ACTIVITY 3: Elements and the Periodic Table-- KEY

Transcription

1 CYCLE 6 Developing Ideas ACTIVITY 3: Elements and the Periodic Table-- KEY Purpose You might imagine that with more than 92 elements, scientists have developed classification schemes (besides just gas, liquid, and solid as in Cycle 5) to group elements according to the similarities and differences in their physical properties and chemical changes. In fact, early scientists devised a simple classification system for the two major types of elements metals and nonmetals and later came to include a third type of element, metalloids (also called semimetals). In this activity you will investigate the physical properties and chemical changes of a small number of elements in order to duplicate this simple classification system for elements. By the late 1800s, scientists had developed an early version of the periodic table of the elements a more complex classification scheme based on trends in certain physical properties and chemical interactions of elements. In this activity we will use similar information to that which was available in the late 1800s to create our own periodic table of the elements. In the next activity we will examine how atom structure relates to the modern periodic table. How are elements classified? Collecting and Interpreting Evidence Experiment #1: How can physical properties and chemical interactions distinguish between metals, nonmetals, and metalloids? In this activity you will examine the appearance, conductivity, malleability, and chemical interactions (sometimes called reactivity) of various elements PSET 6-37

2 Cycle 6 You will need: an assortment of elements from the list: carbon (coal and graphite forms), iron, magnesium, silicon, sulfur, tin, zinc; a pea-sized lump or ~1 cm if using wire or ribbon 3.0 M Hydrochloric acid solution in dropper bottle 1.0 M Copper (II) chloride solution in dropper bottle Small hammer Conductivity apparatus Magnifying glass Well plates, 2 Forceps/tweezers CAUTION: Wear safety glasses or goggles. CAUTION: Do NOT handle any element samples with your fingers use forceps or wear gloves to ensure that your skin is not contaminated by these materials. CAUTION: Be sure to follow standard safety rules for using acids. Do NOT use or store magnesium ribbon around open flames. STEP 1. Place a sample of each element in a separate well on your well plate. Observe the appearance of each element. Record the appearance of each element in the table below. Include descriptions of color, luster (shiny or dull), and form crystalline form (like table salt), noncrystalline form (like baby powder), or metallic form (like stainless steel). 6-38

3 Activity 3: Elements and the Periodic Table Table 1. Element Properties Element Appearance Conductive Malleable Interacts with Hydrochloric Acid Interacts with Copper Chloride Carbon (coal) Black, noncrys no brittle none none Carbon (graphite) Gray, noncrys-metallic looking yes brittle none none Iron Silvery, metallic yes malleable bubbles turns color Magnesium Silvery, metallic yes malleable bubbles turns color Silicon Silvery, noncrys-metallic looking yes brittle none none Sulfur Yellow, noncrys no brittle none none Tin Silvery, metallic yes malleable bubbles turns color Zinc Silvery, metallic yes malleable bubbles turns color STEP 2. Test each element sample with the conductivity apparatus. If the bulb or diode lights or flashes, electricity is being conducted through the sample and the sample is considered conductive. Record the results of the conductivity test in the table. STEP 3. Tap each element sample with a small hammer. (Note: you may want to transfer each element sample from the well plate to a clean sheet of paper for gentle hammering, and then return the element to its original well.) If the sample is malleable, it will flatten or change shape without shattering when struck by the hammer. If the sample shatters when struck, it is considered to be brittle. Record the results of the malleability test in the table. 6-39

4 Cycle 6 STEP 4. Divide each element sample in half, transferring half of each sample to a new well. Half of each sample will be used to test the interaction with an acid, the other half for the interaction with copper chloride. STEP 5. Add drops of hydrochloric acid solution to each well containing half of each element sample. Use the magnifying glass to make careful observations of each sample. What evidence do you have that a chemical interaction occurs for some of the elements? Bubbles form Record the results of the hydrochloric acid test in the table. STEP 6. Add drops of copper chloride solution to each well containing the other half of each element sample. Use the magnifying glass to make careful observations of each sample over 3-5 minutes. Changes may be slow. What evidence do you have that a chemical interaction occurs for some of the elements? Elements change colors 6-40

