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. o 63 Cu is 69.15% of copper samples. o 65 Cu 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. o 63 Cu is 69.15% of copper samples. o 65 Cu is 30.85% of copper samples. To find the average atomic mass o multiply the mass number by the percentages (as a decimal value)

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

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

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

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

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

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

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

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

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

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

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