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4.3 Distinguishing Among Atoms

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chem_TE_ch04.fm Page 110 Saturday, April 15, 2006 10:00 AM 4.3 4.3 Distinguishing Among Atoms 1 FOCUS Fruits and vegetables come in Objectives Guide for Reading different varieties. For example, a grocery store might sell three varieties 4.3.1 Explain what makes elements of apples: Granny Smith, Red Delicious, and Golden Key Concepts Delicious. Apple varieties can differ in color, and isotopes different from • What makes one element size, texture, aroma, and taste. Just as apples each other. different from another? • How do you find the number of come in different varieties, a chemical ele- 4.3.2 Calculate the number of neu- neutrons in an atom? ment can come in different “varieties” trons in an atom. • How do isotopes of an element called isotopes. In this section, you will differ? 4.3.3 Calculate the atomic mass of • How do you calculate the learn how one isotope of an element atomic mass of an element? differs from another. an element. • Why is a periodic table useful? 4.3.4 Explain why chemists use the Vocabulary periodic table. atomic number Atomic Number mass number Atoms are composed of protons, neutrons, and electrons. Protons and neu- isotopes trons make up the nucleus. Electrons surround the nucleus. How, then, are Guide for Reading atomic mass unit (amu) atoms of hydrogen, for example, different from atoms of oxygen? Look at atomic mass Table 4.2. Notice that a hydrogen atom has one proton, but an oxygen atom Build Vocabulary L2 periodic table has eight protons. Elements are different because they contain differ- period ent numbers of protons. Graphic Organizer Have students use group The atomic number of an element is the number of protons in the the vocabulary for this section to build Reading Strategy nucleus of an atom of that element. Because all hydrogen atoms have one a concept map that links and relates Building Vocabulary As you proton, the atomic number of hydrogen is 1. Similarly, because all oxygen the terms. read the section, write a definition atoms have eight protons, the atomic number of oxygen is 8. The atomic of each vocabulary term in your number identifies an element. For each element listed in Table 4.2, the own words. Reading Strategy L2 number of protons equals the number of electrons. Remember that atoms Visualize Have students revisit Figure are electrically neutral. Thus, the number of electrons (negatively charged particles) must equal the number of protons (positively charged particles). 4.10 when they are tackling Sample Prob- lem 4.2. Ask how the visual relates to the Table 4.2 problem. Atoms of the First Ten Elements 2 Atomic Mass Number of INSTRUCT Name Symbol number Protons Neutrons* number electrons Hydrogen H 1 1 0 1 1 Helium He 2 2 2 4 2 Lithium Li 3 3 4 7 3 Have students study the photograph Beryllium Be 4 4 5 9 4 and read the text that opens the sec- Boron B 5 5 6 11 5 Carbon C 6 6 6 12 6 tion. Ask, What characteristics can Nitrogen N 7 7 7 14 7 you use to classify different apples? Oxygen O 8 8 8 16 8 (Sample answers: color, size, taste, tex- Fluorine F 9 9 10 19 9 ture, cooking qualities) Which of these Neon Ne 10 10 10 20 10 *Number of neutrons in the most abundant isotope. Isotopes are introduced later in Section 4.3. involve the apple’s chemistry? (taste, cooking qualities) 110 Chapter 4 Atomic Number Use Visuals L1 Table 4.2 Point out that the atomic Section Resources number is equal to the number of pro- tons for each element. Ask, Why must Print Technology the number of electrons equal the •Guided Reading and Study Workbook, •Interactive Textbook with ChemASAP, number of protons for each ele- Section 4.3 Problem-Solving 4.15, 4.17, 4.20, 4.21, ment? (Atoms are electrically neutral.) •Core Teaching Resources, Section 4.3 4.24, Assessment 4.3 What is the relationship between Review •Go Online, Section 4.3 the number of protons and the num- •Transparencies, T48–T56 ber of neutrons? (The number of neu- •Small-Scale Chemistry Laboratory trons tends to rise with the number of Manual, Lab 6 protons.) 110 Chapter 4 chem_TE_ch04_FPL.fm Page 111 Thursday, August 5, 2004 3:59 AM CONCEPTUAL PROBLEM 4.1 CONCEPTUAL PROBLEM 4.1 Understanding Atomic Number Answers The element nitrogen (N), shown here in liquid form, has an atomic 15. a. 19 b. B c. 5 d. 5 e. 16 f. 16 number of 7. How many protons and electrons are in a neutral nitro- g. 23 h. 23 gen atom? 16. a. 9 protons and 9 electrons Analyze Identify the relevant concepts. Solve Apply the concepts to this problem. b. 20 protons and 20 electrons The atomic number gives the number of The atomic number of nitrogen is 7, which c. 13 protons and 13 electrons protons, which in a neutral atom equals the means that a neutral nitrogen atom has Practice Problems Plus L2 number of electrons. 