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.

1. Atomic Radius—the atomic radius is defined to be the distance 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.

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 energy implies that energy is released rather than required when an electron is added. The trends here are exactly the same as for ionization energy. From left to right the energy is more negative—high affinity--and from top to bottom it is less negative—lower affinity. The reasoning is very similar to those for ionization energy.

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.

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 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
(Pauling Units)
H / 1 / 53 / 1312 / -73 / 2.1
He / 2 / 31 / 2372 / 0 / NA
Li / 3 / 167 / 520 / -60 / 1.0
Be / 4 / 112 / 899 / 240 / 1.5
B / 5 / 87 / 801 / -27 / 2.0
C / 6 / 67 / 1086 / -122 / 2.5
N / 7 / 56 / 1402 / 0 / 3.0
O / 8 / 48 / 1314 / -141 / 3.5
F / 9 / 42 / 1681 / -328 / 4.0
Ne / 10 / 38 / 2081 / 0 / NA
Na / 11 / 190 / 496 / -53 / 0.9
Mg / 12 / 145 / 738 / 230 / 1.2
Al / 13 / 118 / 578 / -44 / 1.5
Si / 14 / 111 / 787 / -134 / 1.8
P / 15 / 98 / 1012 / -72 / 2.1
S / 16 / 88 / 1000 / -200 / 2.5
Cl / 17 / 79 / 1251 / -349 / 3.0
Ar / 18 / 71 / 1521 / 0 / NA

Graphs are for teacher to check students ’work:(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.

Graphs

Atomic Radius Complete 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.