Use this calculator to determine the average atomic mass of an element based on the masses and natural abundances of its isotopes.
Enter the mass (in atomic mass units, amu) and the percentage abundance for each known isotope.
Understanding Average Atomic Mass
The average atomic mass of an element is a weighted average of the atomic masses of its naturally occurring isotopes.
Unlike the mass number (which is a whole number representing the sum of protons and neutrons in a specific isotope),
the average atomic mass is typically not a whole number because it accounts for the varying masses and relative abundances of all isotopes of that element found in nature.
This value is crucial in chemistry for stoichiometric calculations, as it represents the mass of one mole of the element.
The average atomic mass is the number typically listed for each element on the periodic table.
How is it Calculated?
The calculation for average atomic mass involves summing the products of each isotope's mass and its fractional abundance.
The formula is as follows:
Average Atomic Mass = (Isotope1 Mass × Isotope1 Abundance) + (Isotope2 Mass × Isotope2 Abundance) + …
Where 'Abundance' is expressed as a decimal (e.g., 75% becomes 0.75).
The sum of all isotopic abundances for an element should ideally equal 100%.
Example: Chlorine (Cl)
Chlorine has two major isotopes:
Chlorine-35: Atomic mass of approximately 34.96885 amu with a natural abundance of 75.77%.
Chlorine-37: Atomic mass of approximately 36.96590 amu with a natural abundance of 24.23%.
Using the formula:
Average Atomic Mass = (34.96885 amu × 0.7577) + (36.96590 amu × 0.2423)
Average Atomic Mass = 26.4959 amu + 8.9568 amu
Average Atomic Mass = 35.4527 amu
This calculated value matches the average atomic mass of Chlorine found on the periodic table.
var isotopeCount = 2; // Start with 2 pre-filled isotopes
function addIsotopeRow() {
isotopeCount++;
var container = document.getElementById('isotopeInputs');
var newRow = document.createElement('div');
newRow.id = 'isotopeRow_' + isotopeCount;
newRow.style.marginBottom = '10px';
newRow.style.paddingBottom = '10px';
newRow.style.borderBottom = '1px dashed #eee';
var massLabel = document.createElement('label');
massLabel.htmlFor = 'isotopeMass_' + isotopeCount;
massLabel.textContent = 'Isotope ' + isotopeCount + ' Mass (amu):';
massLabel.style.display = 'inline-block';
massLabel.style.width = '150px';
massLabel.style.marginBottom = '5px';
newRow.appendChild(massLabel);
var massInput = document.createElement('input');
massInput.type = 'number';
massInput.id = 'isotopeMass_' + isotopeCount;
massInput.step = 'any';
massInput.min = '0';
massInput.style.width = '120px';
massInput.style.padding = '8px';
massInput.style.border = '1px solid #ccc';
massInput.style.borderRadius = '4px';
massInput.style.marginRight = '15px';
newRow.appendChild(massInput);
var abundanceLabel = document.createElement('label');
abundanceLabel.htmlFor = 'isotopeAbundance_' + isotopeCount;
abundanceLabel.textContent = 'Abundance (%):';
abundanceLabel.style.display = 'inline-block';
abundanceLabel.style.width = '120px';
abundanceLabel.style.marginBottom = '5px';
newRow.appendChild(abundanceLabel);
var abundanceInput = document.createElement('input');
abundanceInput.type = 'number';
abundanceInput.id = 'isotopeAbundance_' + isotopeCount;
abundanceInput.step = 'any';
abundanceInput.min = '0';
abundanceInput.max = '100';
abundanceInput.style.width = '100px';
abundanceInput.style.padding = '8px';
abundanceInput.style.border = '1px solid #ccc';
abundanceInput.style.borderRadius = '4px';
newRow.appendChild(abundanceInput);
container.appendChild(newRow);
}
function calculateAverageAtomicMass() {
var totalWeightedMass = 0;
var totalAbundance = 0;
var resultDiv = document.getElementById('result');
resultDiv.innerHTML = "; // Clear previous results
resultDiv.style.backgroundColor = '#d4edda'; // Default success color
resultDiv.style.color = '#155724';
var allInputsValid = true;
var hasAtLeastOneIsotope = false;
for (var i = 1; i <= isotopeCount; i++) {
var massInput = document.getElementById('isotopeMass_' + i);
var abundanceInput = document.getElementById('isotopeAbundance_' + i);
// Check if the row exists and has values
if (massInput && abundanceInput && (massInput.value !== '' || abundanceInput.value !== '')) {
hasAtLeastOneIsotope = true;
var mass = parseFloat(massInput.value);
var abundance = parseFloat(abundanceInput.value);
if (isNaN(mass) || isNaN(abundance) || mass < 0 || abundance < 0) {
resultDiv.innerHTML = 'Error: Please enter valid positive numbers for all isotope masses and abundances.';
resultDiv.style.backgroundColor = '#f8d7da'; // Error color
resultDiv.style.color = '#721c24';
allInputsValid = false;
return;
}
if (abundance > 100) {
resultDiv.innerHTML = 'Error: Abundance percentages cannot exceed 100%.';
resultDiv.style.backgroundColor = '#f8d7da'; // Error color
resultDiv.style.color = '#721c24';
allInputsValid = false;
return;
}
totalWeightedMass += mass * (abundance / 100);
totalAbundance += abundance;
}
}
if (!hasAtLeastOneIsotope) {
resultDiv.innerHTML = 'Error: Please enter at least one isotope\'s mass and abundance to calculate.';
resultDiv.style.backgroundColor = '#f8d7da'; // Error color
resultDiv.style.color = '#721c24';
return;
}
if (!allInputsValid) {
return; // Should already have returned, but as a safeguard
}
var abundanceTolerance = 0.1; // Allow for slight rounding errors
if (Math.abs(totalAbundance – 100) > abundanceTolerance) {
resultDiv.innerHTML += 'Warning: The sum of abundances (' + totalAbundance.toFixed(2) + '%) is not exactly 100%. This might lead to an inaccurate average atomic mass if the values are not precise.';
resultDiv.style.backgroundColor = '#fff3cd'; // Warning color
resultDiv.style.color = '#856404';
}
resultDiv.innerHTML += 'The Average Atomic Mass is: ' + totalWeightedMass.toFixed(5) + ' amu';
}