This document is the answer key for the Periodic Trends Gizmo, created by Dan Ruddy. It provides detailed explanations and answers to questions related to atomic radius, ionization energy, and electron affinity, structured as an educational resource for students. The guide is designed to enhance understanding of periodic trends in chemistry.
2018
Periodic Trends Answer Key
Vocabulary: atomic radius, electron affinity, electron cloud, energy level, group, ion, ionization
energy, metal, nonmetal, nucleus, period, periodic trends, picometer, valence electron
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
[Note: The purpose of these questions is to activate prior knowledge and get students thinking.
Students are not expected to know the answer to the Prior Knowledge Question.]
1. On the image at right, the two magnets are the same.
Which paper clip would be harder to remove? B
2. Which magnet would be most likely to attract additional
paper clips? B
3. What is the relationship between the thickness of the book and the ability of the magnet to
hold on to and attract paper clips?
Answers will vary. [The thicker the book, the greater the distance between the magnet and
the paper clip. This increased distance lessens the attractive force, not only causing the
magnet to hold the paper clips more loosely but also decreasing the ability of the magnet to
attract additional items.]
Gizmo Warm-up
Just as the thickness of a book changes how strongly a magnet attracts a
paper clip, the size of an atom determines how strongly the nucleus
attracts electrons. In the Periodic Trends Gizmo, you will explore this
relationship and how it affects the properties of different elements.
The atomic radius is a measure of the size of the electron cloud, or the region where
electrons can be found. To begin, check that H (hydrogen) is selected in Group 1 on the left.
Turn on Show ruler. To measure the radius, drag one end of the ruler to the proton in the
nucleus and the other end to the electron. Click Save radius to record the value.
1. What is the radius of hydrogen? 53 pm
Notice that the radius is measured in picometers (pm). A picometer is one trillionth of a
meter.
2. On the right side of the Gizmo, select Li. Connect the right side of the ruler to the outermost
electron, or valence electron. What is the radius of lithium? 167 pm
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2018
Activity A:
Atomic radius
Get the Gizmo ready:
Check that Atomic radius is selected from the
drop-down menu.
Question: What factors affect the radius of an atom?
1. Predict: How do you think the radius of an atom will change as you move down a group
(vertical column) in the periodic table? Predictions will vary.
2. Collect data: Use the ruler to measure the atomic radii of the group 1 elements. As you do
so, count the energy levels (shown as rings of electrons) in each atom. Record in the table.
Element
H
Li
Na
K
Rb
Cs
Fr
Number of energy levels
1
2
3
4
5
6
7
Atomic radius (pm)
53
167
190
243
265
298
265
3. Observe: What happens to the radius as you move down group 1? It increases. [Except for
Fr, which is smaller than Cs.]
4. Explore: Turn off Show ruler. Select Li, and then select Be. Observe the radii of the
elements in group 2. Then look at other groups. What pattern do you see?
The radius tends to increase as you move down a group.
5. Draw a conclusion: In general, what is the effect of the number of energy levels on the
radius of an atom? As the number of energy levels increases, the radius also increases.
6. Predict: How do you think the radius of an atom will change as you move across a period
(horizontal row) in the periodic table? Predictions will vary.
7. Collect data: Beginning with Na, record the number of energy levels, number of protons, and
atomic radius for each element in period 3.
Element
Na
Mg
Al
Si
P
S
Cl
Ar
Number of energy levels
3
3
3
3
3
3
3
3
Number of protons
11
12
13
14
15
16
17
18
Atomic radius (pm)
190
145
118
111
98
88
79
71
(Activity A continued on next page)
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Activity A (continued from previous page)
8. Observe: What happens to the radius as you move across a period?
It tends to decrease.
9. Explore: Investigate other periods in the periodic table. Does the same trend occur? Yes.
Hypothesize why this trend occurs:
Answers will vary. [As you move across a period, the number of protons increases while the
number of energy levels stays constant. As a result, the cloud is pulled in more tightly.]
10. Analyze: Consider how the number of protons might affect the size of the electron cloud.
A. As you move across a period, are new energy levels added? No.
B. What happens to the number of protons in the nucleus as you move from one
element to the next across a period? The proton number increases.
C. If the proton number increases while the number of energy levels remains constant,
what happens to the attractive force between the nucleus and the electrons?
The attractive force becomes greater.
D. How does your answer to the previous question explain the trend in radii across a
period?
The attractive force is greater across a period, while the number of energy levels is
the same. Therefore, the electrons are pulled in more tightly.
11. Extend your thinking: The Gizmo enables you to examine ions, or atoms that have gained
or lost electrons. Select Na and turn on Show ion. Compare the radius of the neutral atom
to that of the ion. Repeat with Cl. Then look at other ions. See if you can find a pattern.
A. Why do you think the Na
+
ion is smaller than a neutral Na atom?
Answers will vary. [Fewer electrons minimize electron repulsions. Since there are
now more protons than electrons, the electron cloud is pulled in more tightly.]
B. Why do you think the Cl
-
ion is larger than a neutral Cl atom?
Answers will vary. [Increased number of electrons results in greater electron
repulsions, expanding the electron cloud. Since the number of protons has remained
constant, the nucleus is not able to pull in the cloud as tightly.]
Unlike neutral atoms, ions have a different number of electrons than protons. The electrons
are attracted to the protons and repelled by other electrons. If another electron is added, the
repulsion between electrons increases while the attractive force of the nucleus stays the
same. As a result, the electron cloud expands. If an electron is lost, the cloud is pulled in
more tightly. Electron repulsions decrease while the number of protons remains constant.
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