Why Do They Call It a Periodic Table? Investigating and Graphing Periodic Trends

Transcription

1 Why Do They Call It a Periodic Table? Investigating and Graphing Periodic Trends About this Lesson This activity has two parts. Students arrange a set of cards in order before coming to class and then discuss the arrangement with others. This task is designed to give students appreciation for the organization of the periodic table that is currently used. Several of the trends are then graphed and examined. This lesson is included in the LTF Chemistry Module 5. Objectives Students will: Level Chemistry Graph various periodic properties. Examine the trends of the elements as they are arranged on the periodic table. Common Core State Standards for Science Content LTF Science lessons will be aligned with the next generation of multi-state science standards that are currently in development. These standards are said to be developed around the anchor document, A Framework for K 2 Science Education, which was produced by the National Research Council. Where applicable, the LTF Science lessons are also aligned to the Common Core Standards for Mathematical Content as well as the Common Core Literacy Standards for Science and Technical Subjects. (MATH) S-ID.6c Code Standard Level of Thinking (LITERACY) W. Represent data on two quantitative variables on a scatter plot, and describe how the variables are related. Fit a linear function for a scatter plot that suggests a linear association. Write arguments to support claims in an analysis of substantive topics or texts, using valid reasoning and relevant and sufficient evidence. Apply Apply Depth of Knowledge II II

2 Connections to AP* AP Chemistry: I. Structure of Matter 5. Periodic relationships including for example, atomic radii, ionization energies, electron affinities, oxidation states IV. Descriptive Chemistry 2. Relationships in the periodic table: horizontal, vertical, and diagonal with examples from alkali metals, alkaline earth metals, halogens, and the first series of transition elements *Advanced Placement and AP are registered trademarks of the College Entrance Examination Board. The College Board was not involved in the production of this product. Materials and Resources Each lab group will need the following: paper, copy paper, graph scissors tape, clear Assessments The following types of formative assessments are embedded in this lesson: Assessment of prior knowledge. Sharing data with explanations. Visual observation as students complete graphing and examination. The following additional assessments are located on the LTF website: Chemistry Assessment: Structure of Matter Short Lesson Assessment: Periodic Trends 2007 Chemistry Posttest, Free Response Question 2; 200 Chemistry Posttest, Free Response Question AP Style Free Response

3 Teaching Suggestions The day before the activity, distribute the page containing the elements with the atomic masses and the physical/chemical changes. Instruct the students to cut the cards apart and, using the properties, arrange the cards in some order. They should tape the cards to a piece of paper to secure the order for the in-class discussion. For the graphing activities, students should work in pairs and turn in one lab report for each student group. *An alternate method for doing this activity is to use computers and computer graphing software such as Graphical Analysis, LoggerPro, or Excel. This reduces the time needed for graphing and allows more time for discussion. Computer availability and software may dictate which method you choose. Students need a thorough understanding of atomic structure and periodic trends to be successful on the AP* Chemistry examination and indeed, to be successful in the field of chemistry in general. This exercise allows you to explain the reasons behind the trends. Emphasize that a trend is NOT an explanation, but rather an observation. When explaining the reasons behind the trends, it is helpful to start with atomic radii or size of the atoms. This is the easiest for students to understand and, once students grasp this concept, other concepts such as ionization energy, electron affinity, and electronegativity are easily understood. Size: As you move down a family, additional energy levels are added and the atoms get larger. As more levels are added, there are more electrons between the nucleus and the outer energy level. These electrons shield the nucleus from holding tightly to the outer electrons. As you move across a period from left to right, the electrons are in the same energy level, but each successive atom has one more proton in the nucleus. The additional proton increases the nuclear charge, or it increases the Z eff (Z effective), without increasing any inner shielding. This causes the electrons to be drawn closer to the nucleus. : Defined as the amount of energy needed to remove an electron from an atom (in the gaseous state) forming a + ion. Ionization energy can be expressed in equation form: M M + + e. The reasons for the general trends in IE are the same reasons as the trend for size. As you move down a family, electrons are further from the nucleus and have more inner electrons shielding them from the nuclear pull, therefore the amount of energy required to remove an electron decreases. As you move across a period from left to right, the Z eff for that period gets greater, the electrons are held more tightly and therefore electrons are more difficult to remove. IE gets larger. Emphasize that explanations of IE should be tied to the energy required to remove an electron.

