NSES Grades 9-12: Content Standard B: Students should develop an understanding of structures and properties of matter (p. 178)

Transcription

1 Trend Setter MI HSCE C1.1D Identify patterns in data and relate them to theoretical models. C4.9A Identify elements with similar chemical and physical properties using the periodic table. C4.9b Identify metals, non-metals, and metalloids using the periodic table. C4.9c Predict general trends in atomic radius, first ionization energy, and electronegativity of the elements using the periodic table. NSES Grades 9-12: Content Standard B: Students should develop an understanding of structures and properties of matter (p. 178) TYPE OF INQUIRY Guided Inquiry students will proceed through each activity with tasks that are given. The teacher will check for student understanding at specific points as groups work together. A discussion will follow to help clarify ideas. The assessment will summarize the lab concepts as individual students build their own version of the periodic table. TIME The students should read the beginning of the lab and complete Procedure Part A before class begins. Introduction & Discussion of Prediction: 10 minutes Procedure Part A, B & C: 50 minutes Assign Discussion questions as homework. Discussion over previous day and homework: 15 minutes The assessment may be completed as a homework assignment or in class. The Going Further activity will require about 0 minutes. EDUCATIONAL OBJECTIVES The student will be able to identify trends of electronegativity, atomic radii and ionization energy on the periodic table. The student will be able to explain the periodicity of the trends of electronegativity, ionization energy and atomic radii found on the periodic table. The student will be able to identify elements with similar properties based upon their location on the periodic table. The student will be able to predict properties of elements based upon where that element is found on the periodic table. 1

2 PREREQUISITE KNOWLEDGE Students should have a working knowledge of subatomic particles (protons, neutrons, electrons and their locations in an atom). Students should be able to write the expected noble gas electron configurations of all the elements on the periodic table. Students may know the definition and have a basic understanding of ionization energy, electronegativity, and atomic radii or this activity can be used to introduce these concepts. TEACHER BACKGROUND The period table contains a wealth of information and is a very important tool for chemists; however, students often fail to realize how much information the periodic table really holds. Mendeleev developed his ideas for the periodic table when he was working on writing a chemistry textbook. As part of his notes for writing the textbook, he had data for each of the elements on cards. Mendeleev realized that if he arranged these cards by increasing atomic weight, elements with similar properties occurred in regular intervals. However, organization of the cards was more difficult than one would think as at the time, only 60 elements were known and some of the data available about the elements were wrong. As such, Mendeleev often had to look at several pieces of data to correctly place the elements in the periodic table. Although other scientists had similar ideas around the same time as Mendeleev, Mendeleev is generally viewed as the father of the periodic table as using multiple pieces of data he was able to place the elements in the correct positions on the periodic table allowing for the correction of some of the incorrectly calculated atomic masses and prediction of properties of elements not yet discovered (Royal Society of Chemistry, 2004). The purpose of this activity is to give students an opportunity to try and organize elements into the periodic table in much the same way the Mendeleev did. To do this, students will need to identify trends in the data presented for the elements. When looking at the cards you will notice that not all data is presented for all of the elements. This will require students to use more than one trend to organize the cards correctly. Doing so will allow students to identify many of the trends in physical and chemical properties inherent in the organization of the periodic table. If students discover the trends themselves, they are more likely to remember the trends rather than to just put them into short-term memory for the test. The main trends that this activity focuses on are electronegativity, atomic radii, reactivity, and ionization energy. It is important to allow students to discover the format and trends by themselves. Allow students to be frustrated. Lead them to the correct answers through questioning such as Is there another piece of evidence that you could use to answer that question? or What is the trend you used to organize this row/column? Does this element fit both of those? Is there someplace else it fits better? Don t just give them the answers. This lab can be used as an introduction to teaching the specific details of atomic radii, ionization energy, effective nuclear charges and shielding and how these all relate to reactivity. The main goal of the going further activity is for students to discover relationships between electronic structure and the periodicity of element properties. 2

