PERIODIC TRENDS
Strand A: The Nature of Matter
Standard 2: The student understands the basic principles of atomic theory.
Benchmark SC.A.2.4.5: Knows that elements are arranged into groups and families based on similarities in
electron structure and that their physical and chemical properties can be predicted.
Task Analysis: The student…
 explains the organization of the periodic table.
 interprets graphs that show the relationship between physical properties and electron structures of
elements.
 compares and contrasts chemical properties of elements from different families.
Teacher Background
The Periodic Table was the outcome of several chemists working to make some sense out of the knowledge
they were learning about the elements. John Newlands, Dmitri Mendeleev, and Henry Mosley all worked to
give us the periodic table that we have today. John Newlands contribution to the periodic table was his “law of
octaves” noted a pattern in the structures of atoms of elements with similar chemical properties. D. Mendeleev
was a chemistry teacher who was eagerly trying to find a new way to show his students a better way to learn
the properties of elements. In his attempts he wrote each element on a separate card and started to arrange
these elemental cards based on similarities in physical and chemical properties and as a result ended up with
a table that looked somewhat like our table today—except there were holes. According to Mendeleev, these
holes represented elements that existed but were not yet found. The major factor Mendeleev used to arrange
his elements was atomic mass (average mass of the atom). But there were problems where mass said
elements should go one way and chemical properties said they should go another. He rationalized that the
properties were more accurate than the masses since technology used to determine the mass was still
improving. Later once technology improved and we learned that each atom has an integral positive charge
(protons in the nucleus) in other words, each atom has a unique charge in its nucleus. Henry Mosley
discovered that each element in Mendeleev’s table was arranged in an order such that their integral positive
charge increased numerically from left to right and top to bottom. Also, this new discovery fixed Mendeleev’s
problem spots. Now, the periodic table had a new basis of organization: atomic number (number of protons in
the nucleus). This is basically still the organization except we have also added electron configuration (the
placement of electrons into energy levels) to the organization. So now everything with the same valence shell
electron configuration (outer shell electron configuration) is in the same family (column), where as properties
are different from one period (row) to another.
Trends: the periodic table not only provides us with atomic numbers of elements and masses of elements, but
it also provides us with a tool to determine general trends in chemical and physical properties.
Smallest
atom
Largest atom
1. Atomic
Radius—the
atomic
radius is
defined to
be the
distance
increases
decreases
increases
decreases
Largest
value
increase
Smallest value
from the nucleus to the outer energy level where the outermost electrons are. The atomic radius
decreases as you move from left to right across a period and increases as you move from top to bottom in
a family. But why? For a period, you are in the same energy level which itself describes the size of the
electron cloud (the area where the electrons are most probably located), so based on this elements should
have the same radius in the same period. But as you move across the period, the total positive charge in
the nucleus increases thereby increasing the total force attracting the electrons in the outer energy level
thereby making the cloud contract. As you go down a family it is much easier to understand the trend. As
you go down a family the energy level increases thereby increasing the radius of the cloud so the atomic
radius increases.
2. Ionization Energy—this is defined as the amount of energy needed to remove an electron from the outer
energy level of an atom. Here the trend is to increase from left to right across a period and to decrease
from top to bottom in a family. Why? As previously stated, the size decreases as you move from left to
right as the total positive charge increases. By the inverse square law (F=kQ1Q2/r2) this means the force
attracting the electron increases as size decreases and as the positive charge increases. As you move
from top to bottom again using the inverse square law, you get farther from nucleus thus reducing the
attractive force on the electron.
decreases
3. Electron Affinity—this is defined as the amount of energy required to add an electron. For all but one
column this value is either negative or zero. The 2nd family is
the only one that is consistently positive. Here the negative
increases
energy implies that energy is released rather than required
when an electron is added. The trends here are exactly the
Most
same as for ionization energy. From left to right the energy
Negative
is more negative—high affinity--and from top to bottom it is
Value
less negative—lower affinity. The reasoning is very similar to
those for ionization energy.
Least Negative Value
or Most Positive Value
increases
decreases
4. Electro negativity—this
is defined as the
attraction of one atom in a molecule for the shared electrons. The trend
is exactly the same as for electron affinity and ionization energy. The
reasoning is also the same. Fluorine is set at the highest at 4.0 and all
other atoms are lower than this.
Largest
value
5. Reactivity—this is defined as how readily an atom reacts. For metals
this is based on ionization energy where the most reactive metals are
Smallest value
those with the lowest ionization energies. For nonmetals this is based
on electron affinity where the most negative electron affinity is the most reactive. This has to do with the
fact that metals form positive ions (lose electrons) and nonmetals for negative ions (gains electrons).
