Ion Formation Grade Nine

Transcription

1 Ohio Standards Connection: Physical Sciences Benchmark A Describe that matter is made of minute particles called atoms and atoms are comprised of even smaller components. Explain the structure and properties of atoms. Indicator 5 Describe how ions are formed when an atom or a group of atoms acquire an unbalanced charge by gaining or losing one or more electrons. Lesson Summary: In this lesson, students will review atomic structure using two-color counters (plastic disks that are red on one side and yellow on the other) to represent positive and negative charges in the formation of ions. Estimated Duration: One hour Commentary: This lesson was designed to introduce students to an abstract concept by working with concrete models. Students may have encountered two-color counters in mathematics when studying positive and negative numbers, so it may be an easy transition to use them to represent protons and electrons. After using the counters to represent several examples of ion formation, students should find it easier to mentally determine the charge on an ion without using the models. This lesson was reviewed by educators across the state. Some of their comments were: Having students draw diagrams and then use their writing skills to describe ion formation is an excellent learning activity. Ion formation is an abstract idea. Any hands-on activity will help. 1

2 Pre-Assessment: Based on what students have previously learned, select and ask appropriate questions from the following questions. 1. What is an atom? 2. What are its parts? 3. What is an electron? 4. What is a proton? 5. What is a neutron? 6. What does it mean to charge an atom? 7. Describe the structure of atoms. 8. What does the atomic number of an atom represent? 9. What does the mass number of an atom represent? 10. What does the periodic table tell us about the atoms of various elements? 11. Can atoms lose electrons? Gain electrons? 12. What are compounds? 13. How are compounds formed? 14. What is a valence electron? 15. What is the Bohr electronic structure of an atom? 16. How is the Lewis dot configuration used to represent atoms? 17. How are ions formed? 18. What is a positive ion? 19. What is a negative ion? 20. What is a polyatomic ion? Ask some questions to ensure that students know how to add negative numbers. Provide a review as needed. Scoring Guideline: This pre-assessment is intended for the teacher s use in determining students knowledge of background information about atoms, compounds, charging and related concepts and deciding which topics need to be reviewed before or during this lesson. If students do not understand the parts of an atom, atomic number, mass number, valence electrons and electronic structure, then these topics should be taught before going into the instructional procedures of this lesson. Post-Assessment: Ask students to prepare essays and related diagrams or other graphics that describe how atoms become ions (both cations and anions). Have students include in their discussions the types and numbers of subatomic particles (electron, protons and neutrons) that affect the charge on the ion. See Attachment B, Post-Assessment, for a student handout. 2

3 Scoring Guidelines: A good response should include the following key points about each ion: 1. The number of electrons, protons and neutrons in the neutral atom; 2. The number of electrons that are lost or gained; 3. A justification for the charge on the ion; 4. A statement that indicates the neutrons have no effect on the charge. Instructional Procedures: Part I: Introductory prompt 1. Tell students the following joke. Two atoms have been traveling separately and when they meet at an intersection, they have this conversation: First atom: I think I just lost an electron. Second atom: Are you sure? First atom: Yes, I m rather positive. Second atom: I m getting a negative feeling about this! 2. Ask the students, What causes some atoms to respond this way? 3. After pausing to see how many students have the background to understand the humor, cover some scientific topics related to the simple story that extend their knowledge. Part II: Modeling Ions 4. Have students use two-color counters (plastic disks that are red on one side and yellow on the other) to represent positive and negative charges. (Two-color counters are commonly used mathematics manipulatives and may be borrowed from the mathematics department.) Similar disks that can be used on the overhead are available. Some students may benefit by marking the counters with positive or negative signs to represent positive and negative charges. 5. Ask students, What does the atomic number represent? (The number of protons or the number of electrons in a neutral atom) Then ask, What is meant when an atom is considered neutral? (There is no net charge; the number of positively charged particles is equal to the number of negatively charged particles.) 6. Have the students complete the following steps: Select an element with a small atomic number, such as lithium. Find its atomic number on the periodic table. Represent the electrons and protons with the two-color counters. (For lithium, students will use six two-color counters, three with the red side up to represent the protons and three with the yellow side up to represent the electrons.) 7. Check to see that the counters are being used correctly, and then ask, Why is lithium neutral? 3

