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Average Atomic Mass. Average of Isotopes. Every element typically exists in several isotope forms Isotopes are atoms of the same element with different numbers of neutrons
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Average of Isotopes • Every element typically exists in several isotope forms • Isotopes are atoms of the same element with different numbers of neutrons • Even though there are several isotopes of the same element, we only see one square given to each element on the periodic table
The mass recorded on the periodic table is a weighted average of all the naturally occurring isotopes of the element
Example • Chlorine is found in two isotope forms. One has a mass of 34.97 amu, the other a mass of 36.97 amu. The first makes up 75.5% of the sample. What is the average atomic mass of the chlorine?
To solve, multiply the mass of each isotope with its percent abundance, then add • (34.97)(.755) + (36.97)(.245) = 35.5 amu • The average atomic mass is 35.5 amu
Example 2 • Zinc has 5 stable isotopes as shown 48.89% 63.9291 amu 27.81% 65.9260 amu 4.11% 66.9271 amu 18.54% 67.9249 amu 0.62% 69.9253 amu Find the average atomic mass of Zn.