Calculating Average Atomic Mass

1. Calculating Average Atomic Mass

2. Introduction • The value for atomic mass for a particular element is the average atomic mass. • Each sample of an element is composed of a mixture of different isotopes. • Therefore, we need to average the masses of the isotopes to find the average atomic mass.

3. Calculations • Copper has two common isotopes. • 63Cu is 69.15% of copper samples. • 65Cu is 30.85% of copper samples. • We would expect the average atomic mass to be somewhere in between 63 and 65.

4. Calculations • Copper has two common isotopes. • 63Cu is 69.15% of copper samples. • 65Cu is 30.85% of copper samples. • To find the average atomic mass • multiply the mass number by the percentages (as a decimal value)

5. Calculations • Copper has two common isotopes. • 63Cu is 69.15% of copper samples. • 65Cu is 30.85% of copper samples. • To find the average atomic mass • multiply the mass number by the percentages (as a decimal value) • 63 × 0.6915 = 43.56 • 65 × 0.3085 = 20.05

6. Calculations • Copper has two common isotopes. • 63Cu is 69.15% of copper samples. • 65Cu is 30.85% of copper samples. • To find the average atomic mass • multiply the mass number by the percentages (as a decimal value) • 63 × 0.6915 = 43.56 • 65 × 0.3085 = 20.05

7. Calculations • Copper has two common isotopes. • 63Cu is 69.15% of copper samples. • 65Cu is 30.85% of copper samples. • To find the average atomic mass • multiply the mass number by the percentages (as a decimal value) • 63 × 0.6915 = 43.56 • 65 × 0.3085 = 20.05

8. Calculations • Copper has two common isotopes. • 63Cu is 69.15% of copper samples. • 65Cu is 30.85% of copper samples. • To find the average atomic mass • multiply the mass number by the percentages (as a decimal value) • 63 × 0.6915 = 43.56 • 65 × 0.3085 = 20.05 • and add the values

9. Calculations • Copper has two common isotopes. • 63Cu is 69.15% of copper samples. • 65Cu is 30.85% of copper samples. • To find the average atomic mass • multiply the mass number by the percentages(as a decimal value) • 63 × 0.6915 = 43.56 • 65 × 0.3085 = 20.05 • and add the values • 43.56 + 20.05 =

10. Calculations • Copper has two common isotopes. • 63Cu is 69.15% of copper samples. • 65Cu is 30.85% of copper samples. • To find the average atomic mass • multiply the mass number by the percentages(as a decimal value) • 63 × 0.6915 = 43.56 • 65 × 0.3085 = 20.05 • and add the values • 43.56 + 20.05 =

11. Calculations • Copper has two common isotopes. • 63Cu is 69.15% of copper samples. • 65Cu is 30.85% of copper samples. • To find the average atomic mass • multiply the mass number by the percentages(as a decimal value) • 63 × 0.6915 = 43.56 • 65 × 0.3085 = 20.05 • and add the values • 43.56 + 20.05 = 63.61

12. Calculations • Iron has three common isotopes. • 54Fe is 5.85% of iron samples. • 56Fe is 91.75% of iron samples. • 58Fe is 2.82% of iron samples. • We would expect the average atomic mass to be somewhere in between 54 and 58.

13. Calculations • Iron has three common isotopes. • 54Fe is 5.85% of iron samples. • 56Fe is 91.75% of iron samples. • 58Fe is 2.82% of iron samples. • multiply the mass number by the percentages • 54 × 0.0585 = 3.16 • 56 × 0.9175 = 51.38 • 58 × 0.0282 = 1.61

14. Calculations • Iron has three common isotopes. • 54Fe is 5.85% of iron samples. • 56Fe is 91.75% of iron samples. • 58Fe is 2.82% of iron samples. • multiply the mass number by the percentages • 54 × 0.0585 = 3.16 • 56 × 0.9175 = 51.38 • 58 × 0.0282 = 1.61 • and add the values

15. Calculations • Iron has three common isotopes. • 54Fe is 5.85% of iron samples. • 56Fe is 91.75% of iron samples. • 58Fe is 2.82% of iron samples. • multiply the mass number by the percentages • 54 × 0.0585 = 3.16 • 56 × 0.9175 = 51.38 • 58 × 0.0282 = 1.61 • and add the values • 3.16 + 51.38 + 1.61 = 56.15

16. Try one on your own • Magnesium has three common isotopes. • 24Mg is 78.99% of magnesium samples. • 25Mg is 10.00% of magnesium samples. • 26Mg is 11.01% of magnesium samples. • What would be the average atomic mass of Mg? Do not move on until you try this problem.

17. Try one on your own • Magnesium has three common isotopes. • 24Mg is 78.99% of magnesium samples. • 25Mg is 10.00% of magnesium samples. • 26Mg is 11.01% of magnesium samples. • What would be the average atomic mass of Mg? • 24 × 0.7899 = 18.96 • 25 × 0.1000 = 2.50 • 26 × 0.1101= 2.86

18. Try one on your own • Magnesium has three common isotopes. • 24Mg is 78.99% of magnesium samples. • 25Mg is 10.00% of magnesium samples. • 26Mg is 11.01% of magnesium samples. • What would be the average atomic mass of Mg? • 24 × 0.7899 = 18.96 • 25 × 0.1000 = 2.50 • 26 × 0.1101= 2.86 • 18.96 + 2.50 + 2.86 = 24.32

19. Try one on your own • Chlorine has two common isotopes. • 35Cl is 75.76% of chlorine samples. • 37Clg is 24.24% of chlorine samples. • What would be the average atomic mass of Cl? Do not move on until you try this problem.

20. Try one on your own • Chlorine has two common isotopes. • 35Cl is 75.76% of chlorine samples. • 37Clg is 24.24% of chlorine samples. • What would be the average atomic mass of Cl? • 35 × 0.7576 = 26.52 • 37 × 0.2424 = 8.97

21. Try one on your own • Chlorine has two common isotopes. • 35Cl is 75.76% of chlorine samples. • 37Clg is 24.24% of chlorine samples. • What would be the average atomic mass of Cl? • 35 × 0.7576 = 26.52 • 37 × 0.2424 = 8.97 • 26.52 + 8.97 = 35.48

22. Summary • To find the average atomic mass of an element, we need to know the percent abundance of each isotope. • We multiply the mass number of each isotope by the percentages (as a decimal number) and add the values for each isotope to get the average atomic mass.