Ionic and Metallic Bonds
Have you ever noticed that some
solid substances can be easily crushed while others bend under pressure? You
can crush brittle solids such as salt into a fine powder, but applying pressure
to a piece of copper wire would only bend it. The reasons for these differences
lie in the types of bonds that hold atoms together in the two solids.
Ions and Ionic Bonds
Atoms have no charge because they
contain equal numbers of protons and electrons.
If an atom loses or gains
electrons, it becomes an ion carrying an electrical charge. An ion has a
positive charge if it has lost one or more electrons. Positively charged ions
are called cations. An ion has a negative charge if it has gained
one or more electrons. Negatively charged ions are called anions.
The magnitude of the charge on an ion depends on how many electrons have been
lost or gained by the atom.
For example, suppose that an atom
with 19 protons and 19 electrons loses one electron. The atom becomes an ion
with 19 protons and 18 electrons. As the number of protons exceeds the number
of electrons by one, the charge on the ion is (+19) + (−18) = +1.
Now consider an example of an
atom that gains two electrons. Suppose the neutral atom has 8 protons and 8
electrons. This atom gains two electrons to have a total of 10 electrons and a
charge of (+8) + (−10) = −2.
An ion is represented using the
chemical symbol for its element followed by a plus or minus sign, expressed as
a superscript, to indicate the charge on the ion. If the charge is +1 or −1, no
number is shown. If the magnitude of the charge is greater than 1, the number
is shown to the left of the plus or minus sign. Some examples are Na+, Cl−, Ba2+, and N3−.
Ions of opposite charge attract
one another. An ionic bond forms from the strong attraction
between a cation and an anion. An ionic compound contains ionic bonds.
Figure 7 represents one unit of an ionic compound formed from a positively
charged ion (sodium ion) and a negatively charged ion (chloride). The sodium
ion is a result of a sodium atom losing one electron. A chloride ion is a
result of a chlorine atom gaining one electron. The ionic compound that results
from the combination between a sodium ion and a chloride ion is called sodium
chloride.
Fill in the blank.
A neutral atom loses two electrons. What is the
charge of the ion thus formed?
The charge is
A sulfur atom gains two electrons. How is the resulting sulfur
ion represented?
S+
S2−
S
S3−
Fill in the blank.
Two chloride ions do not form an ionic compound
because the two ions have the
charge.
Elements and Ion Formation
From your study of the periodic
table, you learned that only the outermost electrons—the valence electrons—in
an atom are gained or lost. You also learned that the noble gases have the most
stable electron configuration, with either 2 or 8 electrons in their valence
shell. With these ideas in mind, you can predict how different elements will
ionize. Elements form ions in order to acquire the stable electron
configuration of the nearest noble gas.
For example, sodium is a group 1
metal that has one valence electron. To achieve a noble gas configuration, a
sodium atom could either gain 7 electrons to acquire the electron configuration
of argon, or it could lose 1 electron to acquire the electron configuration of
neon. It is far easier for the sodium atom to lose 1 electron than to gain 7
electrons, so it loses its single valence electron. The diagram below
illustrates how a neutral sodium atom achieves this stable electron
configuration when it loses its valence electron.
Figure 8 A
sodium atom loses its one valence electron to become positively charged
and acquire the same electron configuration as that of neon.
and acquire the same electron configuration as that of neon.
All metals in group 1 have only
one valence electron, so they tend to lose it to achieve the configuration of
the nearest noble gas. Group 1 elements form cations with a +1 charge. Metals
in group 2 have two valence electrons and tend to lose them both to achieve the
configuration of the nearest noble gas. Group 2 elements form cations with a +2
charge. In each case, the ions are left with a full valence shell of electrons,
which is more stable than having one or two electrons in the outermost shell.
Figure 9 A
neutral chlorine atom achieves a noble gas configuration when it gains one
electron.
In a similar pattern, nonmetals
in group 16 have six valence electrons and tend to gain two electrons to form
anions with a −2 charge.
Transition metals are more
variable in cation formation than other metals. Many transition metals can
exist in more than one cation form, so it is not as useful to use the periodic
table to predict the charges of these ions as it is for other groups. For
instance, silver and gold are examples of transition metals. Although
they both belong to group 11, gold atoms lose either one or three electrons to
form Au+ or Au3+ ions, while silver atoms
lose only one electron to form Ag+ ions.