5 Activity 3: Elements and the Periodic Table Record the results of the copper chloride test in the table. Which properties tested were physical properties? Which properties tested involved chemical interactions? Explain your reasoning. Appearance, conductivity, malleability were physical could be observed/measured without changing composition; interaction with hydrochloric acid and copper chloride were chemical they involved chemical changes (evident by bubbles forming, or color change, or formation of black solid) Sort the elements into two groups metals and nonmetals based on their physical properties and chemical interactions. List the elements in each group. Metals: iron, magnesium, tin, zinc Nonmetals: carbon (coal), sulfur What physical properties and chemical interactions do most of the metals exhibit? Which do most of the nonmetals exhibit? Metals: shiny, silvery appearance; conductive; malleable; interactions with hydrochloric acid and copper chloride Nonmetals: dull, noncrystalline appearance; nonconductive; brittle; no interactions with hydrochloric acid and copper chloride Which element(s) could belong to either group? Explain your reasoning. Silicon and graphite have appearance and conductivity properties like metals but chemical properties like nonmetals Exloration #2: How can physical properties and chemical interactions be used to organize the elements? How did Dmitri Mendeleev organize the elements? In 1869 Dmitri Mendeleev (a Russian chemistry teacher) published what is recognized to be the first periodic table--his model for organizing the elements that were known in his day. A reproduction of Mendeleev s periodic table is shown below. As you examine the table, think about and answer the questions. 6-41

6 Cycle 6 What are some organizational features of Mendeleev s table? Columns (groups) and rows (periods) What do you think the letters, numbers, and blanks represent? Letters: abbreviations of names (chemical symbols); numbers: atomic mass/weight; blanks: undiscovered elements How do you think he organized the elements (i.e. why did he choose the order that he did?) Increasing atomic mass/weight 6-42

7 Activity 3: Elements and the Periodic Table Mendeleev s Periodic Table was the first to demonstrate periodic trend patterns when melting or boiling points and specific heat values were graphed against atomic weight (at that time, a very crude estimate of the average mass of an atom of an element, also called relative atomic mass) for 60 elements. He also considered the periodicity of chemical interactions the combining power (we ll discuss later), the ratios of elements in compounds formed, and the reactivity (e.g. with water, air, or metals). His Periodic table was even able to predict the presence and properties and interactions of elements that had not yet been discovered. How will we organize the elements? In this activity you will create your own periodic table using element cards containing information similar to that available to Mendeleev. Your goal is to organize the elements in a way that makes sense of element properties and chemical interactions. You will need: Element cards Element card sort mat (to be distributed by instructor at STEP 2) Modern Periodic Table (to be distributed by instructor at STEP 5) STEP 1. Remove the element cards from the pack. Find Lithium, Beryllium, Magnesium, Calcium, and Strontium, and Boron. Arrange them according to the figure below. STEP 2. Carefully study this arrangement, making mental note of some of the patterns that you observe. Arrange the remaining cards around these, using the information provided on the cards to guide your organizing strategies. 6-43

8 Cycle 6 As you work, record your ideas for organizing the cards in the space below. The objective is not necessarily coming to a right answer, but rather to utilize emerging patterns to develop strategies in this process of solving a problem. Did your group have spaces or holes in your element card organization? Participate in a class discussion and share your organizing strategies. STEP 2. Your instructor will now distribute the element card sort mat. This mat most closely resembles Mendeleev s Periodic Table, but with some additions and deletions. Compare your organization of the element cards with the element card sort mat. How is your organization similar? How is it different? STEP 3. Rearrange your cards to match the organization shown on the element card sort mat. As you examine this new arrangement of the element cards, think about and answer the following questions. 6-44

