Isotopes and Atomic Mass Atoms and Elements 3.6 Isotopes and Atomic Mass 24Mg 25Mg 26Mg 12 12 12 Copyright © 2009 by Pearson Education, Inc.

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

Atomic Symbol An atomic 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 Atomic Symbols The atomic symbol for a specific atom of an element gives the number of protons (p+), number of neutrons (n), and number of electrons (e-). 4

Information from Atomic Symbols Examples of number of subatomic particles for atoms Atomic symbol 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 12C 13C 14C 6 6 6 protons 6 p+ 6 p+ 6 p+ neutrons 6 n 7 n 8 n 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 Write the atomic 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 A. 8 p+, 8 n, 8 e- 16O 8 B. 17p+, 20 n, 17e- 37Cl 17 C. 47p+, 60 n, 47 e- 107Ag 47

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

Solution B. 12X 14X 6 6 The atomic symbols in “B.” represent isotopes of carbon with 6 protons each, but one has 6 neutrons and the other has 8. C. 15X 16X 7 8 These isotopes of nitrogen and oxygen have 8 neutrons.

Isotopes of Magnesium

Isotopes of Magnesium

Copyright © 2009 by Pearson Education, Inc. Isotopes of Sulfur A sample of naturally occurring sulfur contains several isotopes with the following abundances Isotope % abundance 32S 95.02 33S 0.75 34S 4.21 36S 0.02 32S, 33S, 34S, 36S 16 16 16 16 Copyright © 2009 by Pearson Education, Inc.

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. Na 22.99

Isotopes of Some Elements and Their Atomic Mass Most elements have two or more isotopes that contribute to the atomic mass of that element.

Atomic Mass for Cl The atomic mass of chlorine is due to all the Cl isotopes. not a whole number. the average of two isotopes: 35Cl and 37Cl.

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

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 of isotope(1)x (%) + mass of isotope(2) x (%) + 100 100

Calculating Atomic Mass for Cl 35Cl has atomic mass 34.97 amu (75.76%) and 37C has atomic mass 36.97 amu (24.24%). Use atomic mass and percent of each isotope to calculate the contribution of each isotope to the weighted average. 34.97 x 75.76 = 26.49 amu 100 36.97 x 24.24 = 8.961 amu Sum is atomic mass of Cl 35.45 amu

Calculating Atomic Mass Mg Isotope Mass Abundance 24Mg = 23.99 amu x 78.70/100 = 18.88 amu 25Mg = 24.99 amu x 10.13/100 = 2.531 amu 26Mg = 25.98 amu x 11.17/100 = 2.902 amu Atomic mass (average mass) Mg = 24.31 amu

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.

Learning Check Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.10% of 69Ga (atomic mass 68.926) atoms and 39.90% of 71Ga (atomic mass 70.925) atoms. What is the atomic mass of gallium?

Solution 69Ga 68.926 amu x 60.10 = 41.42 amu (from 69Ga) 100 71Ga Atomic mass Ga = 69.72 amu 31 Ga 69.72