3 SAFETY Wear safety goggles in the lab. Phenolphthalein is harmful if swallowed and will irritate skin and eyes. MATERIALS, PREPARATION, AND DISPOSAL (Amounts are per group. Suggested group size is two to three students.) Part A & C Part B Assessment Element card set Goggles Colored pencils, pens and/or markers 0.50 gram magnesium ribbon Periodic table outline 0.50 gram calcium turnings** ml beakers ml beakers Phenolphthalein indicator solution **Calcium does not have a good shelf life. It easily oxidizes. If fresh calcium is not available, cut a calcium turning in half or find a picture of fresh calcium to show students what pure calcium should look like. PRELAB ENGAGEMENT / QUESTIONS Students should complete before class. On the day of the activity, have students compare answers and discuss any differences. Ask groups to report definitions. Discuss if clarification is necessary. Group: A vertical column on the periodic table of elements. Period: A horizontal row on the periodic table of elements. Atomic Mass: The mass of an atom in atomic mass units. At the time Mendeleev produced his table, the atomic weight was calculated by using proportions in which elements combined. Ionization Energy: The energy required to remove the most loosely held electron from a neutral atom. Electronegativity: The attraction an atom has for the shared electrons between bonded atoms. Atomic Radii: Half the distance between two adjacent nuclei. Emphasize that atomic radii of an individual atom is difficult to measure, so we use the bond radius in this activity. Bond Radii: One-half the distance from center to center of two like atoms bonded together. List the properties of the following: You may wish to address the terms ductile, malleable, luster, and brittle in the discussion of the following properties. Metal: good conductor of heat and electricity, have luster, malleable and ductile, tend to lose electrons Nonmetal: poor conductor of heat and electricity, brittle, tends to gain electrons Metalloid: Exhibit properties of both metals and nonmetals

4 Prediction: Emphasize that properties within a group may be exactly the same, but more often, there will be a trend of similar properties. Possible student answer: Elements A, B and D can be grouped together. They are all metals and they combine with chlorine in a ratio of 1 to 2. Element C is a nonmetal and combines with chlorine in a ratio of 1 to 1. Some students may just group B and D together saying that A is a liquid and does not react with water so it should be grouped separately. A=Hg, B=Zn, C=I, D=Cd. Element C (iodine) does not belong with elements A, B & D which belong to the same group on the periodic table of elements. You may wish to ignore the correct answer and focus more upon how we decide if elements have similar properties. Emphasize the idea of trends, that properties won t necessarily be identical for all elements in a group. Use this to lead into what will be done in the procedure. PROCEDURE -- Part A A set of element cards is provided in this document. If you wish to reuse the decks, copy onto card stock, laminate, and cut. Store the decks in plastic storage bags. There is some color used in the original to help students, but it is acceptable to use black and white if color is not an option. Sample student answers and facilitation ideas are provided. Fig. 1. Outline of Main Group Elements 1. Obtain a set of element cards. The cards represent the actual main group elements, but the element symbols have been replaced with a random code. Less than half of the cards show the atomic mass of the element represented on the card. 2. Obtain cards Gg, Jj, Pp, Rr, Ss, Tt, Vv, Ww, Yy, Zz. Place them in order of increasing atomic mass, using Fig. 1 as a reference. II III IV V VI VII VIII Zz Yy Gg Jj Pp Vv Tt Ww Ss Rr Use key to check for understanding after steps 1 and 2. Initial. Check Point -- Have your teacher check your table before you move on. 4