9 Activity 3: Elements and the Periodic Table List as many patterns as you can find that occur horizontally. Left to right trends: atomic number increases, atomic mass increases, combining power increases from 1-4 then decreases from 4-1, the ratio of Cl to element increases to 1-3, the ratio of H to element decreases from 4-1, conductivity is > 1 for metals and metalloids and extremely low (<< 1) for nonmetals List as many patterns as you can find that occur vertically. Top to bottom trends: atomic number increases, atomic mass increases, combining power is the same for a column, melting/boiling pt decreases for columns 1-4, melting/boiling pt increases for columns 5-8, density increases, hardess decreases (softness increases), reactivity increases for columns 1-6 but decreases for columns 7, for columns 7 and 8 conductivity decreases. At room temperature, what physical state are most of the elements in? Solid metals Where are most of the gases located on the periodic table? Right side, to the right of the metalloids Where are the metals located on the periodic table? The nonmetals? What divides them? The metalloids divide metals and nonmetals. Metals are to the left of the metalloids, nonmetals are to the right of the metalloids. Where are the most highly reactive elements located on the periodic table? The least reactive elements? Column 1 increasing down the group and Column 7 decreasing going down the group. What do you notice about the differences in relative atomic mass between calcium and gallium, and between strontium and indium? How does this compare to the differences in relative atomic mass between other elements such as nitrogen and oxygen, or sodium and magnesium? Calcium and gallium/strontium and indium have a difference of ~20. The other elements differ by ~

10 Cycle 6 Mendeleev left blanks on his periodic table because he did not force the known elements to fit any preconceived pattern. These blanks allowed him to make predictions of the chemical and physical properties of undiscovered elements, which guided the search for these new elements. STEP 4. Compare the element cards with Mendeleev s Periodic Table. Think about and answer the questions. Mendeleev s periodic table has blanks for =68 and =72. Make careful comparisons in the arrangements of both tables. What element does =68 correspond to in your element cards? (Remember, his atomic masses were very crude estimates!) Gallium Are there elements in Mendeleev s periodic table with atomic masses between those of calcium and gallium (Mendeleev s =68), and between strontium and indium? If so, where did Mendeleev put them? Yes. He put 10 elements between calcium and gallium, in rows 4 and 5. He put 10 elements between strontium and indium, in rows 6 and 7. He put four in group 8, and one of those same elements he put in group 8 is also put in group 1. Where might you put these additional elements in the element cards? Explain your reasoning. If we made a big gap between columns 2 and 3, we could put ten elements across. STEP 5. Your instructor will now distribute the modern periodic table. Compare the element cards and Mendeleev s periodic table to the modern periodic table. Think about and answer the questions. Where are the additional elements found in the modern periodic table? Explain your reasoning. Ten columns between Group 2A and Group 3A. Would you expect these elements to be gases, liquids, or solids? Why? Solid metals, since they are left of the metalloids. 6-46

11 Activity 3: Elements and the Periodic Table Your element cards and the modern periodic table include a vertical column (8A) of inert (nonreactive) gases. Why might Mendeleev not have included any of the inert gases in his table? If they are colorless, odorless, and nonreactive, how would he have known that they existed? He probably had no way to detect them. According to the modern Periodic Table, what element corresponds to the blank between gallium (Mendeleev s =68) and arsenic in Mendeleev s Periodic Table? Germanium In the space below, create an element card that predicts the properties and chemical interactions of this element. Students should come up with something like: Moderately hard/soft, gray solid atomic mass: ~73 Metalloid Density: ~5 g/cm 3 Melting point: ~ K Boiling point: ~ K Conductivity: responses may vary Reacts very slowly with oxygen Forms GeH 4 gas Combining power: 4 The Periodic Table Each vertical column on the periodic table is known as a group, or family, of elements that have similar physical and chemical properties. A number has been assigned to each group. The elements included in the eight groups in your card sort are considered representative elements. They are often given group numbers of 1A-8A, or in Roman numerals IA-VIIIA, increasing from left to right across the modern periodic table. The elements in the ten groups in the center of the modern periodic table are the transition elements and are designated with the letter B after their group number. Each horizontal row is known as a period. The number of the period increases from top to bottom on the modern periodic table. Notice that the number of elements in a period increases going down the table. While Mendeleev used groups and periods in his original periodic table, it wasn t until scientists better understood atom structure that they could make full use of these organizational schemes. 6-47

