Page 1 of 4
Ions
Objectives: To describe the formation of ions from their parent atoms,
and learn to name them.
To predict which ion a given element forms by using the periodic
table.
W
e have seen that an atom has a certain number of protons in its nucleus and an equal number of electrons in the space around the nucleus. This results in an exact balance of positive and negative charges. We
say that an atom is a neutral entity—it has zero net charge.
We can produce a charged entity, called an ion, by taking a neutral atom
and adding or removing one or more electrons. For example, a sodium atom
(Z 11) has eleven protons in its nucleus and eleven electrons outside its
nucleus.
If one of the electrons is lost, there will be eleven positive charges but
only ten negative charges. This gives an ion with a net positive one (1)
charge: (11) (10) 1. We can represent this process as follows:
In shorthand form,
Na → Na e
where Na represents the neutral sodium atom, Na represents the 1 ion
formed, and e represents an electron.
11 electrons
(11–)
10 electrons
(10–)
1 electron
lost
11+
11+
e–
Neutral sodium
atom (Na)
Sodium ion
(Na+)
Loses
1 electron
CHEMISTRY in ACTION
Elementary, My Dear Watson!
1. Make flashcards with the names of the elements in Groups 1 through
8 on one side and their symbols on the other side.
2. Group the elements according to:
a. their families on the periodic table
b. the similarity between their names and their symbols
c. the first letter of their symbol
d. the state of the element at 25 °C
e. whether the element is a metal, a nonmetal, or a metalloid
3. Use your flashcards to learn the names and symbols for these elements.
3.10
Ions
71
Page 2 of 4
Cations and Anions
A positive ion, called a cation (pronounced cat’ eye on), is produced when
one or more electrons are lost from a neutral atom. We have seen that sodium
loses one electron to become a 1 cation. Some atoms lose more than one
electron. For example, a magnesium atom typically loses two electrons to
form a 2 cation:
We usually represent this process as follows:
Mg → Mg2 2e
12 electrons
10 electrons
2 electrons
lost
12+
12+
2e–
Neutral magnesium
atom (Mg)
Magnesium ion
(Mg2+)
Loses
2 electrons
Aluminum forms a 3 cation by losing three electrons:
That is,
Al → Al3 3e
13 electrons
10 electrons
3 electrons
lost
13+
13+
3e–
Neutral aluminum
atom (Al)
Aluminum ion
(Al3+)
Loses
3 electrons
C H E M I S T RY
An ion has a net positive or
negative charge.
72
Chapter 3
A cation is named using the name of the parent atom. Thus Na is called
the sodium ion (or sodium cation), Mg2 is called the magnesium ion (or
magnesium cation), and Al3 is called the aluminum ion (or aluminum
cation).
When electrons are gained by a neutral atom, an ion with a negative
charge is formed. A negatively charged ion is called an anion (pronounced
an’ ion). An atom that gains one extra electron forms an anion with a 1
charge. An example of an atom that forms a 1 anion is the chlorine atom,
which has seventeen protons and seventeen electrons.
Chemical Foundations: Elements, Atoms, and Ions
Page 3 of 4
That is,
Cl e → Cl
17 electrons
1 electron
18 electrons
Plus
1 electron
e–
17+
17+
Neutral chlorine
atom (Cl)
Chloride ion
(Cl–)
Note that the anion formed by chlorine has eighteen electrons but only seventeen protons, so the net charge is (18) (17) 1. Unlike a cation,
which is named for the parent atom, an anion is named by taking the root
name of the atom and changing the ending. For example, the Cl anion produced from the Cl (chlorine) atom is called the chloride ion (or chloride anion). Notice that the word chloride is obtained from the root of the atom name
(chlor-) plus the suffix -ide. Other atoms that add one electron to form 1
ions include the following:
C H E M I S T RY
The name of an anion is
obtained by adding -ide to
the root of the atom name.
fluorine
F e → F
bromine
iodine
(fluoride ion)
Br e → Br
Ie →I
(bromide ion)
(iodide ion)
Note that the name of each of these anions is obtained by adding -ide to the
root of the atom name.
