Atomic Structure and Nuclear Chemistry
Average Atomic Mass from Isotopes Calculator
Use the entered measurements to determine weighted atomic mass. This average atomic mass from isotopes page keeps the governing relationship visible and shows intermediate quantities so the output can be checked.
Why this quantity is calculated
Average Atomic Mass from Isotopes combines isotope masses according to their natural or measured abundances. It is particularly useful for isotope tables, mass-spectrometry summaries, and introductory atomic chemistry. The requested output is weighted atomic mass, so interpret it as a chemistry-specific model, not as a generic unit converter.
When copying the answer, retain the chemical identity, measurement basis, and units beside it. Without that context, a copied number can be arithmetically sound yet answer the wrong problem.
Setting up the relationship
The governing relationship is Σ(mass × fractional abundance). The inputs on this page are isotope 1 mass, isotope 1 abundance, isotope 2 mass, isotope 2 abundance, isotope 3 mass, isotope 3 abundance. Every entry has a specific place in the formula, so revising one should move the answer as the equation predicts.
Σ(mass × fractional abundance)
Avoid rounding working quantities merely to match the visible answer. Carry adequate working precision and round only to the level justified by the chemistry measurements.
Worked values from the opening example
The calculator opens with isotope 1 mass 35 u, isotope 1 abundance 75 %, isotope 2 mass 37 u, isotope 2 abundance 20 %. Using 35 u at 75%, 37 u at 20%, and 36 u at 5% produces a weighted average of 35.5 u.
The opening entries form a repeatable case rather than a universal reference. Substitute problem-specific entries drawn from one specimen, isotope distribution, material, or trial before carrying the answer into other work.
Reading the answer
The primary answer is identified as weighted atomic mass. Interpret its name and unit together because the page can present an amount, count, proportion, percentage, energy, mass, or duration, and these different kinds of answer cannot be substituted for one another.
The size of the answer supplies a useful clue. Before trusting every displayed digit, predict whether the answer should sit above or below the dominant entry. A major disagreement can expose percent entered as a decimal share, a wrong sign, or inconsistent time units.
A direct check
Multiply the reported average by 100 and compare it with the sum of each mass times its percentage. This path checks the relationship backward instead of duplicating the same button press.
A second verification is to alter one input deliberately and watch how the answer responds. A proportional equation should change by the same factor, whereas exponential or weighted relationships have their own recognizable response.
Precision supported by the entries
The reported precision cannot be stronger than the measurements entered. Isotopic masses, specimen masses, percentage measurements, molar masses, and time intervals need not share one uncertainty. Carry extra figures through the arithmetic, then round the answer to the precision supported by the weakest required measurement.
Unit labels belong to the recorded data, not merely the page design. Percent needs to be separated from fractional abundance; the atomic-mass unit and gram per mole are dimensionally different despite a numerical correspondence. All durations in one decay equation must share a time unit.
Limits of the model
Abundances must describe the same sample and total 100 percent; mass number is not a substitute for a measured isotope mass.
The calculator evaluates only the explicitly stated equation. It cannot infer a substance identity, validate an experiment, establish uncertainty limits, or provide exposure and handling advice not represented by the entries.
Where this chemistry result can lead
After finishing average atomic mass from isotopes, a natural next calculation may be Two-isotope abundance. Open a related calculator only when this answer truly supplies one of its required inputs; otherwise treat the two problems independently.
Save the starting entries before continuing to another calculator. An input record lets the chemistry sequence be repeated without trying to reverse rounded answers.
Questions about average atomic mass from isotopes
What does the average atomic mass from isotopes result represent?
It represents weighted atomic mass under the formula Σ(mass × fractional abundance) and the assumptions described on this page.
How can I check this average atomic mass from isotopes calculation?
Multiply the reported average by 100 and compare it with the sum of each mass times its percentage.
Why might another average atomic mass from isotopes answer differ?
Review what each input means, its units, the constants used, and the rounding method. A different chemical basis or assumption can alter weighted atomic mass despite correct arithmetic.