Part I: Atomic Inventory and Classification of Matter
Objectives and Strategies
The first part of the unit, which addresses atomic inventory and classification of matter, will be presented at beginning of the course where students are being reacquainted with the elements and the periodic table. Although students have discussed elements in previous courses, upon entering chemistry class, most do not make a connection to the Earth systems and living systems that they have explored in previous science courses. Students will investigate the primary elements in the Earth's crust, atmosphere, oceans and in living things. Through this investigation, they will gain familiarity with the periodic table and review the factors that make the elements different (protons, neutrons and electrons). In addition, we will compare and contrast elements, compounds and mixtures as they relate to Earth and biological systems. The purpose of this section is to build a bridge between the students' prior knowledge and to begin to give the students a relationship with the Earth and build the foundation for understanding climate change.
Many students fail to realize that all matter, from the soil they are standing on, to the air that they are breathing, is composed of elements. In addition, the complex systems of the Earth and life are highly dependent upon a handful of very important elements. Most students do not know that their own bodies are primarily carbon, nitrogen, oxygen, phosphorus, sulfur and hydrogen. They are baffled to find that the same carbon that is present in their bodies is "the same" carbon that they would find in a piece of coal or in a tree. This leads to the question of what makes elements the same and what makes them different. This question will serve as the basis for the introduction to atomic inventory where students will learn that the primary similarities and differences between elements can be traced to the number of protons, neutrons and electrons. When asked to look at the periodic table through this lens, the intimidating chart on the wall becomes a living tool for understanding the world in which we live.
The poetic nature of science will be reinforced through the use of Primo Levi's fictional piece, The Periodic Table [1]. Primo Levi was a Jewish Italian chemist who suffered through the Holocaust and used his writing as an outlet for his internal turmoil that ultimately resulted in his suicide. Although most of Levi's writings are centered on his experiences in Auschwitz, The Periodic Table is slightly more autobiographical in nature as he uses the nature of several elements in the periodic table as metaphors for various portions of his life story.
My students will focus on the "Carbon" chapter in Levi's book. In this chapter, Levi traces an atom of carbon through the carbon cycle. It begins with carbon as a component of limestone and chronicles its journey into the air in the form of carbon dioxide, its entry into the living world through the process of photosynthesis, continuing through the body in the form of glucose, and ultimately its return to the Earth as a result of the inevitable demise of the host.
The Periodic Table will be a difficult read for most students and will need to be supported through pre-reading and guided reading activities. Although the battle will be arduous, it is well worth the effort. Reading this piece not only makes a tie to biology-based chemistry but also to literature. It allows the reader to take a glimpse into the emotional and spiritual nature of the relationship between a chemist and his life's work through metaphors that transcend the communicative capabilities of a traditional lecture on the periodic table.
In addition to using the Earth to demonstrate the nature of the elements, the Earth can also be used to explain how matter is classified. Matter is classified into two categories: mixtures and pure substances; where pure substances are elements and compounds and mixtures are physical combinations of the latter. Through the discussions of The Periodic Table, students should already be familiar with several elements and have been exposed to compounds including calcium carbonate, carbon dioxide, and glucose. This discussion can be extended to mixtures on the Earth. Air makes a great example of a homogeneous mixture of nitrogen, oxygen, argon and carbon dioxide because it includes elements and compounds so students will be less likely to develop a misconception that homogeneous mixtures are limited to only elements or compounds. Another great example of mixtures in Earth systems is a heterogeneous mixture such as granite.
In support of the mixture portion of the unit, students will learn how to use paper chromatography to separate the ink in markers into the component dyes, and then apply this technique to separating the pigments present in a leaf. Leaves appear green because the pigment chlorophyll reflects green light. Other pigments that are often present and masked by the green are carotenoids (orange), xanthophylls (yellow), and anthocyanins (red, purple). These colorful compounds differ in both size and polarity, and in the presence of the right solvent will make their way up the strip of chromatography paper at different rates and separate into distinct colorful bands [2]. This is quite a popular separation and many students may have performed it in middle school. However, students of all ages love pretty colors and the discussion can be extended into more sophisticated topics such as, conjugated double bonds, the structures and chemical properties of the pigments, the electromagnetic spectrum and polarity.
Instructional Plan
This section of the unit should take approximately two weeks to teach. It is important that at this stage students get the time to explore the elements and their properties so that they begin to build a personal relationship with chemistry. In addition to the group discussions around all the topics in the unit, students will read Primo Levi's Periodic Table chapter on carbon and complete an accompanying worksheet to scaffold the reading. After reading the story, students will research an element that we have discussed and write a story about the "life" of that element using the Levi text as a model. The pre-reading, during reading, and after reading activities can be found in Sample Activity 1 at the end of this unit.
After discussing classification of matter, students will first separate the dyes of water-soluble markers into their components [3]. They will then apply the same concept to complete a separation of pigment mixtures in a leaf through paper chromatography [4,5,6].
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