The weight of a large # of different isotopes that makes up a sample is termed the Atomic weight of the sample. It is calculated as a weighted average. The procedure for this calculation is as follows.
- Express the percentage of each isotope as a decimal number (divide by 100).
- Multiply each percentage as a decimal by the corresponding atomic mass (in amu's) of the isotope (isotope fraction).
- Add up the isotope fractions.
Example:
What is the atomic weight of a sample of Lithium that consists of the isotopes listed in the table below?
| Isotope | Atomic Mass(amu) | Abundance (%) |
| Lithium - 6 | 6.015 | 7.68 |
| Lithium - 7 | 7.016 | 92.32 |
Calculations:
| | Atomic Mass | | Decimal Percent | | |
| Lithium - 6 | 6.015 | X | .0768 | = | 0.461952 |
| Lithium - 7 | 7.016 | X | .9232 | = | 6.477171 |
| Atomic Weight
| 6.94 |
(answer rounded to 3 significant figures)
Assignment #3: Calculation of Atomic Weight:
Calculate the Atomic weight of the following samples using the information given.
1) A Sample of Potassium
| Isotope | Atomic Mass (amu) | Abundance (%) |
| potassium - 39 | 38.964 | 93.12 |
| potassium - 41 | 40.962 | 6.88 |
2) A Sample of Boron
| Isotope | Atomic Mass (amu) | Abundance (%) |
| Rubidium -85 | 85.04 | 72.15 |
| Rubidium -87 | 86.94 | 27.85 |
3) A Sample of Magnesium
| Isotope | Atomic Mass (amu) | Abundance (%) |
| Magnesium - 24 | 23.986 | 78.60 |
| Magnesium - 25 | 24.985 | 10.11 |
| Magnesium - 26 | 25.983 | 11.29 |
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