4 As you look at the graph of ionization energies across a period, there are clearly some anomalies. Looking at the second period, the first observed irregularity is the lower ionization energy from Be to B. The simplest explanation is that the first electron to be removed from boron is coming from a 2p orbital which is higher energy than the electron from beryllium which is in a 2s and is of lower energy. There are certainly more sophisticated arguments which may be dealt with in the AP* course. Under no circumstance should a student be given the impression that the reason the 2s electron is harder to remove is that it is being taken from a full subshell that has an intrinsic stability! The second anomaly occurs when you move from nitrogen to oxygen. The increasing nuclear charge should dictate that the first IE of oxygen should be higher than that of nitrogen. However, oxygen has a lower first ionization energy than nitrogen due to the spin-spin repulsion of two electrons in the same orbital. Again, there is no increased stability due to a half-filled subshell. Even though this is written in many textbooks, it is not true and students should not be taught that there is some magic stability to half and totally filled subshells! Electron affinity: Defined as the energy change involved in forcing a gaseous atom to accept an electron to form a ion. Electron affinity is represented by X + e X. Again, the trends are dictated by the same factors distance from the nucleus, inner electron shielding, and increasing nuclear charge (Z eff in the same period). Therefore, electron affinity decreases as you move down a family and increases as you move from left to right across a period. (Pauling s): actually combines several factors and gives a relative number expressing attraction for electrons within a chemical bond. It is relative and, while it is very useful, there are many factors which affect this attraction for electrons within compounds. You may choose to introduce it at this point, because the factors which influence the electronegativity trend are the same as the others that we have been discussing. Or you may choose to introduce it when you talk about bonding. Metallic trends: Metals are really defined by loosely held, mobile electrons. The elements with the most loosely held electrons, of course, are in the lower left portion of the periodic table (largest and furthest from the nucleus with the lowest Z eff ) and the trend gets less as you move diagonally from lower left to upper right. After students understand the other periodic trends, and what really defines a metal, it will be easier for them to see why the zigzag line divides the metals from the non-metals. In teaching students to discuss various periodic trends, it is very important that they not answer a why question with a what answer. For example, if they are asked why the first ionization energy for magnesium is greater than the first ionization energy for sodium, they must NOT say that the trend is for ionization energies to increase from left to right across the periodic table. Instead they must address the fact that the electrons are in the same energy level, but the number of protons in magnesium is greater than the number of protons in sodium and therefore the Z eff is greater. They must also clearly address both chemical species (i.e. magnesium and sodium) in their answer.

5 POSSIBLE ANSWERS TO THE CONCLUSION QUESTIONS AND SAMPLE DATA After the students have made the arrangement of their cards and discussed their ideas with their partner, you will need to tell them the actual name and atomic number represented by each of the cards. The chart below will help you do so without having to actually match each element card with the periodic table. You may find it easiest to make a transparency of this page. Atomic Mass 36 Atomic Mass 9 Chlorine Beryllium Atomic number: 7 Atomic number: 4 Atomic Mass 40 Atomic Mass 7 Argon Lithium Atomic number: 8 Atomic number: 3 Atomic Mass 2 Atomic Mass 4 Carbon Helium Atomic number: 6 Atomic number: 2 Atomic Mass Atomic Mass 28 Boron Silicon Atomic number: 5 Atomic number: 4 Atomic Mass 20 Neon Atomic number: 0 Atomic Mass 32 Sulfur Atomic number: 6 Atomic Mass 40 Calcium Atomic number: 20 Atomic Mass 24 Magnesium Atomic number: 2 Atomic Mass Hydrogen Atomic number: Atomic Mass 27 Aluminum Atomic number: 3 Atomic Mass 23 Sodium Atomic number: Atomic Mass 3 Phosphorus Atomic number: 5 Atomic Mass 9 Fluorine Atomic number: 9 Atomic Mass 6 Oxygen Atomic number: 8 Atomic Mass 39 Potassium Atomic number: 9 Atomic Mass 4 Nitrogen Atomic number: 7

6 DATA AND OBSERVATIONS. Compare the indicated graphs. Do any of f the graphs show a repeating, or cyclic, pattern? Focus on elements with very large or veryy small values. a. Oxygen Atoms vs. Atomic Number and Chlorine Atoms vs. Atomic Number These graphs show a rise and fall within the period, but definitely a repeating pattern. b. Melting Point vs. Atomic Number andd Boiling Point vs. Atomic Number Again, these graphs show a rise and fall within the period with the highest meltingg points and boiling points being toward the middle of the period.

7 c. vs. Atomic Number and vs. Atomic Number These graphs show an increase across the period and falling as a new period starts. 2. Are these graphs consistent with patterns found in your earlier grouping of elements? Explain. Student answers will vary based on their original arrangements. Focus on the logic of the answers and discourage any assignment of right or wrong patterns at this time. After all, there are still many versions of periodic tables that are being used. You might evenn encourage students to find other models of the periodic table to share with the class. 3. Based on your graphs, why is the chemist s organization of elements called a periodic table? Because the trends repeat themselves inn an organized repeating pattern, or periodically. 4. Wheree are the elements with the highest oxide numbers located on the periodic table? How about the elements with the highest chloride numbers? Highest oxide and chloride numbers are located toward the middle of the periods. 5. Predict which element should have the lowest boiling point: selenium (Se), bromine (Br), or krypton (Kr)? Explain your prediction.. Krypton will have the lowest boiling point. It is a noble gas and is located at the end of o the period. The boiling point trend shows a decrease in boiling point toward the end of a period.

