+
H
H
H H
H
Hydrogen atom
H
Hydrogen atom
H
Hydrogen atom
H
Hydrogen atom
H 2
Hydrogen molecule
T
I
M
E
present. The strong attractive force that
holds two hydrogen atoms together results
from a covalent bond, a chemical bond
formed by the sharing of a pair of electrons
between atoms.
Imagine two hydrogen atoms (each
with one proton and one electron)
approaching one another so that their
electron clouds overlap (FIGURE 2.8). Once
they meet, the electron configuration will
change so that both electrons will primarily
occupy the space between the atoms. In
other words, the two electrons are shared
by both hydrogen atoms and attracted
simultaneously by the positive charge of the
proton in the nucleus of each atom. The
attraction between the electrons and both
nuclei holds these atoms together.
Although ions do not exist in hydrogen
molecules, the force that holds these atoms
together arises from the attraction of oppositely charged particles—protons in the
nuclei and electrons shared by the atoms.
Metallic Bonds:
Electrons Free to Move
In metallic bonds, the valence electrons are
free to move from one atom to another so
that all atoms share the available valence
electrons. This type of bonding is
found in metals such as copper, gold, aluminum, and
silver, and in alloys such as
brass and bronze. Metallic
bonding accounts for the high
electrical conductivity of metals, the ease with which metals
are shaped, and numerous other
special properties.
43
Why Atoms Bond
D I D Y O U K N O W ?
The names of precious gems often
differ from the names of parent
minerals. For example, sapphire is one
of two gems that are varieties of the
same mineral, corundum. Tiny amounts
of the elements titanium and iron in
corundum produce the most prized
blue sapphires. When the mineral
corundum contains chromium, it
exhibits a brilliant red color and the
gem is called ruby.
After electron transfer takes place, the
atoms are no longer electrically neutral. By
giving up one electron, a neutral sodium
atom becomes positively charged (with
11 protons and 10 electrons). Similarly, by
acquiring one electron, a neutral chlorine
atom becomes negatively charged (with
17 protons and 18 electrons). We know
that ions with like charges repel, and those
with unlike charges attract. Thus, an ionic
bond is the attraction of oppositely charged
ions to one another, producing an electrically neutral compound.
FIGURE 2.7B illustrates the arrangement
of sodium and chlorine ions in ordinary
table salt. Notice that salt consists of alternating sodium and chlorine ions, positioned in such a manner that each positive
ion is attracted to and surrounded on all
sides by negative ions, and vice versa. This
arrangement maximizes the attraction
between ions with opposite charges while
minimizing the repulsion between ions
with identical charges. Thus, ionic compounds consist of an orderly arrangement
of oppositely charged ions assembled in a
definite ratio that provides overall electrical
neutrality.
The properties of a chemical compound are dramatically different from the
properties of the various elements comprising it. For example, sodium is a soft silvery
metal that is extremely reactive and poisonous. If you were to consume even a small
amount of elemental sodium, you would
need immediate medical attention.
Chlorine, a green poisonous gas, is so toxic
that it was used as a chemical weapon
during World War I. Together, however,
these elements produce sodium chloride, a
harmless flavor enhancer that we call table
salt. Thus, when elements combine to form
compounds their properties change
significantly.
Covalent Bonds:
Electrons Shared
Sometimes the forces that hold atoms
together cannot be understood on the basis
of the attraction of oppositely charged ions.
One example is the hydrogen molecule
(H 2 ), in which the two hydrogen atoms
are held together tightly and no ions are
C O N C E P T C H E C K 2 . 3
What is the difference between an atom
and an ion?
What occurs in an atom to produce a
positive ion? What occurs in an atom to
produce a negative ion?
Briefly distinguish between ionic and
covalent bonding and the role that
electrons play in both.
3
2
1
FIGURE 2.8 Formation of a covalent bond
between two hydrogen atoms (H) to form a
hydrogen molecule (H 2 ). When hydrogen
atoms bond, the electrons are shared by both
hydrogen atoms and attracted simultaneously
by the positive charge of the proton in the
nucleus of each atom. The attraction between
electrons and both nuclei holds (bonds) these atoms
together.
