Chapter 6, Section 2
Key Concept: Chemical bonds hold compounds together.
BEFORE, you learned
• Elements combine to form
compounds
• Electrons are located in a cloud
around the nucleus
• Atoms can lose or gain
electrons to form ions
NOW, you will learn
• How electrons are involved
in chemical bonding
• About the different types
of chemical bonds
• How chemical bonds affect
structure
THINK ABOUT
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.
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.
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.
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
metals, like the transition metals, also always form positive
ions, but the number of electrons they may lose varies.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Covalent Compounds
Unlike ionic compounds, covalent compounds exist as individual
molecules. Chemical bonds give each molecule a specific, three-dimensional
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.
Iodine (I2)
water (H2O)
Ammonia (NH3)
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.
Questions for Chapter 6, Section 2
KEY CONCEPTS
1. What part of an atom is involved in chemical bonding?
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.
CRITICAL THINKING
4. Analyze Would you expect the bonds in ammonia to be polar
covalent? Why or why not?
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.