes
• Describe the crystals you see forming on the paper clip. Do the
crystals look different as they get larger?
• Compare your crystals to those of other groups. What
similarities do you see among them? What differences?
CHALLENGE Try growing larger crystals by selecting one
of the crystals from your paper clip, tying it to a piece of
string, and sinking it into a solution of the same crystal-growing substance.
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Covalent Compounds
Unlike ionic compounds, covalent compounds exist as individual
molecules. Chemical bonds give each molecule a specific, threedimensional shape called its molecular structure. Molecular structure
can influence everything from how a specific substance feels to the
touch to how well it interacts with other substances.
A few basic molecular structures are shown below. Molecules
can have a simple linear shape, like iodine (I2), or they can be bent,
like a water molecule (H2O). The atoms in an ammonia molecule
(NH3) form a pyramid, and methane (CH4) molecules even have a
slightly more complex shape. The shape of a molecule depends on
the atoms it contains and the bonds holding it together.
reading tip
To help yourself appreciate
the differences among these
structures, try making threedimensional models of them.
H
I
H
H
iodine
(I2)
N
O
I
water
(H2O)
H
H
C
H
H
ammonia
(NH3)
H
H
methane
(CH4)
Molecular shape can affect many properties of compounds. For
example, there is some evidence to indicate that we detect scents
because molecules with certain shapes fit into certain smell receptors
in the nose. Molecules with similar shapes, therefore, should have similar smells. Molecular structure also plays an essential role in how our
bodies respond to certain drugs. Some drugs work because molecules
with certain shapes can fit into specific receptors in body cells.
KEY CONCEPTS
CRITICAL THINKING
1. What part of an atom is
involved in chemical bonding?
4. Analyze Would you expect
the bonds in ammonia to
be polar covalent? Why or
why not?
2. How are ionic bonds and
covalent bonds different?
3. Describe two ways that crystal
and molecular structures affect
the properties of ionic and
covalent compounds.
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54 Unit: Chemical Interactions
5. Infer What kind of bond
would you expect atoms of
strontium and iodine to form?
Why? Write the formula and
name the compound.
CHALLENGE
6. Conclude Is the element
silicon likely to form ionic or
covalent bonds? Explain.