Figure
10 The periodic table can be used to predict the charges of most ions
of elements.
The symbol of calcium ion is
Ca−
Ca2+
Ca+
Ca2−
Choose the charge of the ion formed from each of the following
elements.
K
Mg
F
O
N
Elements and Ion Formation - Cont.
In addition to the ions mentioned
earlier, there are ions called polyatomic ions. A polyatomic ion is
an ion composed of two or more atoms that are chemically bonded together. The
charge on a polyatomic ion indicates the net charge on the entire ionic
particle.
An example of a polyatomic ion is
the hydroxide ion, OH−. The hydroxide ion consists of one atom of oxygen (having 8
protons and 8 electrons) and one atom of hydrogen (having 1 proton and 1
electron) bonded together as shown in Figure 11. When these two atoms combine
to form the hydroxide ion, they gain one additional electron. Thus, the ion has
9 positive charges and 10 negative charges. The sum of these charges is 9 +
(−10) = −1, which means that the ion has a charge of −1. Notice that this
ion has a stable noble gas configuration just like the ions formed from
single atoms.
Another example of
a polyatomic ion is the ammonium ion, shown in Figure 12. A nitrogen
atom has 7 protons and 7 electrons, and each atom hydrogen
atom has 1 proton and 1 electron. Since four hydrogen atoms combine with one
nitrogen atom, the ion has 7 + 4 = 11 protons. As ammonium ion forms, one
electron is lost. Thus, there are 11 positive charges and 10 negative charges
in the final form of the ion. Therefore, the charge on this ion is 11 + (−10) =
+1.
2.2 Ionic and Metallic Bonds
Elements and Ion Formation - Cont.
In addition to the ions mentioned
earlier, there are ions called polyatomic ions. A polyatomic ion is
an ion composed of two or more atoms that are chemically bonded together. The
charge on a polyatomic ion indicates the net charge on the entire ionic
particle.
An example of a polyatomic ion is
the hydroxide ion, OH−. The hydroxide ion consists of one atom of oxygen (having 8
protons and 8 electrons) and one atom of hydrogen (having 1 proton and 1
electron) bonded together as shown in Figure 11. When these two atoms combine
to form the hydroxide ion, they gain one additional electron. Thus, the ion has
9 positive charges and 10 negative charges. The sum of these charges is 9 +
(−10) = −1, which means that the ion has a charge of −1. Notice that this
ion has a stable noble gas configuration just like the ions formed from
single atoms.
Another example of
a polyatomic ion is the ammonium ion, shown in Figure 12. A nitrogen
atom has 7 protons and 7 electrons, and each atom hydrogen
atom has 1 proton and 1 electron. Since four hydrogen atoms combine with one
nitrogen atom, the ion has 7 + 4 = 11 protons. As ammonium ion forms, one electron
is lost. Thus, there are 11 positive charges and 10 negative charges in the
final form of the ion. Therefore, the charge on this ion is 11 + (−10) = +1.
Fill in the blank.
Both
ammonium ions and hydroxide ions are ________________.
Match.
It forms when an atom either loses or gains electrons.
If an atom loses one or more electrons it forms a positively
charged ion called
If an atom gains one or more electrons it forms a negatively
charged ion called
Match.
Your answer :
It forms when an atom either loses or gains
electrons.
= ion
If an atom loses one or more electrons it forms a
positively charged ion called
= ion
If an atom gains one or more electrons it forms a
negatively charged ion called
= ion
|
Fill in the blank.
The magnitude of the charge depends on how many
have been exchanged by the atom.
Ionic Bonds Link Metals and Nonmetals
Metals tend to form cations and
nonmetals tend to form anions; consequently, ionic bonds typically occur
between a metal and a nonmetal. In the ionic compound sodium chloride, a group
1 metal cation (Na+) bonds to a group 17 nonmetal anion (Cl−). When
anions and cations combine to form an ionic compound, the net charge of the
compound must be zero. Since Na+ and
Cl− ions have charges of the same magnitude, it takes one of
each for the charges to cancel and produce a neutral compound.
Ionic compounds can also form
between ions having different numerical charges. For example, magnesium is a
group 2 metal that forms a cation with +2 charge. This cation can bond with
chloride anions having a −1 charge. The sum of the positive charges must always
be equal to the sum of the negative charges. Thus, one magnesium ion bonds with
two chloride ions to form a neutral ionic compound [(+2) + [2 × (−1)] = 0].
