s8pe-20701-ca
12/19/05
4:40 PM
MAZER
Page 204
Atoms form ions.
MAIN IDEA WEB
Make a main idea web to
organize what you know
about ions.
A neutral atom has an equal number of electrons and protons. Since
each electron has one negative charge and each proton has one positive
charge, atoms have no overall electric charge. An ion is formed when
an atom loses or gains one or more electrons. Because the number of
electrons in an ion is different from the number of protons, an ion
does have an overall electric charge.
Formation of Positive Ions
Consider how a positive ion can form from an atom. The left side of
the illustration below represents a sodium (Na) atom. Its nucleus
contains 11 protons and some neutrons. Because the electron cloud
surrounding the nucleus consists of 11 electrons, there is no overall
charge on the atom. If the atom loses one electron, however, the
charges are no longer balanced. There is now one more proton than
there are electrons. The ion formed, therefore, has a positive charge.
11 electrons
(11–)
10 electrons
(10–)
Loses 1
electron
11+
11+
Sodium Atom (Na )
Sodium Ion (Na+)
A positive ion is
smaller than the
atom that formed
it because it has
fewer electrons.
Notice the size of the positive ion. It is smaller than the neutral
atom. One reason the ion is smaller is because it has one less electron.
The electron that was lost is usually the one farthest away from the
nucleus. The size of the electron cloud, therefore, is decreased.
Positive ions are represented by the symbol for the element with a
raised plus sign to indicate the positive charge. In the above example,
the sodium ion is represented as Na+.
Some atoms form positive ions by losing more than one electron.
In those cases, the symbol for the ion also indicates the number of
positive charges on the ion. For example, calcium loses two electrons
to form an ion Ca2+, and aluminum loses three electrons to form Al3+.
Check Your Reading
204 Unit 2: The Structure of Matter
What must happen to form a positive ion?
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