5 . Use the information on each of the remaining cards to arrange them into eight groups and four periods (see fig. 1). Also place the UNKNOWN ELEMENT where you think it belongs. If students struggle with step, ask what main properties Mendeleev used besides atomic mass to create his original table. This may lead them to look at the XO ratio. They will also need to consider other properties and their apparent trends to place all the cards. Lead students to focus on similarities within groups by asking students what trend they notice about all elements they have placed in a certain column. If students ask questions like, does reacts violently mean more reactive than reacts vigorously? or should the ionization energy of the elements increase as you go up a group? ask them if there is another piece of data they could use to help them place their cards/answer their question. Students may have trouble placing Qq and Cc. The atomic masses for these elements have been included to help with placing them. 4. a) Record the properties you used to arrange the cards. Explain how they helped in your arrangement of the cards. (1) Reactivity with oxygen was noted, atomic radii, and electronegativity. Each group contains the same element to oxygen ratio. (2) Atomic radii seemed to decrease moving to the right and increased going down the groups, so we used that to help. () Atomic mass was used when provided on the cards (not all cards list the atomic mass) to help arrange when it was difficult to see a trend in electronegativity. (4) Electronegativity was used because it seemed to increase going to the right and decrease going down a group. I II III IV V VI VII VIII Zz Yy Gg Jj Pp Vv Tt Ww Ss Rr Mm Hh Ii Nn Oo Uu Ll Dd Bb Kk Cc Xx Qq Ee Aa Ff Use completed key to check for understanding after Step #4. Check Point -- Have your teacher check your table before you move on. 5. Look at the table you created. List other patterns and/or trends you see in the arrangement of the element cards that you did not already list in #4. (You should discuss at least five properties between questions #4 & #5.) Ionization energy seems to increase going to the right and decrease going down each group. 6. Identify where the metals, metalloids, and nonmetals are found on the table. Metals are on the left. Nonmetals are on the right. Metalloids are in a stair-step type line on the table between metals and nonmetals. Students may say the Metalloids are in between the metals and nonmetals. 7. Identify where most of the gases are found on the table. Most gases are found on the right most side of the table. Check Point -- Have your teacher check your work before you move on. 5

6 Part B (If fresh calcium is not available, cut a calcium turning in half or find a picture of fresh calcium to show students what pure calcium should look like.) 1. Place about 100 ml of tap water in each of two 250 ml beakers. 2. Add two drops of phenolphthalein to the water in each beaker. Phenolphthalein turns hot pink in a basic solution (ph 7-14).. Add about 0.50 grams of calcium turnings to one beaker. 4. Add about 0.50 grams of magnesium to the other beaker. You may wish to provide students with a small scoop of appropriate size instead of taking the time to use the balance. 5. Record your observations. Calcium: Magnesium: Water quickly turns pink. After several minutes students should see a slight May see bubbling pink color. Use white paper behind beaker to help show color change. Ca + 2H 2 O Ca(OH) 2 + H 2 Mg + 2H 2 O Mg(OH) 2 + H 2 Note that the color change is caused by the phenolphthalein indicating for the base produced during the reaction. Thus, greater color change means more reactivity. Answer the following questions. 1. The elements calcium and magnesium belong in group 2 under card Jj on the table of cards you developed. Given your observations, predict which element belongs directly under Jj and describe your reasoning. Mg (magnesium) -- Kk reacts to water, but Hh only slightly reacts with water (per cards). Calcium showed a greater reactivy with water, so it matches the Kk card. Magnesium reacts only slightly, so it matches the Hh card. 2. a) Is there a trend of reactivity in group 2? Reactivity increases moving down the group. b) Look at other groups on your table. Do you see a consistent trend of reactivity within groups and periods on your table? Describe what you see. Within metals, the trend is an increase in reactivity moving down the groups and to the bottom left corner. Within nonmetals, the trend is an increase in reactivity moving up the groups and to the upper right corner. Note that students tend to incorrectly state that the trend is increasing reactivity going down every group. Check student work for understanding and initial. (Help students focus on specific parts of the periodic table if they are having difficulty identifying trends.) Part C Students look at the unknown element card on the table. They should use the trends they have discovered on their table to predict the properties of this unknown element. 1. Predict what phase this element will be in at room temperature and explain your reasoning. Solid the other elements on the left side of the table are solids. 6

