Isotopes and Atomic Mass Atoms and Elements 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 isotope of an element. 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 can help you find the number of protons (p+), neutrons, (n), and electrons (e-) in a particular atom. 16 31 65 O P Zn 8 15 30 In atoms, the protons = the electrons. If you know one, you know the other. Mass number – # protons = neutrons (so neutrons + protons = Mass number)

Information from Nuclear Symbols 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 ______ ______ ______ (notice the ONLY difference in the nuclear symbols is the mass number)

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 Which of the following pairs are isotopes of the same element? 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.

Learning Check 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 2. C. 15X 16X 7 8 The atom of nitrogen (7) and the atom of oxygen (8) each have 8 neutrons. We know they are nitrogen and oxygen because nitrogen is 7 on the PT and oxygen is 8 on the PT.

Atomic Mass…a bit more complicated Na 22.99 Atomic Mass…a bit more complicated Atomic Mass is the average mass of all isotopes of an element. Its rarely a whole number ATOMIC MASS vs. MASS NUMBER Mass Number is the mass of one particular isotope. Its always a whole number 12 x 0.98892 + 13 x 0.01108

Calculating Atomic Mass The calculation for atomic mass requires multiplying the… Percent(%) abundance of each isotope. Mass of each isotope of that element. Then add the results for each isotope to get the atomic mass for that element: [Mass isotope 1 x (% ) ] + [Mass isotope 2 x (% )] (100) (100) IMPORTANT: to turn a percent into a decimal, just move the decimal point two places to the left…avoid the % / 100 calculation.

Atomic Mass of Magnesium The atomic mass of Mg on the periodic table… The most abundant isotope contributes the most; least abundant contributes the least Can’t be found by just adding the masses and dividing by 3…is not a “straight average”

Correctly Calculating Atomic Mass Isotope Mass Abundance 24Mg = 24 amu x .7870 = 18.89 amu 25Mg = 25 amu x .1013 = 2.53 amu 26Mg = 26 amu x .1117 = + 2.90 amu Atomic mass (average mass) Mg = 24.32 amu Wrong way: 24+ 25 + 26 amu 3 isotopes = 24.98 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. 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 35 (75.78%) and 37C has atomic mass 37 (24.22%). The atomic mass and percent of each isotope are used to calculate the contribution of each isotope to the weighted average. 35 x .7578 = 26.52 amu 37 x .2422 = + 8.96 amu 35.48 amu The sum is the weighted average or atomic mass of Cl. 35.45 amu on the Periodic Table

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 .6011 = 41.43 amu (from 69Ga) 71Ga Atomic mass Ga = 69.72 amu 31 Ga 69.72