Periodic Trends: Ionization
Energy
Ck12 Science
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Printed: November 30, 2015
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
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Chapter 1. Periodic Trends: Ionization Energy
1
Periodic Trends: Ionization
Energy
Define ionization energy.
Describe factors affecting ionization energy.
Describe how ionization energy changes across a period.
Describe how ionization energy changes down a group.
Why do sheep travel in herds?
Like many other animals, sheep travel in herds. The tendency is for each individual sheep to stay with the herd.
However, a sheep may sometimes wander off, depending on how strong the attraction is for a particular food or
water supply. At other times, a sheep may become frightened and run off. Whether a sheep chooses to stay with the
herd or go its own way depends on the balance between attraction to the herd and attraction to the outside influence.
There is an on-going tension between the electrons and protons in an atom. Reactivity of the atom depends in
part on how easily the electrons can be removed from the atom. We can measure this quantity and use it to make
predictions about the behaviors of atoms.
Ionization Energy
Ionization energy is the energy required to remove an electron from a specific atom. It is measured in kJ/mol,
which is an energy unit, much like calories. The ionization energies associated with some elements are described
in the Table 1.1. For any given atom, the outermost valence electrons will have lower ionization energies than the
inner-shell kernel electrons. As more electrons are added to a nucleus, the outer electrons become shielded from the
nucleus by the inner shell electrons. This is called electron shielding.
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TABLE 1.1: Ionization Energies (kJ/mol) of the First 18 Elements
Element
H
He
Li
Be
B
C
N
O
IE1
1312
2373
520
899
801
1086
1400
1314
IE2
IE3
IE4
IE5
IE6
5251
7300
1757
2430
2350
2860
3390
11,815
14,850
3660
4620
4580
5300
21,005
25,000
6220
7500
7470
32,820
38,000
9400
11,000
47,261
53,000
13,000
If we plot the first ionization energies vs. atomic number for the main group elements, we would have the following
trend
FIGURE 1.1
Ionization energy and atomic number.
Moving from left to right across the periodic table, the ionization energy for an atom increases. We can explain this
by considering the nuclear charge of the atom. The more protons in the nucleus, the stronger the attraction of the
nucleus to electrons. This stronger attraction makes it more difficult to remove electrons.
Within a group, the ionization energy decreases as the size of the atom gets larger. On the graph, we see that the
ionization energy increases as we go up the group to smaller atoms. In this situation, the first electron removed is
farther from the nucleus as the atomic number (number of protons) increases. Being farther away from the positive
attraction makes it easier for that electron to be pulled off.
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Chapter 1. Periodic Trends: Ionization Energy
Summary
• Ionization energy refers to the amount of energy needed to remove an electron from an atom.
• Ionization energy decreases as we go down a group.
• Ionization energy increases from left to right across the periodic table.
Practice
Questions
Use the following link to answer the questions below:
http://dl.clackamas.cc.or.us/ch104-07/ionization_energy.htm
1. What is a “gaseous atom?”
2. Why would the ionization energy for O be somewhat less than that for N?
3. Why is a third-level electron easier to remove than a first-level one?
Review
Questions
1.
2.
3.
4.
5.
6.
Define “ionization energy.”
Do valence electrons have larger or smaller ionization energies that the inner-shell kernel electrons?
What is electron shielding?
Describe the trends in ionization energy from left to right across the periodic table.
Describe the trends in ionization energy from top to bottom of a group in the periodic table.
Why is the second ionization energy for lithium so much larger than the first ionization energy?
• electron shielding: As more electrons are added to a nucleus, the outer electrons become shielded from the
nucleus by the inner shell electrons
• ionization energy: The energy required to remove an electron from a specific atom. It is measured in kJ/mol,
which is an energy unit, much like calories.
References
1. Paul Englefield. http://commons.wikimedia.org/wiki/File:Sheep_herd.jpg .
2. User:RJHall/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Ionization_energies.svg .
3. . Periodic Table Trends.
3