7 2. Predict what the appearance of this element will be and explain your reasoning. The element is probably a metal, like the other elements on the left side of the table. Metals exhibit luster (are shiny).. Predict the ratio that this element will combine with oxygen and explain your reasoning. XO 2 -- based on the fact that the other elements in this group have the ratio XO Predict the ratio between this element and fluorine when they combine and explain your reasoning. XF 4 based on the fact that the other elements in this group have the ratio XF 4. Check student work for understanding and initial. DISCUSSION Students will use the periodic table they created in class. The discussion of this section and the questions that follow should take place the next day. Discuss tables with arrows denoting an increase in property. Have students use the right side of the paper to take notes relating to an explanation of each trend that you will lead them to during the discussion time if you choose to address at this time. Electronegativity: Electronegativity is the ability of an atom to attract electrons towards itself in a covalent bond. When two atoms with different EN s come together one typically has a greater pull on the 1. _Electronegativity electrons than the other. In some cases, like that between many metal and nonmetal atoms, the pull is so great that the electron is transferred and an ionic bond is formed. The strong attraction of electrons by nonmetals explains why they typically form negative ions. Relative reactivity of nonmetals can partially be explained by how easily they gain electrons (Electron Affinity); however, it is more complicated in groups as F has a lower EA than Cl, yet is more reactive ( Ionization energy: decreases down the group of elements because the 2. _Ionization Energy_ valence electron that is removed upon ionization is farther from the nucleus and attraction decreases exponentially as the distance increases. The ease in removing electrons as you go down a family explains why metals lower in a family are more reactive. Moving from left to right across the period, ionization energy increases. Across a period the valence electrons are in the same shell and thus approximately the same distance from the nucleus; however, there are more protons in the nucleus resulting in a stronger attraction for the valence electrons. The nucleus holds the valence electrons more tightly. _Atomic Radii and more energy is required to remove it. The atomic radii increases moving down the groups because an extra energy level of electrons is added with each period thus making the atom larger. Moving from left to right across the period the number of protons within the nucleus increases but the valence electrons are in the same shell; therefore, the valence electrons are pulled in more tightly and atomic radii decrease. 7

8 4. Atomic Mass Depending on the level of your class you may also want to discuss shielding and effective nuclear charge with respect to ionization energy and atomic radii. The atomic mass of the elements, in general, increases as the atomic number increases because the majority of the mass of atoms comes from the protons & neutrons. There are just a few places on the table that don t follow this trend. Students may wish to find them. This section is intended as homework. Allow students to use a periodic table for this. Questions 1. (a) Which group of elements will combine with fluorine in a 1 to 1 ratio? Give an example of a compound formula. Group I NaF and KF are examples. You may wish to discuss the Na +1 and F -1 charges at this point or save for a later lesson. (b) Which group of elements will combine with fluorine in a 1 to 2 ratio? Give an example of a compound formula. Group II MgF 2 and CaF 2 are examples. 2. In which two areas of the table do you find the most reactive elements? What trend(s) from the ones you identified above with the arrows correspond to these trends in reactivity? Predict an explanation for this (hint consider the trends of properties in terms of gaining and losing electrons). The most reactive elements are found at the bottom left and the top right of the table. The elements in the bottom left have large atomic radii and low ionization energies. This results in these atoms losing electrons more easily. The elements in the top right have small atomic radii and high electronegativity which corresponds to them having greater attraction for electrons or gaining electrons more easily. Note: ease with which atoms gain electrons is actually electron affinity (EA) energy change resulting when an atom gains an electron. However, this is often not discussed in high school chemistry. It is actually more complex than just relative EA as more energy is given off when Cl accepts an electron than when F accepts an electron. Actual relative reactivity also must consider energy changes associated with other parts of a reaction. For example, when NaF is formed there is energy given off when F gains an electron and when Na + and F - ions come together to form a lattice.. Find strontium on the periodic table. Considering Part B of the procedure, predict the reactivity of strontium with water. The reactivity of the elements of group II with water increases moving down the group. It would be expected that strontium will react vigorously with water. 4. Groups on the periodic table are also called families. Explain the use of that alternative term. The groups contain the elements that are most similar to each other, just as family members are similar. 8

