function calculateAverageAtomicMass() {
var isotope1Mass = parseFloat(document.getElementById('isotope1Mass').value);
var isotope1Abundance = parseFloat(document.getElementById('isotope1Abundance').value);
var isotope2Mass = parseFloat(document.getElementById('isotope2Mass').value);
var isotope2Abundance = parseFloat(document.getElementById('isotope2Abundance').value);
var isotope3Mass = parseFloat(document.getElementById('isotope3Mass').value);
var isotope3Abundance = parseFloat(document.getElementById('isotope3Abundance').value);
var totalWeightedMass = 0;
var totalAbundanceSum = 0;
var resultDiv = document.getElementById('averageAtomicMassResult');
resultDiv.innerHTML = "; // Clear previous results
var isValidInput = true;
// Process Isotope 1
if (!isNaN(isotope1Mass) && isotope1Mass > 0 && !isNaN(isotope1Abundance) && isotope1Abundance >= 0) {
totalWeightedMass += isotope1Mass * (isotope1Abundance / 100);
totalAbundanceSum += (isotope1Abundance / 100);
} else if (document.getElementById('isotope1Mass').value !== " || document.getElementById('isotope1Abundance').value !== ") {
isValidInput = false;
resultDiv.innerHTML = 'Please enter valid positive numbers for Isotope 1 Mass and non-negative for Abundance.';
}
// Process Isotope 2
if (isValidInput && !isNaN(isotope2Mass) && isotope2Mass > 0 && !isNaN(isotope2Abundance) && isotope2Abundance >= 0) {
totalWeightedMass += isotope2Mass * (isotope2Abundance / 100);
totalAbundanceSum += (isotope2Abundance / 100);
} else if (isValidInput && (document.getElementById('isotope2Mass').value !== " || document.getElementById('isotope2Abundance').value !== ")) {
isValidInput = false;
resultDiv.innerHTML = 'Please enter valid positive numbers for Isotope 2 Mass and non-negative for Abundance.';
}
// Process Isotope 3 (Optional)
if (isValidInput && !isNaN(isotope3Mass) && isotope3Mass > 0 && !isNaN(isotope3Abundance) && isotope3Abundance >= 0) {
totalWeightedMass += isotope3Mass * (isotope3Abundance / 100);
totalAbundanceSum += (isotope3Abundance / 100);
} else if (isValidInput && (document.getElementById('isotope3Mass').value !== " || document.getElementById('isotope3Abundance').value !== ")) {
isValidInput = false;
resultDiv.innerHTML = 'Please enter valid positive numbers for Isotope 3 Mass and non-negative for Abundance.';
}
if (!isValidInput) {
return;
}
if (totalAbundanceSum === 0) {
resultDiv.innerHTML = 'Please enter at least one valid isotope\'s mass and abundance.';
return;
}
var averageAtomicMass = totalWeightedMass / totalAbundanceSum;
resultDiv.innerHTML = 'The Average Atomic Mass is:
';
if (Math.abs(totalAbundanceSum * 100 – 100) > 0.1 && totalAbundanceSum > 0) { // Check if sum is not close to 100%
resultDiv.innerHTML += '
';
}
}
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Understanding Average Atomic Mass
The average atomic mass of an element is a weighted average of the atomic masses of its naturally occurring isotopes. It's the value you typically see on the periodic table. Unlike the mass number (which is a whole number representing the sum of protons and neutrons in a specific isotope), average atomic mass is usually not a whole number because it accounts for the varying masses and relative abundances of an element's different isotopes.
What are Isotopes?
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This difference in neutron count leads to different atomic masses for each isotope. For example, Carbon-12 and Carbon-14 are isotopes of carbon; both have 6 protons, but Carbon-12 has 6 neutrons while Carbon-14 has 8 neutrons.
Natural Abundance
Natural abundance refers to the percentage of a particular isotope found in a naturally occurring sample of an element. These percentages are relatively constant for most elements across the Earth. For instance, about 75.77% of all chlorine atoms found naturally are Chlorine-35, while 24.23% are Chlorine-37.
How is Average Atomic Mass Calculated?
The calculation of average atomic mass involves a simple weighted average formula:
Average Atomic Mass = (Mass of Isotope 1 × Abundance of Isotope 1) + (Mass of Isotope 2 × Abundance of Isotope 2) + ...
Where:
- Mass of Isotope: The atomic mass unit (amu) of a specific isotope.
- Abundance of Isotope: The natural abundance of that isotope, expressed as a decimal (percentage divided by 100).
If the sum of the abundances entered does not equal 100%, the calculator will normalize the values to provide a true weighted average based on the relative proportions you've provided.
Example Calculation: Chlorine (Cl)
Let's calculate the average atomic mass of Chlorine, which has two main isotopes:
- Chlorine-35: Mass = 34.96885 amu, Abundance = 75.77%
- Chlorine-37: Mass = 36.96590 amu, Abundance = 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 result matches the value typically found on the periodic table for chlorine.
Why is this important?
Understanding average atomic mass is fundamental in chemistry. It's crucial for:
- Stoichiometry: Calculating the masses of reactants and products in chemical reactions.
- Molar Mass: Determining the molar mass of compounds, which is essential for converting between mass and moles.
- Understanding Elemental Properties: Providing insight into the composition and behavior of elements in various contexts.
This calculator simplifies the process of determining average atomic mass, allowing you to quickly verify calculations or explore hypothetical isotopic compositions.