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Periodic Trends in Ionic Size
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Printed: November 25, 2013
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C ONCEPT
Concept 1. Periodic Trends in Ionic Size
1
Periodic Trends in Ionic
Size
Lesson Objectives
The student will:
•
•
•
•
•
explain what an ion is.
describe how cations and anions are formed.
describe the factors that determine the trend in ionic size.
describe the trend in ionic size for elements.
use the general trends to predict the relative sizes of ions.
Vocabulary
• anion
• cation
• ion
Introduction
An atom is electrically neutral, which means that the number of protons is equal to the number of electrons. In
chemical reactions, however, atoms can gain or lose electrons. This results in the formation of an ion. An ion is an
atom with a positive or negative charge.
Atoms and Ions
Atoms of metallic elements tend to form positive ions by losing one or more electrons. A positive ion is called
a cation (pronounced CAT-ion) and has fewer electrons than an electrically neutral atom. For example, an atom
of sodium has eleven protons and eleven electrons. Its electron configuration is [Ne]3s1 . Sodium has one valence
electron surrounding a stable core of ten electrons. In chemical reactions, a sodium atom tends to lose its one valence
electron to become a sodium cation. Because this sodium ion has eleven protons and only ten electrons, it has a net
charge of +1. An atom that loses two electrons will become an ion with a charge of +2, and an atom that loses three
electrons will become an ion with a charge of +3.
Atoms of nonmetallic elements tend to form negative ions by gaining one or more electrons. A negative ion is called
an anion (pronounced AN-ion). For example, an atom of fluorine has seventeen protons and seventeen electrons. Its
electron configuration is 1s2 2s2 2p5 , and it has seven valence electrons. In chemical reactions, a fluorine atom tends
to gain one valence electron, becoming a fluoride anion. (Notice that the name of anions typically end in “-ide.”)
Because the fluoride ion has seventeen positive protons and eighteen negative electrons, it has a net charge of −1.
1
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An atom that gains two electrons will become an ion with a charge of −2, and an atom that gains three electrons will
become an ion with a charge of −3.
Group and Period Trends in Ionic Size
Cations are smaller than the atoms from which they are formed. The loss of outer shell electrons results in increased
attraction between the nucleus and the remaining electrons. This results in less electron-electron repulsion and
allows the nucleus and the electrons to come closer together. When compared to a neutral atom of sodium, Na, a
sodium cation, symbolized by Na+ , has a smaller size.
Anions are larger than the atoms from which they are formed. The gain of outer shell electrons results in decreased
attraction between the nucleus and the remaining electrons, and electron-electron repulsion forces them to spread
apart. When compared to a neutral atom of fluorine, F, a fluoride anion, symbolized by F− , has a larger size.
If we examine the ionic sizes of just the metals in the main group, we will find that the trends are the same as the
trends in atomic sizes for the neutral elements. All the elements for each group of metals lose the same number of
electrons, which means that the ionic sizes will be primarily affected by the number of energy levels in the electron
cloud. Since the number of energy levels still increases from top to bottom, the ionic size also increases down a
group of elements in the periodic table. For similar reasons, the trend across a period is the same for both ions and
neutral atoms. All the metal elements in a given period will lose their outer shell electrons but still have the same
number of core electrons. As a result, the nuclear charge increases from left to right, while the number of core
electrons remains the same. This means that the ion size will decrease from left to right across a period.
Nonmetals also see the same trends in size as the neutral elements. The negative ions increase in size as you move
down a group and decrease in size as you move from left to right across a period. In other words, as you go from
top to bottom down a group or left to right across a period, the ionic size decreases as long as you are comparing all
metals or all nonmetals. Between the metals and the nonmetals, the ionic size increases because you are switching
from cations, which lose electrons, to anions, which gain electrons.
Lesson Summary
• In chemical reactions, atoms can gain or lose electrons. This results in the formation of an ion. An ion is
basically an atom with a positive or negative charge.
• Atoms of metallic elements tend to form positive ions (cations) by losing one or more electrons.
• Atoms of nonmetallic elements tend to form negative ions (anions) by gaining one or more electrons.
• Cations are smaller than the atoms from which they are formed.
• Anions are larger than the atoms from which they were formed.
• Ionic size increases from top to bottom down a group of elements in the periodic table.
• From left to right across a period, the ionic size decreases as long as you are comparing all metals or all
nonmetals. Between the metals and nonmetals, the ionic size increases as you switch from cations to anions.
Review Questions
1.
2.
3.
4.
2
How is the size of a cation different from the size of the atom from which it was formed? Why?
How is the size of an anion different from the size of the atom from which it was formed? Why?
Mg2+ has the same number of electrons as F− (they are said to be isoelectric). Which ion is larger and why?
Which of the following has the smallest ionic radius?
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a.
b.
c.
d.
Concept 1. Periodic Trends in Ionic Size
O2−
S2−
Mg2+
Ca2+
5. Which of the following has the largest ionic radius?
a.
b.
c.
d.
Ba2+
Cs+
I−
Te2−
3