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12/19/05
4:40 PM
MAZER
Page 205
Formation of Negative Ions
The illustration below shows how a negative ion is formed. In this
case the atom is chlorine (Cl). The nucleus of a chlorine atom contains
17 protons and some neutrons. The electron cloud has 17 electrons,
so the atom has no overall charge. Chlorine has a tendency to gain
one electron. When an electron is added to the chlorine atom, a negatively charged ion is formed. Notice that a negative ion is larger than
the neutral atom that formed it. An additional electron has been
added to the electron cloud. The electron has gone into the space in
the electron cloud that is farthest from the nucleus. The electron
cloud is, therefore, larger.
17 electrons
(17-)
18 electrons
(18-)
Gains 1
electron
A negative ion is
larger than the
atom that formed
it because it has
more electrons.
17+
17+
Chlorine Atom (Cl )
Chloride Ion (Cl–)
Negative ions are represented by placing a minus sign to the right
and slightly above the element’s symbol. The negative chloride ion in
the example, therefore, would be written as Cl–. If an ion has gained
more than one electron, the number of added electrons is shown by a
number in front of the minus sign. Oxygen (O), for example, gains
two electrons when it forms an ion. Its symbol is O2–.
KEY CONCEPTS
CRITICAL THINKING
1. Describe how elements are
named. (8.7.b)
4. Infer Magnesium and sodium
atoms are about the same size.
How does the size of a magnesium ion with a 2+ charge
compare with that of a sodium
ion with a single + charge?
2. What determines the identity
of an atom? (8.3.a)
3. What happens when an atom
forms an ion? (8.3.a)
CHALLENGE
6. Analyze When determining
the mass of an atom, the
electrons are not considered.
Why can scientists disregard
the electrons?
5. Compare The atomic number
of potassium is 19. How does
potassium-39 differ from
potassium-41?
Chapter 7: Atomic Structure and the Periodic Table 205
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