Try this average atomic mass calculator to determine the weighted mean of the masses of all isotopes of an element.
- Carbon-12 (98.93% abundance): 12.0000 amu
- Carbon-13 (1.07% abundance): 13.0034 amu
The calculator would weigh these masses based on their natural abundance, yielding an average atomic mass of approximately 12.0107 amu for carbon.
This calculation is crucial in chemistry as it provides a single, representative mass for elements that exist as multiple isotopes in nature.
Average Atomic Mass Calculator
| Element | Isotope 1 | Abundance 1 | Isotope 2 | Abundance 2 | Isotope 3 | Abundance 3 | Average Atomic Mass |
|---|---|---|---|---|---|---|---|
| Lithium | Li-6: 6.0151 amu | 7.59% | Li-7: 7.0160 amu | 92.41% | – | – | 6.9410 amu |
| Oxygen | O-16: 15.9949 amu | 99.757% | O-17: 16.9991 amu | 0.038% | O-18: 17.9992 amu | 0.205% | 15.9994 amu |
| Copper | Cu-63: 62.9296 amu | 69.17% | Cu-65: 64.9278 amu | 30.83% | – | – | 63.5460 amu |
| Silver | Ag-107: 106.9051 amu | 51.839% | Ag-109: 108.9048 amu | 48.161% | – | – | 107.8682 amu |
Average Atomic Mass Formula
The formula for calculating average atomic mass is:
Average Atomic Mass = Σ(Isotope Mass × Isotope Abundance)Where:
- Σ represents the sum of all isotopes
- Isotope Mass is measured in atomic mass units (amu)
- Isotope Abundance is expressed as a decimal fraction
Let’s illustrate this with boron:
- Boron-10 (19.9% abundance): 10.0129 amu
- Boron-11 (80.1% abundance): 11.0093 amu
Calculation:
(10.0129 × 0.199) + (11.0093 × 0.801) = 10.811 amuHow do you calculate the average atomic mass?
To calculate the average atomic mass, follow these steps:
- Identify isotopes: List all naturally occurring isotopes of the element.
- Gather data: Obtain the mass and abundance of each isotope.
- Convert abundances: Ensure abundances are in decimal form (e.g., 75% = 0.75).
- Apply formula: Multiply each isotope’s mass by its abundance.
- Sum results: Add all products from step 4.
Let’s apply this to magnesium:
- Mg-24 (78.99%): 23.9850 amu
- Mg-25 (10.00%): 24.9858 amu
- Mg-26 (11.01%): 25.9826 amu
Calculation:
(23.9850 × 0.7899) + (24.9858 × 0.1000) + (25.9826 × 0.1101) = 24.3050 amuThus, the average atomic mass of magnesium is approximately 24.3050 amu.
How to calculate the average atomic mass of chlorine?
Chlorine serves as an excellent example for calculating average atomic mass due to its two naturally occurring isotopes.
- Identify chlorine isotopes:
- Chlorine-35
- Chlorine-37
- Gather isotope data:
- Cl-35: Mass = 34.9689 amu, Abundance = 75.76%
- Cl-37: Mass = 36.9659 amu, Abundance = 24.24%
- Convert abundances to decimals:
- Cl-35: 0.7576
- Cl-37: 0.2424
- Apply the formula:
(34.9689 × 0.7576) + (36.9659 × 0.2424) - Calculate:
26.4924 + 8.9598 = 35.4522 amu
The average atomic mass of chlorine is approximately 35.4522 amu.
Sources / Reference URLs
- National Institute of Standards and Technology (NIST) – Atomic Weights and Isotopic Compositions
- Royal Society of Chemistry – Periodic Table
- International Union of Pure and Applied Chemistry (IUPAC) – Atomic Weights of the Elements
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