H
H
H H
H
Hydrogen atom
H
Hydrogen atom
H
Hydrogen atom
H
Hydrogen atom
H 2
Hydrogen molecule
T
I
M
E
present. The strong attractive force that
holds two hydrogen atoms together results
from a covalent bond, a chemical bond
formed by the sharing of a pair of electrons
between atoms.
Imagine two hydrogen atoms (each
with one proton and one electron)
approaching one another so that their
electron clouds overlap (FIGURE 2.8). Once
they meet, the electron configuration will
change so that both electrons will primarily
occupy the space between the atoms. In
other words, the two electrons are shared
by both hydrogen atoms and attracted
simultaneously by the positive charge of the
proton in the nucleus of each atom. The
attraction between the electrons and both
nuclei holds these atoms together.
Although ions do not exist in hydrogen
molecules, the force that holds these atoms
together arises from the attraction of oppositely charged particles—protons in the
nuclei and electrons shared by the atoms.
Metallic Bonds:
Electrons Free to Move
In metallic bonds, the valence electrons are
free to move from one atom to another so
that all atoms share the available valence
electrons. This type of bonding is
found in metals such as copper, gold, aluminum, and
silver, and in alloys such as
brass and bronze. Metallic
bonding accounts for the high
electrical conductivity of metals, the ease with which metals
are shaped, and numerous other
special properties.
43
Why Atoms Bond
D I D Y O U K N O W ?
The names of precious gems often
differ from the names of parent
minerals. For example, sapphire is one
of two gems that are varieties of the
same mineral, corundum. Tiny amounts
of the elements titanium and iron in
corundum produce the most prized
blue sapphires. When the mineral
corundum contains chromium, it
exhibits a brilliant red color and the
gem is called ruby.
After electron transfer takes place, the
atoms are no longer electrically neutral. By
giving up one electron, a neutral sodium
atom becomes positively charged (with
11 protons and 10 electrons). Similarly, by
acquiring one electron, a neutral chlorine
atom becomes negatively charged (with
17 protons and 18 electrons). We know
that ions with like charges repel, and those
with unlike charges attract. Thus, an ionic
bond is the attraction of oppositely charged
ions to one another, producing an electrically neutral compound.
FIGURE 2.7B illustrates the arrangement
of sodium and chlorine ions in ordinary
table salt. Notice that salt consists of alternating sodium and chlorine ions, positioned in such a manner that each positive
ion is attracted to and surrounded on all
sides by negative ions, and vice versa. This
arrangement maximizes the attraction
between ions with opposite charges while
minimizing the repulsion between ions
with identical charges. Thus, ionic compounds consist of an orderly arrangement
of oppositely charged ions assembled in a
definite ratio that provides overall electrical
neutrality.
The properties of a chemical compound are dramatically different from the
properties of the various elements comprising it. For example, sodium is a soft silvery
metal that is extremely reactive and poisonous. If you were to consume even a small
amount of elemental sodium, you would
need immediate medical attention.
Chlorine, a green poisonous gas, is so toxic
that it was used as a chemical weapon
during World War I. Together, however,
these elements produce sodium chloride, a
harmless flavor enhancer that we call table
salt. Thus, when elements combine to form
compounds their properties change
significantly.
Covalent Bonds:
Electrons Shared
Sometimes the forces that hold atoms
together cannot be understood on the basis
of the attraction of oppositely charged ions.
One example is the hydrogen molecule
(H 2 ), in which the two hydrogen atoms
are held together tightly and no ions are
C O N C E P T C H E C K 2 . 3
What is the difference between an atom
and an ion?
What occurs in an atom to produce a
positive ion? What occurs in an atom to
produce a negative ion?
Briefly distinguish between ionic and
covalent bonding and the role that
electrons play in both.
3
2
1
FIGURE 2.8 Formation of a covalent bond
between two hydrogen atoms (H) to form a
hydrogen molecule (H 2 ). When hydrogen
atoms bond, the electrons are shared by both
hydrogen atoms and attracted simultaneously
by the positive charge of the proton in the
nucleus of each atom. The attraction between
electrons and both nuclei holds (bonds) these atoms
together.