1. Estimating the Mass Number:
Isotope
Mass (amu)
% abundance
Mass Number 1
37.765
9.67 2
39.056
78.68 3
40.003
11.34 4
41.060
0.31

2. Estimating the Mass Number:
Isotope
Mass (amu)
% abundance
Mass Number 1
37.765
9.67 38 2
39.056
78.68 39 3
40.003
11.34 40 4
41.060
0.31 41

3. Estimating the Mass Number:
1) What is the most common isotope of element X?_________.
Isotope
Mass (amu)
% abundance
Mass Number
1 (X-38)
37.765
9.67 38
2 (X-39)
39.056
78.68 39
3 (X-40)
40.003
11.34 40
4 (X-41)
41.060
0.31 41

4. Estimating the Mass Number:
1) What is the most common isotope of element X?__X-39 (isotope 2)_.
Isotope
Mass (amu)
% abundance
Mass Number
1 (X-38)
37.765
9.67 38
2 (X-39)
39.056
78.68 39
3 (X-40)
40.003
11.34 40
4 (X-41)
41.060
0.31 41

5. Calculating Atomic Mass:
2) Calculate the average atomic mass of element X. _________.
Isotope
Mass (amu)
% abundance
Mass Number
1 (X-38)
37.765
9.67 38
2 (X-39)
39.056
78.68 39
3 (X-40)
40.003
11.34 40
4 (X-41)
41.060
0.31 41

6. Atomic Mass:
Atomic Mass: Is the weighted average mass of the isotopes of the element.
Calculating Atomic Mass:
Mass contribution =
Atomic mass = The sum of all mass
contributions.

7. Calculating Atomic Mass:
Isotope Mass contribution
1 (X-38)
2 (X-39)
3 (X-40)
4 (X-41) = = = =
_______
39.05
Average Atomic Mass

8. Calculating Atomic Mass:
2) Calculate the average atomic mass of element X. __39.05 amu_.
The isotope which exist in greater abundance has the greater effect in determining the atomic mass of the element.
Isotope
Mass (amu)
% abundance
Mass Number
1 (X-38)
37.765
9.67 38
2 (X-39)
39.056
78.68 39
3 (X-40)
40.003
11.34 40
4 (X-41)
41.060
0.31 41

9. Calculating Atomic Mass:
2) Calculate the average atomic mass of element X. __39.04 amu_.
3) Use your periodic table to identify element X. _______________.

10. Calculating Atomic Mass:
2) Calculate the average atomic mass of element X. __39.04 amu_.
3) Use your periodic table to identify element X.__Potassium (K)_.
4) What is the atomic number of this element?_____________.

11. Calculating Atomic Mass:
2) Calculate the average atomic mass of element X. __39.05 amu_.
3) Use your periodic table to identify element X.__Potassium (K)_.
4) What is the atomic number of this element?_ 19_.

12. K 19 39.04 Atomic Mass Is not a whole number because it is an average.
are the decimal numbers on the periodic table. 19 K
39.04

13. Exercises:
1) Calculate the atomic mass of copper, it has two isotopes: 69.1% Copper-24 with a mass of amu and the rest (Copper-25) has a mass of amu.
Answer:
Isotope Mass contribution
Copper-24
Copper-25 = =
_____
63.54
Average Atomic Mass

14. Exercises:
2) Magnesium has three isotopes: 78.89% magnesium-24 with a mass of amu, 10.00% magnesium-25 with a mass of amu, and the rest magnesium-26 with a mass of amu. What is the atomic mass of magnesium?
Data: Atomic mass % Abundance
Magnesium %
Magnesium %
Magnesium
11.01

15. Answers :
Isotope Mass contribution
Magnesium-24
Magnesium-25
Magnesium-26 = = =
______
24.27
Average Atomic Mass