7 protons and 7 electrons. Chapter 4 Assessment problem 47 is Practice Problems related to Conceptual Problem 4.1. 15. Complete the table. 16. How many protons and electrons are in each Atomic atom? Mass Number Element number Protons Electrons a. fluorine (atomic number ϭ 9) Discuss L2 K19(a) 19 b. calcium (atomic number ϭ 20) (b) (c) (d) 5 c. aluminum (atomic number ϭ 13) Discuss why the mass number of an S 16 (e) (f) element is defined as the number of V (g) 23 (h) protons and neutrons. Explain that chemists have arbitrarily assigned a Problem-Solving 4.15 Solve value of one atomic mass unit to Mass Number Problem 15 with the help of an interactive guided tutorial. represent the mass of one twelfth of a You know that most of the mass of an atom is concentrated in its nucleus and depends on the number of protons and neutrons. The total number of withChemASAP carbon-12 atom. Ask, How do you find protons and neutrons in an atom is called the mass number. Look again at the number of neutrons in an atom? Table 4.2 and note the mass numbers of helium and carbon. A helium atom (Subtract the atomic number from the has two protons and two neutrons, so its mass number is 4. A carbon atom, mass number.) which has six protons and six neutrons, has a mass number of 12. If you know the atomic number and mass number of an atom of any element, you can determine the atom’s composition. Table 4.2 shows that an oxygen atom has an atomic number of 8 and a mass number of 16. Because the atomic number equals the number of protons, which equals the number of electrons, an oxygen atom has eight protons and eight elec- trons. The mass number of oxygen is equal to the number of protons plus the number of neutrons. The oxygen atom, then, has eight neutrons, which is the difference between the mass number and the atomic number (16 Ϫ 8 ϭ 8). The number of neutrons in an atom is the difference Figure 4.8 Au is the chemical between the mass number and atomic number. symbol for gold. Applying Number of neutrons ϭ mass number Ϫ atomic number Concepts How many electrons does a gold atom have? The composition of any atom can be represented in shorthand nota- tion using atomic number and mass number. Figure 4.8 shows how an atom of gold is represented using this notation. The chemical symbol Au appears with two numbers written to its left. The atomic number is the subscript. The mass number is the superscript. 197 You can also refer to atoms by using the mass number and the name 197 79 Au of the element. For example, 79 Au may be written as gold-197. Checkpoint How do you calculate mass number? Section 4.3 Distinguishing Among Atoms 111 Differentiated Instruction Less Proficient Readers L1 Have students make a list of familiar elements and describe at least one use for each element. Have students combine their lists into a master list on the chalkboard. Answers to... Figure 4.8 79 electrons Checkpoint Mass number = number of neutrons + atomic number Atomic Structure 111 chem_TE_ch04_FPL.fm Page 112 Thursday, August 5, 2004 3:59 AM Section 4.3 (continued) SAMPLE PROBLEM 4.1 Determining the Composition of an Atom Sample Problem 4.1 How many protons, electrons, and neutrons are in each atom? Answers Atomic number Mass number 17. a. 8 b. 16 c. 61 d. 45 e.125 a. Beryllium (Be) 4 9 b. Neon (Ne) 10 20 12C 19 9 18. a. 6 b. 9F c. 4Be c. Sodium (Na) 11 23 Practice Problems Plus L2 Analyze List the knowns and the unknowns. The element strontium has four iso- Knowns Unknowns topes, strontium-84, strontium-86, • atomic number • number of protons ϭ ? strontium-87, and strontium-88. Given • mass number • number of electrons ϭ ? that strontium has an atomic num- • number of neutrons ϭ ? ber of 38, how many neutrons are in Use the definitions of atomic number and mass number to calculate each of these isotopes? (46, 48, 49, 50) the numbers of protons, electrons, and neutrons. Calculate Solve for the unknowns. Isotopes number of electrons ϭ atomic number a. 4 b. 10 c. 11 number of protons ϭ atomic number CLASS Activity a. 4 b. 10 c. 11 number of neutrons ϭ mass number-atomic number Applications of Isotopes L2 a.
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