4 8. After receiving correct answers or guiding students to the correct answers have the students complete the following steps: Have students remove one electron (one yellow disk). Ask them, Is the atom still neutral? Why or why not? What is the net charge on the atom after it lost one electron? (Positive one) What is the net charge when the neutrons of the atom are also considered? (The charge does not change because the neutrons are neutral and do not add any charge to the atom.) 9. Have students pair up the protons and electrons and point out that when the protons and electrons are paired, there is no net charge on the atom. If there are excess protons or electrons, there is a net charge and it is equal to the number of excess protons (positive) or excess electrons (negative). At this point, consider providing information about the number of electrons that lithium is most likely to lose, telling the students that in most compounds, lithium has lost only one electron. 10. Have the students complete the following steps: Select beryllium, another element with a small atomic number. Find its atomic number on the periodic table. Represent the electrons and protons with the two-color counters. (For beryllium, students will use eight two-color counters, four with the red side up to represent the protons and four with the yellow side up to represent the electrons.) 11. After learning that the students have represented beryllium correctly and view it as neutral, have students remove one electron from the beryllium atom and predict the resulting charge, and then remove a second electron and predict the charge. (Positive one; then positive two). If timely and appropriate, inform students that beryllium is most likely to lose two electrons, and that in most compounds, beryllium has lost two electrons. 12. Ask, What are charged atoms called? (Ions) Tell students that positively charged ions are called cations. 13. When students have mastered the concept of positive ion formation, shift the activity to negative ion formation by completing the following steps: Have students represent fluorine with the two-color counters. Have students add one electron to fluorine. Ask them Is the fluorine atom still neutral? Why or why not? What is the net charge on the atom after it gained one electron? (Negative one) 4

5 If timely and appropriate, inform students that fluorine atoms seek to gain electrons and, in compounds, fluorine gains one electron. You can be assured that fluorine will nearly always take an electron from any atom it meets at the corner. 14. Ask students to find the atomic number of oxygen, an element whose atoms are likely to gain electrons, and represent its number of electrons and protons with the two-color counters. 15. Take these four actions: Tell students to add one electron to oxygen. Ask them Is the oxygen atom still neutral? Why or why not? What is the net charge on the atom after it gained one electron? (Negative one) Tell them to add another electron and predict the net charge. (Negative two) If timely and appropriate, let students know that oxygen is most likely to gain two electrons and in most compounds of oxygen, the oxygen atom has gained two electrons. 16. Continue the activity with hydrogen by completing the following steps: Have students represent hydrogen with the two-color counters. Have students remove one electron from hydrogen. Ask them Is the hydrogen atom still neutral? Why or why not? What is the net charge on the atom after it lost one electron? (Positive one) 17. Have students restore hydrogen to its neutral state and then add an additional electron. Ask them Is the hydrogen atom still neutral? Why or why not? What is the net charge on the hydrogen atom after it gained one electron? (Negative one) Tell the students that hydrogen, like some other elements, may form ions by gaining an electron or by losing an electron. To add real-life examples to the lesson, ask What happens when hydrogen burns? (Two hydrogen atoms each give up an electron and one oxygen atom gains two electrons to form water.) 18. Ask, What are negatively charged atoms called? (All charged atoms are called ions, and negatively charged ions are called anions. ) 19. Point out that, so far, the class has used only elements of Periods 1 and 2 of the Periodic Table (first and second horizontal rows), and then say: How do we represent ion formation by the many other elements that are known to exist? What is the atomic number of iodine? (53) 5

6 How many colored counters would we need to represent the electrons and protons of iodine? (106) Wow! Do we have enough to deal with bromine and other elements with high atomic numbers? (Tell the students that we usually represent only those electrons that are valence electrons.) 20. Review or guide a class discussion of the following topics: Atomic structure; Bohr electronic structures; Valence electrons. Depending on the background of the students, it may be appropriate to: Have students predict how many electrons an atom is most likely to gain or lose. Guide students to an understanding of the relationships with the periodic table and the electrons an element is likely to gain or lose. 21. Choose some elements from the third, fourth and fifth periods and have the students go through abbreviated steps similar to those used above, using valence electrons. 22. Encourage students to determine the charges of the resulting ions without relying on the counters: Iodine gaining one electron (negative one); Calcium losing two electrons (positive two); Sodium losing one electron (positive one). 23. At some point, begin to ask students to use the chemical symbols for the atoms and the ions, as they continue to form the following ions: Sulfur gaining two electrons (negative two); Aluminum losing three electrons (positive three); Iron losing two electrons, then losing another electron; (positive two, then positive three); Argon What would Argon do? (Argon and the other noble gases normally do not form ions and compounds, hence the terms, noble or inert gases!) 24. To add other real-life examples to the lesson, ask What happens when sodium reacts with chlorine to form salt? (A sodium atom gives up an electron to become a Na +1 ion and a chlorine atom gains an electron to become a Cl -1 ion. The two oppositely charged ions are attracted to each other (ionic bond), forming NaCl.) Other examples are iron rusting to become iron oxide, silver tarnishing into silver sulfide and copper tarnishing (oxidizing) into cupric oxide. 25. Conclude the lesson with a review of the concepts and general points covered in class about atoms, electrons, neutral atoms, formation of ions by loss or gain of electrons, etc. 26. Give students some practice questions and ask them to review their notes and their texts; discuss what they learned and what they may not understand with fellow students; ask 6