Identify the pairs of elements that can combine to form ionic
compounds.
sodium and bromine
calcium and sodium
calcium and bromine
sodium and sulfur
calcium and sulfur
Identify the pairs of elements that can combine to form ionic
compounds.
calcium and sulfur
calcium and sodium
sodium and sulfur
calcium and bromine
sodium and bromine
Identify the pairs of elements that can combine to form ionic
compounds.
calcium and sulfur
calcium and sodium
sodium and sulfur
calcium and bromine
sodium and bromine
Lattice Structures in Ionic Compounds
Solid ionic compounds are substances that we use in everyday life. For example, table salt consists of the ionic compound sodium chloride. One crystal of table salt contains millions of sodium chloride units. If you were able to magnify the crystal enough to see the individual ions, you would see them arranged in a three-dimensional structure similar to the one shown in Figure 13. This structure is called a lattice. A lattice is the way ions are arranged in an ionic compound.
In the lattice, each ion is
surrounded by several ions of opposite charge. In the sodium chloride case,
each sodium ion is surrounded by six chloride ions and each chloride ion is
surrounded by six sodium ions.The resulting lattice is a regularly repeating
pattern of positive and negative ions. This structure is extremely stable
because it allows the maximum number of bonds between ions. Because of the very
close spacing of the oppositely charged ions in this arrangement, the ions are
bonded very tightly. This tight bonding of a large number of ion pairs requires
a great deal of energy to disrupt, which accounts for the high melting point
sodium chloride has.
Notice that the image of the NaCl
lattice is designed to show the arrangements of bonds and ions clearly. If we
could see an actual crystal, the electron clouds of the ions would touch each
other, leaving no empty space in between. This would be more realistic but less
informative.
Each ionic compound forms a
specific crystal lattice structure. There are 14 different lattice structures,
divided into 7 different systems. Figure 14 shows the three lattice structures
in the cubic system. Sodium chloride ions are arranged according to the first
crystal lattice (a).
A crystal is a
solid in which the ions are arranged in a lattice. We see evidence of this
lattice structure in the appearance of the crystals. Crystals have smooth,
regular faces that are often shiny. The crystals themselves often have distinct
repeating shapes. These features arise from the regular arrangement of ions in
a lattice.
Salts form crystals in the solid
state. In this state, all ionic compounds do not conduct electricity. This is
because ions are arranged in fixed places in the lattice. Salts lose their
crystal structure, and therefore their lattice structure, when they change from
the solid state to the liquid or gaseous state. They also lose their crystal
structure when the solid salt is dissolved in water. During dissolution, the
water breaks the lattice into individual anions and cations. Thus, when salts
are melted or dissolved in water, anions and cations move freely through the
liquid, which allows the electric current to flow.
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Metallic Bonding
Unlike ionic compounds, which
contain two or more elements, pure metals contain only one element, with the
result that all atoms in a metal are identical.
Most metals are solid at room
temperature. Metal atoms are tightly packed together in the solid state. This
allows them to share valence electrons with one another. If you could observe a
piece of a solid metal at the atomic level, you would see the metal cations
fixed in a rigid lattice structure like that shown in the figure below. You
would also observe a sea of electrons—the valence electrons—moving freely in
between the metal cations.
You have seen how ions in ionic
compounds are bonded together by ionic bonds. In pure metals, another type of
bonding forms as a result of the attractive forces between the metal cations
and the sea of electrons. This type of bonding is called metallic
bonding. Metallic bonding can be described as a lattice of
positive metal ions in a sea of electrons.
Note that metallic bonding occurs
throughout a large group of atoms, and not as individual bonds between pairs of
atoms. That is why this type of bonding is often described as a regular
repeating array of metal cations in a sea of electrons.
Figure 16 At
the atomic scale, a metal is arranged as a lattice of cations
surrounded by a sea of electrons.
surrounded by a sea of electrons.
This type of bonding explains
some of the unique properties of metals. While crystals of ionic compounds
would break and crumble if they were pounded with a hammer, metals only flatten
and bend under pressure. When a metal is pounded, its cations slide past one
another as the sea of electrons flows freely in between, holding the metal
together.
Metallic bonding also allows
metals to conduct electric current. Because the electrons flow freely in
between the metal cations, they can transmit electric current from one end of
the metal to the other. This is what happens in a metal wire used for
conducting electricity.
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