12 Cycle 6 The metals and nonmetals are divided by a stairstep line. The metals are to the left of the line and the nonmetals are to the right of the line. The elements that border the line on either side (except for aluminum) are metalloids (also called semimetals). There are several named groups of representative elements: Group 1A (IA): The alkali metals are soft shiny metals, good conductors of heat and electricity, and have relatively low melting and boiling points. Elements in this group interact quite vigorously with water, and form white waxy products when they interact with oxygen. Even though it is at the top of Group 1A, hydrogen is not considered an alkali metal. It is often positioned above the alkali metals because of similarities in atom structure, as we will see in Activity 4. Group 2A (IIA): The alkaline earth metals have similar properties to the alkali metals except that they are not as reactive. Group 7A (VIIA): The halogens are all nonmetals. These elements, especially fluorine and chloride, are very reactive and form compounds with most of the elements. When halogens react with metals, they form salts. For example, when sodium and chlorine interact, they form sodium chloride. Group 8A (VIIIA): The noble gases are nonreactive and hardly ever combine with other elements. Vibrant colors are visible when electric current is passed through some noble gases, but they are colorless at ambient (room) conditions. The lanthanide series and actinide series (also known as rare earth elements) are often considered families because they are found together in minerals and share so many common properties that they are often difficult to separate and distinguish from one another. They are a special set of transition elements, are silvery grey shiny metals that tarnish quickly in air, and are highly conductive. Summarizing Questions 6-48 Discuss these questions with your group and note your ideas. Leave space to add any different ideas that may emerge when the whole class discusses their thinking. S1. In Exploration #2 you predicted the properties and interactions of germanium, an element that was undiscovered in Mendeleev s day

13 Activity 3: Elements and the Periodic Table but predicted in his Periodic Table. The card below lists the properties and interactions of germanium: moderately soft, silvery white solid density at 298K:5.323 g/cm 3 melting point: 1211 K boiling point: 3093 K conductivity: 60 W m -1 K -1 reacts very slowly with oxygen in air forms GeH 4 gas combining power: 4 Germanium, Ge atomic mass: (a) How do your predictions compare to the information listed on the card? (b) Imagine that you and your classmates experimentally determined that it germanium did not react with hydrochloric acid or copper chloride solutions. Would you classify Germanium as a metal, nonmetal, or metalloid? Explain your reasoning. Metalloid, it has physical properties of metals, but chemical properties of nonmetals. S2. In the space below, create an element card that predicts the properties and interactions for Cesium, Cs (atomic mass 132.9). Students should be able to deduce something like the following: extremely soft, silvery solid metal density at 298K:> 1.5 g/cm 3 melting point: K boiling point: K conductivity: W m -1 K -1 Cesium, Cs atomic mass: reacts explosively with oxygen in air forms CsCl solid combining power:

14 Cycle 6 You may wish to have students perform a web search to find the exact quantities for density, melting point, boiling point, and conductivity (given below). The following site is an adequate reference. S3. Do elements in vertical columns of the Periodic Table generally have similar or different properties and interactions? What evidence/examples do you have? Similar: some elements are reactive with oxygen or with metals but to varying degrees, elements in a column form compounds with same ratios and have same combining power S4. Do elements in horizontal rows of the Periodic Table generally have similar or different properties and interactions? What evidence/examples do you have? Different: reactivities differ greatly, compound ratios and combining power change from element to element, physical properties (appearance, harness, melting/point, conductivity, etc.) may differ greatly from element to element 6-50