7 questions of the teacher as needed; and prepare for an exam on a later class day. See Attachment A, Homework for Ion Formation. 27. As the class ends, give this advice to students; Be careful out there and watch your electrons! You never know what kind of atoms you may meet! 28. During the next class, collect and review the homework. Review the information and concepts as needed. Ensure that students know: Electronic structure of atoms; Valence electrons; Process by which ions are formed by atoms or groups of atoms; Ions may be positively or negatively charged; Symbols used for atoms and ions; Ions typically have charges of 3, -2, -1, +1, +2 or +3; Some elements form positive and negative ions; Noble gases normally do not form ions and compounds. 29. Ask students to reflect on how this lesson increased their understanding of atoms and ions and how they would explain to a family member how atoms become ions. Differentiated Instructional Support: Instruction is differentiated according to learner needs, to help all learners either meet the intent of the specified indicator(s) or, if the indicator is already met, to advance beyond the specified indicator(s). Students may work independently, in pairs or heterogeneous groups. Reinforce understanding of lesson vocabulary by providing accurate definitions of terms. Utilize graphic organizers to categorize information about atomic structure, Bohr electronic structures and valence electrons. Challenge students to identify an ion of interest and conduct research to share with the class. Extensions: Discuss chemical reactivity in relation to the ease of formation of ions. Discuss ionic character and the properties of ionic compounds, like solubility. Show the relationship between the periodic table and types of ions formed. Include the naming of ions in the lesson, especially the more common ones. Discuss the formation and naming of polyatomic ions. Homework Options Examine labels in home or in stores to see which ions may be in common substances. 7

8 Materials and Resources: The inclusion of a specific resource in any lesson formulated by the Ohio Department of Education should not be interpreted as an endorsement of that particular resource, or any of its contents, by the Ohio Department of Education. The Ohio Department of Education does not endorse any particular resource. The Web addresses listed are for a given site s main page, therefore, it may be necessary to search within that site to find the specific information required for a given lesson. Please note that information published on the Internet changes over time, therefore the links provided may no longer contain the specific information related to a given lesson. Teachers are advised to preview all sites before using them with students. For the teacher: Colored plastic disks for the overhead. For the students: Two-color counters (plastic disks that are red on one side and yellow on the other), periodic table. Vocabulary: atoms molecules ions elements chemical symbols cations anions electrons compounds protons neutrons nucleus orbit reactions charges inert polyatomic atomic structure valence periodic table groups periods Technology Connections: Have students visit Internet sites for practice, similar lessons, film clips, etc. Use computer animations to illustrate the process of ion formation and bonding. 8

9 Research Connections: Marzano, R. et al. Classroom Instruction that Works: Research-Based Strategies for Increasing Student Achievement. Alexandria: Association for Supervision and Curriculum Development, Nonlinguistic representations help students think about and recall knowledge. They include the following: Creating graphic representations (organizers); Making physical models; Generating mental pictures; Drawing pictures and pictographs; Engaging in kinesthetic activity. General Tips: Commercial materials are available with shapes that help students match the ions. These materials may help students learn how to form polyatomic ions and, later, to balance ionic compounds. Attachments: Attachment A, Homework for Ion Formation Attachment B, Post-Assessment 9

10 Attachment A Homework for Ion Formation 1. Use your textbook and notes to review these terms and concepts: atoms, molecules, ions, elements, chemical symbols, cations, anions, electrons, compounds, protons, neutrons, nucleus, orbit, reactions, charges, inert, polyatomic, atomic structure, valence, periodic table, groups and periods. 2. In your own words, explain to your relatives, how ions are formed by the loss or gain of electrons. 3. Write a short description of how each of the following ions might have been formed. Use any diagrams and symbols that will help. K +1, H -1, Ca +2, Fe +3, Br -1, O -2, N -3 10

11 Attachment B Post-Assessment Prepare an essay with related diagrams or other graphics that describe how the following became ions (both cations and anions). Include in your discussion the types and numbers of subatomic particles (electron, protons and neutrons) that affect the charge on the ion. Na +1, Mg +2, Fe +3, Br -1, S -2 11