8 6. Which should have the lowest ionization energy: rubidium (Rb) or cesium (Cs)? Explain your prediction. Cesium will have a lower ionization energy because its outer electrons are further from the nucleus. Rb has only 6 energy levels where Cs has 7 energy levels. The extra layer of electrons will cause the nucleus of Cs to be more shielded than in Rb, therefore, the outer electrons will not be held as tightly, resulting in a lower ionization energy. 7. Which would have the greatest ionization energy: arsenic (As) or selenium (Se)? Explain your prediction. This is tricky. Both elements are on the 4th period. Selenium has a higher Z eff, so you would at first expect it to have a higher ionization energy. However, it has a lower ionization energy because Se has paired electrons in its p-orbital and As does not. Spin-spin repulsion due to the pairing of the electrons will cause that electron to be more easily removed and therefore arsenic will have the higher ionization energy.

9 Why Do They Call It a Periodic Table? Investigating and Graphing Periodic Trends Have you ever wondered why chemists call that big chart in the front of the room that contains all of the elements, the periodic table? For years scientists attempted to find some order to the elements that they knew existed. They looked at various properties and tried out many arrangements. You are going to attempt to do a similar task. You will be given a set of cards that contain certain properties. Without referring to the actual periodic table you will attempt to put the cards in some order that makes sense to you. You will be asked to explain your reasoning to your classmates. PURPOSE In this activity you will graph various periodic properties to ascertain the order. In so doing, you will see the trends among the elements of the periodic table. MATERIALS Each lab group will need the following: paper, copy paper, graph scissors tape, clear AT HOME PREPARATION Cut along the lines of the periodic trends cards. Try to arrange them in an order. Try several different patterns to see which makes the most sense to you. When you have an arrangement that you like, tape them in order on a piece of paper. Justify your arrangement on your student answer page.

10 Atomic Mass Atomic Mass Atomic Mass Atomic Mass Atomic Mass Atomic Mass Atomic Mass Atomic Mass Periodic Trend Cards Atomic Mass Atomic Mass Atomic Mass Atomic Mass Atomic Mass Atomic Mass Atomic Mass Atomic Mass Atomic Mass Atomic Mass Atomic Mass Atomic Mass

11 PROCEDURE. Compare your arrangement with your classmates results. Are there differences? Are there similarities? If there are differences, try to resolve them. 2. Now your teacher will give you the atomic number and the symbol for each of your cards. Work with your partner to arrange the cards in order of atomic number. 3. Use two sheets of graph paper. Title each Trends in Chemical Properties. Label the x-axis Atomic Numbers and number it from to 20 on both. One of you should label your y-axis Oxygen Atoms per Atom of Element. The other should label the y-axis Chlorine Atoms per Atom of Element. Determine a proper scale for each. Construct a line graph for each. 4. Get two more sheets of graph paper. Label one Boiling Points vs. Atomic Number. Label the other Melting Points vs. Atomic Number. Number the x-axes with atomic numbers from to 20 as before. Determine an appropriate scale for the temperatures along the y-axes. Construct line graphs for each. 5. Construct two more graphs. Label one vs. Atomic Number. The units for ionization energy are volts. Be sure to include the units on your graph. The other should be labeled vs. Atomic Number. is a relative number and has no units. As you did before, label the x-axis Atomic Number and scale from to 20. Use a line graph to plot these properties. Look at all six graphs, and answer the conclusion questions.

12 Why Do They Call It a Periodic Table? Investigating and Graphing Periodic Trends JUSTIFICATION OF ARRANGEMENT Explain why you arranged the cards in the particular order that you chose. DATA AND OBSERVATIONS Include your graphs with your report. ANALYSIS AND CONCLUSION QUESTIONS. Compare the indicated graphs. Do any of the graphs show a repeating, or cyclic, pattern? Focus on elements with very large or very small values. If the value is not given, skip that number or enter on the computer. a. Oxygen Atoms vs. Atomic Number and Chlorine Atoms vs. Atomic Number b. Melting Point vs. Atomic Number and Boiling Point vs. Atomic Number

13 c. vs. Atomic Number and vs. Atomic Number 2. Are these graphs consistent with patterns found in your earlier grouping of elements? Explain. 3. Based on your graphs, why is the chemist s organization of elements called a periodic table? 4. Where are the elements with the highest oxide numbers located on the periodic table? How about the elements with the highest chloride numbers? 5. Predict which element should have the lowest boiling point: selenium (Se), bromine (Br), or krypton (Kr)? Explain your prediction.

14 6. Which should have the lowest ionization energy: rubidium (Rb) or cesium (Cs)? Explain your prediction. 7. Which would have the greatest ionization energy: arsenic (As) or selenium (Se)? Explain your prediction.