9 ASSESSMENT Provide students with a blank periodic table (figure 2) and colored pencils, markers and/or pens if allowing class time. Students will create a table that summarizes the main points bulleted below. Students will choose how to represent the items/ideas and include a key to explain any coding used. This should be an individual assignment. Students should use this as a review for any written assessment to follow. Insert element symbols. Identify the elements as metals, nonmetals or metalloids. Identify the elements as solids, liquids, gases at room temperature. Include the following family names: Alkali Metals, Alkaline Earth Metals, Halogens and Noble Gases. Identify the Lanthanide Series and the Actinide Series. Identify the s block, p block, d block and f block. Include arrows showing an increasing trend in periods and groups of the following trends: electronegativity, ionization energy and atomic radii. Grading suggestion: 10 points per bulleted item must be complete and clear. Any coding used must be included in a key/explanation on the front of the table. GOING FURTHER Students will need to obtain a periodic table with electron configurations. Most textbooks will contain a periodic table including noble gas configurations. Webelements.com is an internet site with a wealth of periodic table information. Also, a variety of printable versions can be found at sciencegeek.net/tables/tables.shtml. It may be beneficial to discuss the different forms of the periodic table at this point. Students will notice the different ways that the groups are denoted, the difference in where the elements La(57) and Ac(89) are shown on the table, the different information shown and the different places where that information is represented are some common differences. Students may also notice that some tables show more elements than others (probably due to the copyright date.) Using the trends found earlier along with the electron configurations of the elements on the table, complete the following. 1. a) What do you think happens to the size of an atom as the atomic number increases? Students may say that the atom will increase in size or they may know already that the period trend does not support that statement and indicate that it depends whether you are going across a period or down a group. b) Does your explanation correspond to the trend seen for atomic radii? Explain. Atomic radii increases going down the table because each successive element contains an extra energy level containing more electrons. The atomic radii decreases moving across the period because the increase of protons causes an increase in positive charge (effective nuclear charge). There is also an increase of electrons, but they are all added within the same energy level. Thus the nuclei holds the negatively charged electrons closer to the nucleus. 2. The 10 groups added between the main group elements are called Transition Elements. (a) Predict what phase they will be in at room temperature. Explain. 9

10 All of the metals to the left of the metalloids on our main group table we created were solids, so most students will predict that the transition elements are solid. (b) What do the electron configurations of these elements have in common? When writing the electron configurations for the transition elements, they all end with one to ten electrons in a d sublevel.. Look at groups 1 & 2. (a) What do the electron configurations of group 1 have in common? All elements in group I have one valence electron. (b) What do the electron configurations of group 2 have in common? All elements in group II end with two valence electrons. (Students may know that these elements will all have a charge of +2 if they become an ion.) (c) What do the electron configurations of both group 1 and 2 have in common with each other? All the electron configurations of group I and II elements end with electrons in the s sublevel. 4. Look at the rest of the table. Find and explain other similarities like those in questions #1b and #2. All elements in groups 1 through 18 (or - 8 in the main group elements) have electron configurations that end with electrons in a p sublevel. 5. As an atom gets larger, the valence electron(s) are farther from the nucleus. Looking at atoms A and B, which do you think would require more energy to remove its valence electron? (N = nucleus in the diagram.) Explain your answer. e- N e- Atom A Atom B 5. _Atom B_ More energy will be required to remove the valence electron from Atom B, because the negatively charged valence electron is closer to the positive nuclei, thus the attraction between the electron and nuclei is stronger. 6. The transition elements do not show as much change in terms of the trends down the groups or across the table as the main group elements do. Explain. The electron configurations of all the transition elements have electrons in the d sublevel. The number of valence electrons for all the transition elements is two when we follow the textbook rules for writing electron configurations. If you look at the actual electron configurations found on a published table, exceptions to this will be found, but it can be seen that there is less variety within the number of valence electrons within this block of elements. N 10

