Average Atomic Mass Calculator
Calculate weighted average atomic mass from isotope data
Table of Contents
How to Use
- Enter the mass (in amu) for each isotope
- Enter the natural abundance (%) for each isotope
- Add more isotopes if needed using the Add button
- Click Calculate to find the average atomic mass
What is Average Atomic Mass?
Average atomic mass is the weighted average of the masses of all naturally occurring isotopes of an element. It's the value you see on the periodic table for each element.
The calculation accounts for both the mass of each isotope and how commonly it occurs in nature (its abundance).
How to Calculate
The formula for average atomic mass is:
Average Atomic Mass = Σ (isotope mass × abundance / 100)
For example, chlorine has two main isotopes: Cl-35 (75% abundant, 34.97 amu) and Cl-37 (25% abundant, 36.97 amu). The average atomic mass is (34.97 × 0.75) + (36.97 × 0.25) = 35.45 amu.
Frequently Asked Questions
- Why isn't atomic mass a whole number?
- Atomic mass is a weighted average of all isotopes of an element. Since different isotopes have different masses and abundances, the average is typically not a whole number.
- What are isotopes?
- Isotopes are atoms of the same element with different numbers of neutrons, resulting in different atomic masses but the same chemical properties.
- Do all isotope abundances need to add up to 100%?
- Yes, when calculating average atomic mass, the sum of all natural abundances should equal 100% to represent the complete composition of the element.
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