How do I calculate the average atomic mass of isotopes?
Average atomic mass is calculated by multiplying each isotope's mass by its fractional abundance, then summing all contributions. Use the formula: AM = (m₁ × f₁) + (m₂ × f₂) + ... where m is mass and f is fractional abundance (percentage ÷ 100). See also our Ion/Atom Counter Calculator.
What is the average atomic mass of chlorine?
Chlorine has two stable isotopes: Cl-35 (mass 34.969 amu, 75.78% abundance) and Cl-37 (mass 36.966 amu, 24.22% abundance). The average atomic mass is approximately 35.45 amu.
Why is atomic mass not a whole number?
Atomic mass is not a whole number because it represents a weighted average of all isotopes of an element. Each isotope has a different mass, and their natural abundances result in a decimal value when averaged.
What are isotopes?
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This gives them different atomic masses while maintaining the same chemical properties. You might also find our Empirical Formula Calculator useful.
What is fractional abundance?
Fractional abundance is the proportion of each isotope found in nature, expressed as a decimal (percentage divided by 100). For example, if an isotope has 25% abundance, its fractional abundance is 0.25.
Why is average atomic mass used instead of individual isotope masses?
Average atomic mass reflects the actual composition of an element as it appears in nature. Since elements exist as mixtures of isotopes with different abundances, the average provides a more practical value for calculations and the periodic table.
Do all isotope abundances need to add up to 100%?
Yes, the abundances of all naturally occurring isotopes of an element must total 100%. If your abundances don't add up to 100%, check your data or consider if you're missing any isotopes. Check out our calculate Radioactive Decay (Biology) Remaining Activity as well.