Some atoms can add two electrons to form 2 anions. Examples include
oxygen and sulfur:
oxygen
sulfur
O 2e → O2
S 2e → S
2
(oxide ion)
(sulfide ion)
Note that the names for these anions are derived in the same way as those
for the 1 anions.
It is important to recognize that ions are always formed by removing electrons from an atom (to form cations) or adding electrons to an atom (to form
anions). Ions are never formed by changing the number of protons in an atom’s
nucleus.
It is essential to understand that isolated atoms do not form ions on their
own. Most commonly, ions are formed when metallic elements combine with
nonmetallic elements. As we will discuss in detail in Chapter 8, when metals and nonmetals react, the metal atoms tend to lose one or more electrons,
which are in turn gained by the atoms of the nonmetal. Thus reactions between metals and nonmetals tend to form compounds that contain metal
cations and nonmetal anions. We will have more to say about these compounds in Section 3.11.
3.10
Ions
73
Page 4 of 4
Ion Charges and the Periodic Table
We find the periodic table very useful when we want to know what type of
ion is formed by a given atom. Figure 3.19 shows the types of ions formed
by atoms in several of the groups on
1
8
the periodic table. Note that the
Group 1 metals all form 1 ions
2
3
4
5
6
7
(M
), the Group 2 metals all form
+
2+
2–
–
Li Be
O
F
2 ions (M2), and the Group 3 metNa+ Mg2+
Al3+
S2– Cl–
als form 3 ions (M3). Thus for
+
2+
3+
2–
–
Groups 1 through 3 the charges of
K Ca
Ga
Se
Br
the cations formed are identical to
Rb+ Sr2+
In3+
Te2– I–
Transition metals form cations
the group numbers.
with various charges.
Cs+ Ba2+
In contrast to the Group 1, 2,
and 3 metals, most of the transition
metals form cations with various positive charges. For these elements
Figure 3.19
The ions formed by selected members of Groups 1, 2, 3, 6, and 7.
there is no easy way to predict the
charge of the cation that will be
formed.
Note that metals always form positive ions. This tendency to lose electrons is a fundamental characteristic of metals. Nonmetals, on the other hand,
C H E M I S T RY
form negative ions by gaining electrons. Note that the Group 7 atoms all gain
For Groups 1, 2, and 3, the
one electron to form 1 ions and that all the nonmetals in Group 6 gain two
charges of the cations equal the
group numbers.
electrons to form 2 ions.
At this point you should memorize the relationships between the group
number and the type of ion formed, as shown in Figure 3.19. You will understand why these relationships exist after we further discuss the theory of
the atom in Chapter 11.
Consumer Connection
Miracle Coatings
I
magine a pair of plastic-lens sunglasses that are
unscratchable, even if you drop them on concrete
or rub them with sandpaper. Research may make
such glasses possible, along with cutting tools that
never need sharpening, special glass for windshields
and buildings that cannot be scratched by windblown sand, and speakers that reproduce sound
with a crispness unimagined until now. The secret
of all these marvels is a thin diamond coating. Diamond is so hard that virtually nothing can scratch
it. A thin diamond coating on a speaker cone limits resonance and gives a remarkably pure tone.
But how do you coat something with a diamond?
It is nearly impossible to melt diamond (melting
point, 3500 °C). And even if diamond were melted,
the object being coated would itself melt immediately at this temperature. Surprisingly, a diamond
coating can be applied quite easily to something
even as fragile as plastic. First, the surface is bathed
74
Chapter 3
with a mixture of gaseous methane (CH4) and hydrogen (H2). Next, the methane is broken apart into
its component elements by an energy source similar to that used in microwave ovens. The carbon
atoms freed from the methane then form a thin diamond coating on the surface being treated.
The coating of soft, scratchable materials with a
super-tough diamond layer should improve many
types of consumer products in the near future.
Image not available for this
CD-ROM. Please refer to
the image in the textbook.
Chemical Foundations: Elements, Atoms, and Ions