Lesson 9.3 Naming and Writing Formulas for Molecular Compounds

Transcription

1 9.3 Lesson 9.3 Naming and Writing Formulas for Molecular Compounds Objectives Apply the rules for naming and writing formulas for binary molecular compounds. Lesson Links Ch. 9 Core TR: Section 3 Review Chapter 9 Online Student Edition 9.3 Lesson Overview (PowerPoint file) Study WB Chapter 9 Lesson 3 Overview/Materials Overview In this lesson you will cover the guidelines used to write the names and formulas of binary molecular compounds. Standard 1 Block 0.5 Standard There are no items. Chemistry & You Engage Have students read the Chemistry & You text on p Chapter 9 Online Student Edition Ask Do you know of any chemicals that use prefixes in their names? (Sample answers: carbon dioxide, carbon monoxide) Point out that these compounds have prefixes attached to the second component of the chemical, but that prefixes can also be attached to the first component as well. Have students speculate as to what information the prefixes provide about the compounds students named at the beginning of this activity. page 1 of 5

2 Activate Prior Knowledge Engage Remind students that, earlier in this course, they encountered metric prefixes such as milli-, centi- and -kilo. Point out that both - milli and -centi represent fractional amounts, while kilo- represents a whole number amount. Explain that, in this lesson, students will encounter prefixes that represent small whole numbers. Explain Explain how the order of the elements in the names of most molecular compounds is established. Point out that ordinarily, the less electronegative element appears first in the name. For example, a compound containing carbon and sulfur is carbon disulfide (CS2). Tell students that since carbon is less electronegative than sulfur, it appears first. Remind students that electronegativity increases from left to right on the periodic table and decreases from top to bottom. Explain that the first element in the name and formula is the element further to the left. Ask In the compound disulfur dichloride, S2Cl2, why does the sulfur appear first? ( Sulfur is less electronegative than chlorine.) Explain that for elements in the same column, the element nearer the bottom of the table typically appears first, for example, iodine heptafluoride (IF7). Explore Use a class activity to have students convert the subscripts in a formula to prefixes in the name. Have students make a chart with three columns: element name, number of atoms, and prefix. Have them fill in the columns for page 2 of 5

3 N2O and then name the compound. For example: Nitrogen: 2: di- Oxygen: 1: mono- The name is dinitrogen monoxide. Write the molecular formulas for other compounds on the board and have students add them to their charts. Some possibilities are CCl4, PBr5, and P4S3. Explain Discuss how some families name a male offspring the same name as his father. Explain that this is noted by the person's full name followed by either the word "junior," or by the Roman number of his position in the family. For instance Eric Philip Thomas Junior indicates that he is the second male in his family with the name Eric Philip Thomas; whereas, Eric Philip Thomas V indicates that he is the fifth male in his family with this name. Explain that when a person's name is original to the family, such as Wyatt Mitchell Thomas, his name is not followed by a "1st," a Roman numeral I, or the words "the original." When a number or a word does not follow a name, then it is customarily understood that the name is the first occurrence for a male in a family. Use this scenario to help explain why the prefix mono- is not used to indicate one atom of the first element in a binary molecular compound. Explain that when a prefix does not appear on the first element in a binary compound, it is customarily understood that there is only one atom. It would be redundant to add the prefix. Extend Remind students that the Mohs scale rates the hardness of various minerals on a scale of 0 to 10. Discuss that silicon carbide is a binary molecular compound that has a Mohs scale rating of 9.3. Have students research this compound and what its rating means in page 3 of 5

4 terms of its utility in industry. Assess and Remediate Evaluate Write each of the following pairs of compounds on an index card. Chapter 9 Online Student Edition PBr3 and CrBr3 N2O and Na2O Cl4 and PbI4 P2O3 and Fe2O3 Ask groups of four students to approach your desk. Have each student select one card and identify what the two compounds have in common. (Each pair has one element in common.) Then have each student identify which compound in the pair is a binary molecular compound and which one is a binary ionic compound and name each compound. Then have students complete the 9.3 Lesson Check. Remediate Review the guidelines for naming binary molecular compounds. Then, have students write the names of the following compounds: NO, SiO2, N2O4, and ClF3. ( nitrogen monoxide, silicon dioxide, dinitrogen tetroxide, and chlorine trifluoride) have students write the formulas for these compounds: arsenic pentachloride, iodine tribromide, and tetraphosphorus hexoxide. (AsCl5, IBr3, and P4O6) Differentiated Instruction Struggling Students Provide students with a copy of Table 9.4, but include an additional column so students can write a couple of examples of compounds for each row. Instruct students to use the table as a study guide. Study WB Chapter 9 Lesson 3 Ch. 9 Core TR: Section 3 Review Table 9.4 Less Proficient Readers page 4 of 5

5 Have students write out steps for writing formulas for molecular compounds in their own words. Encourage them to use symbols and examples as needed. Advanced Students Increasingly large quantities of the binary molecular compound CO2 are released into the atmosphere annually by the burning of fossil fuels. Once in the atmosphere, carbon dioxide traps infrared energy near Earth's surface. As a result, some scientists predict that over time global temperatures will rise. Ask students to learn more about global warming and prepare an oral or written report or a poster that explains the current understanding or perception of this topic. Focus on ELL Frontload the Lesson Preview the prefixes in Table 9.4. Ask student pairs to make a list of familiar words that use these prefixes. Have each pair share their words (and meanings) with the class. Students should add to their list when another pair states a word that is not on their list. Have students keep these examples in mind as they proceed with the lesson. Study WB Chapter 9 Lesson 3 Ch. 9 Core TR: Section 3 Review Ch. 9 Core TR: Vocabulary Review My Notes Homework page 5 of 5