11 7. Do the noble gases have electronegativities? Explain. The noble gases do not have electronegativities, because they are generally unreactive. This is a measure of the attraction for electrons in a covalent bond, as noble gases do not typically form compounds, they do not have electronegativity values. 8. Hydrogen is not an alkali metal. Why did you think it is placed in group 1? The placement of H has been and is still an issue. Some periodic tables place it with the alkali metals and with the halogens. Mendeleev s original table placed it in the same group with Cu, Ag, and Hg (see: A good student answer would be: The table is arranged so that the main group elements with the same number of valence electrons (and thus similar properties) are in the same group. Hydrogen is not an alkali metal, but it has only one valence electron. The upper left hand corner is not a perfect match, but it is the best place for it given the pattern of electronic structure on the table. 9. All of the noble gases, except helium, have a valence electron configuration of s 2 p 6 valence which we call a full or stable octet. How does having a stable octet affect the reactivity of the noble gases? If helium does not have a stable octet, explain why it exhibits the same reactivity as the rest of the noble gases. A stable octet makes the noble gases unreactive, they don t gain or lose electrons easily. Even though helium does not have a stable octet of valence electrons, it has a completely full outermost energy level (1s 2 ) making it unreactive. Note: When Mendeleev put together his initial periodic table the noble gases had not been discovered. REFERENCES Royal Society of Chemistry (2004). The development of the periodic table (pre-16): Demetri Mendeleev. Retrieved October 25, 2009 from: develop/mendeleev.htm Stacy, Angelica M., Coonrod, Jan, and Claesgens, Jennifer, Living by Chemistry General Chemistry, Alchemy, Key Curriculum Press, 200. Winter, Mark, WebElements: The Periodic Table on the WWW ( University of Sheffield and WebElements Ltd, UK,

12 Zz 1.0 X 2 O XF Colorless, Odorless Nonmetal Gg 6.9 X 2 O XF Silver Metal Yy 4.00 Colorless, Odorless Nonmetal Explodes in air when sparked Reacts with water Unreactive Code Physical Description Atomic mass Ratio w/ oxygen Ratio w/ fluorine 1 st ionization energy Electronegativity Atomic radii Uu Brittle, yellow nonmetal XO 1.0 XF Ii Soft, silvery metal X 2 O XF Reactivity room Reacts slowly with metals Does not react in presence of oxygen Rr Colorless, Odorless Nonmetal Vv Hard, clear solid metalloid OR soft, black nonmetal XO 2 XF Oo Red nonmetal 0.97 X 2 O 1.0 XF Unreactive Does not react with oxygen Does not react with oxygen 12

13 Hh XO XF 2 Moderately hard, silvery metal Ss X 2 O XF Pale yellow nonmetal Pp Hard, black metalloid 10.8 X 2 O 1.8 XF Reacts only slightly with water Explodes upon contact with metals Does not react with oxygen Nn XO 2 XF 4 Moderately hard, silvery metalloid Mm X 2 O XF Soft, silvery metal Ww Colorless, odorless nonmetal XO 1. XF Reacts very slowly with oxygen Reacts vigorously with water Reacts slowly with metals Jj Hard, lead gray metal 9.0 XO 0.9 XF Ee Gray nonmetal XO.94 XF Aa X 2 O XF Reddish brown nonmetal Does not react with water Reacts slowly with metals Reacts vigorously with metals LIQUID 1

14 Qq Brittle, steel gray metalloid X 2 O.95 XF Cc Silvery metal X 2 O.58 XF Kk Moderately hard silvery metal XO 0.6 XF Reacts very slowly with oxygen Does not react with oxygen Reacts with water e Ff Colorless, Odorless Nonmetal Tt Colorless, Odorless Nonmetal X 2 O 1.4 XF.1.07 Xx UNKNOWN ELEMENT Unreactive Not very reactive Bb Very soft, silvery metal Dd Colorless, Odorless Nonmetal Ll Greenish yellow Nonmetal X 2 O XF X 2 O XF Reacts violently with water Unreactive Reacts violently with metals 14

15 THE PERIODIC TABLE OF ELEMENTS I II III IV V VI VII VIII Figure 2: Trend Setter Activity