Chapter 4 Atoms and Elements 4.5 Isotopes and Atomic Mass 24Mg 25Mg 26Mg 12 12 12 Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Isotopes Isotopes Are atoms of the same element that have different mass numbers. Have the same number of protons, but different numbers of neutrons.

Nuclear Symbol A nuclear symbol Represents a particular atom of an element. Gives the mass number in the upper left corner and the atomic number in the lower left corner. Example: An atom of sodium with atomic number 11 and a mass number 23 has the following atomic symbol: mass number 23 Na atomic number 11

Information from Nuclear Symbols The nuclear symbol indicates the number of protons (p+), neutrons, (n), and electrons (e-) in a particular atom. 16 31 65 O P Zn 8 15 30 8 p+ 15 p+ 30 p+ 8 n 16 n 35 n 8 e- 15 e- 30 e-

Learning Check Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following. 12C 13C 14C 6 6 6 protons ______ ______ ______ neutrons ______ ______ ______ electrons ______ ______ ______

Solution Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following. 12C 13C 14C 6 6 6 protons 6 p+ 6 p+ 6 p+ neutrons 6 n 7 n 8 n electrons 6 e- 6 e- 6 e-

Learning Check A. 8 p+, 8 n, 8 e- ___________ Write the nuclear symbols for atoms with the following subatomic particles: A. 8 p+, 8 n, 8 e- ___________ B. 17p+, 20n, 17e- ___________ C. 47p+, 60 n, 47 e- ___________

Solution Write the nuclear symbols for atoms with the following subatomic particles: 16O A. 8 p+, 8 n, 8 e- 8 B. 17p+, 20 n, 17e- 37Cl 17 C. 47p+, 60 n, 47 e- 107Ag 47

Learning Check 1. Which of the following pairs are isotopes of the same element? 2. In which of the following pairs do both atoms have 8 neutrons? A. 15X 15X 8 7 B. 12X 14X 6 6 C. 15X 16X 7 8

Solution 1. B. 12X 14X 6 6 Both nuclear symbols represent isotopes of carbon with six protons each, but one has 6 neutrons and the other has 8. 2. C. 15X 16X 7 8 The atom of nitrogen (7) and the atom of oxygen (8) each have 8 neutrons.

Atomic Mass Na 22.99 The atomic mass of an element Is listed below the symbol of each element on the periodic table. Gives the mass of an “average” atom of each element compared to 12C. Is not the same as the mass number.

Isotopes of Magnesium In naturally occurring magnesium, there are three isotopes. Table 4.7 24Mg 25Mg 26Mg 12 12 12 Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Examples of Isotopes and Their Atomic Masses Most elements have two or more isotopes that contribute to the atomic mass of that element. Table 4.8 Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Calculating Average Weight of “da pineapple” The calculation for the average weight of a pineapple requires Percent(%) abundance of each kine of pineapple. Atomic mass of each isotope of that element. Sum of the weighted averages. mass isotope(1) x (%) + mass isotope(2) x (%) + … 100 100

Calculating Atomic Mass The calculation for atomic mass requires the Percent(%) abundance of each isotope. Atomic mass of each isotope of that element. Sum of the weighted averages. mass isotope(1) x (%) + mass isotope(2) x (%) + … 100 100

Atomic Mass of Magnesium The atomic mass of Mg Is due to all the Mg isotopes. Is a weighted average. Is not a whole number. Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Calculating Atomic Mass Isotope Mass Abundance 24Mg = 23.985 amu x 78.70/100 = 18.88 amu 25Mg = 24.986 amu x 10.13/100 = 2.531 amu 26Mg = 25.983 amu x 11.17/100 = 2.902 amu Atomic mass (average mass) Mg = 24.31 amu Mg 24.31

Learning Check Using the periodic table, specify the atomic mass of each element: A. calcium __________ B. aluminum __________ C. lead __________ D. barium __________ E. iron __________

Solution Using the periodic table, specify the atomic mass of each element: A. calcium 40.08 amu B. aluminum 26.98 amu C. lead 207.2 amu D. barium 137.3 amu E. iron 55.85 amu

Atomic Mass for Cl The atomic mass of chlorine is the weighted average of two isotopes 35Cl and 37Cl. Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Calculating Atomic Mass for Cl 35Cl has atomic mass 34.97 (75.78%) and 37Cl has atomic mass 36.97 (24.22%). The atomic mass and percent of each isotope are used to calculate the contribution of each isotope to the weighted average. 34.97 x 75.78 = 26.50 amu 100 36.97 x 24.22 = 8.953 amu 100 35.45 amu The sum is the weighted average or atomic mass of Cl. 35.45 amu

Learning Check Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.11% of 69Ga (atomic mass 68.93) atoms and 39.89% of 71Ga (atomic mass 70.93) atoms. What is the atomic mass of gallium?

Solution 69Ga 68.93 amu x 60.11 = 41.43 amu (from 69Ga) 100 71Ga Atomic mass Ga = 69.72 amu 31 Ga 69.72