Materials
 graph paper
 activity series for metals and nonmetals (found in any chemistry text or on the internet)
Description
Note: This activity is intended to be an activity used before the students are taught the trends. It is to
be used to have the students recognize the repeating patterns in the graphs. After the completion of
the activity, the explanations of the trends may be addressed depending on the level of the students.
For the following data, plot the atomic number on the x-axis and the property on the y-axis. Each property
should be graphed on a separate sheet of graph paper.
Element
Atomic
Number
Atomic Radius
(pm)
1st Ionization
Energy (kJ/mol)
Electron Affinity
(kJ/mol)
Electro
negativity
H
He
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
53
31
167
112
87
67
56
48
42
38
190
145
118
111
98
88
79
71
Graphs are for teacher to check students
’work:
1312
2372
520
899
801
1086
1402
1314
1681
2081
496
738
578
787
1012
1000
1251
1521
-73
0
-60
240
-27
-122
0
-141
-328
0
-53
230
-44
-134
-72
-200
-349
0
(Pauling Units)
2.1
NA
1.0
1.5
2.0
2.5
3.0
3.5
4.0
NA
0.9
1.2
1.5
1.8
2.1
2.5
3.0
NA
(note on
the Electro negativity graph, the 3 noble gases are omitted because values are not available)
Analysis
1. Based on your graphs, what is the trend in atomic radius across a period? down a family?
Atomic radius decreases from left to right and increases from top to bottom.
2. Based on your graphs, what is the trend in ionization energy across a period? down a family?
Ionization energy increases from left to right and decreases from top to bottom.
3. Based on your graphs, what is the trend in electron affinity across a period? down a family?
Electron affinity increases (becomes more negative) as you move from left to right and decreases
(becomes less negative) as you move from top to bottom).
4. Based on your graphs, what is the trend in electro negativity across a period? down a family?
Electro negativity increases as you move from left to right and decreases as you move from top to bottom.
5. Using an activity series, what can you deduce about the relationship between ionization energy and
reactivity of metals?
The lower the ionization energy the more reactive the metal.
6. Using an activity series, what can you deduce about the relationship between electron affinity and reactivity
of nonmetals?
The more negative the electron affinity is for the nonmetal, the more reactive the nonmetal.
7. Explain in 3-4 paragraphs the organization and usefulness of the modern periodic table based on what you
have learned in class and what you have learned from this activity. (See teacher notes).
Periodic Trends
Description
For the following data, plot the atomic number on the x-axis and the property on the y-axis. Each property should be
graphed on a separate Excel chart. You must graph both Atomic Radius and 1st ionization energy. Electron affinity and
Electronegativity graphs are optional. Show each graph to your teacher when it is completed.
Element
H
He
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
Atomic
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Graphs
Atomic Radius
Atomic Radius (pm)
53
31
167
112
87
67
56
48
42
38
190
145
118
111
98
88
79
71
Complete
1st Ionization Energy
(kJ/mol)
1312
2372
520
899
801
1086
1402
1314
1681
2081
496
738
578
787
1012
1000
1251
1521
Electron Affinity
(kJ/mol)
-73
0
-60
240
-27
-122
0
-141
-328
0
-53
230
-44
-134
-72
-200
-349
0
Electro negativity
(Pauling Units)
2.1
NA
1.0
1.5
2.0
2.5
3.0
3.5
4.0
NA
0.9
1.2
1.5
1.8
2.1
2.5
3.0
NA
Partial (missing title or 1 axis label) Incomplete (missing 2 things)
Ionization Energy
Complete
Partial
Incomplete
Electron Affinity
Complete
Partial
Incomplete
Electronegativity
Complete
Partial
Incomplete
Analysis
1. Based on your graphs, what is the trend in atomic radius across a period? down a family?
2. Based on your graphs, what is the trend in ionization energy across a period? down a family?
3. Based on your graphs, what is the trend in electron affinity across a period? down a family?
4. Based on your graphs, what is the trend in electro negativity across a period? down a family?
5. Using an activity series, what can you deduce about the relationship between ionization energy and
reactivity of metals?
Using an activity series, what can you deduce about the relationship between electron affinity and reactivity of
nonmetals? Explain in 3-4 paragraphs the organization and usefulness of the modern periodic table based on
what you have learned in class and what you have learned from this activity.
Extra Credit: Print your charts.
Excel Hints
Label Each Column
To select columns that are not next to each other, hold down the control key while selecting the second column
To start your graph (chart) click on the icon that looks like a bar graph
All graphs should be an XY (scatter)
Make sure that each graph has a title and both X and Y labels with units where appropriate.