Calculate the mass number or the average atomic mass of an element.
Calculate Mass Number (A)
Calculates the total mass of a specific isotope (Protons + Neutrons).
The Mass Number (A) is: amu
Note: 1 amu ≈ 1.66 × 10-24 grams.
Calculate Average Atomic Mass
Calculate the weighted average mass based on natural abundance.
The Average Atomic Mass is: amu
Warning: Abundance does not equal 100%.
function calculateMassNumber() {
var p = parseFloat(document.getElementById('numProtons').value);
var n = parseFloat(document.getElementById('numNeutrons').value);
var resDiv = document.getElementById('resMassNumber');
var resVal = document.getElementById('valMassNumber');
if (isNaN(p) || isNaN(n) || p < 0 || n 0.01) {
warn.style.display = 'block';
warn.innerText = "Warning: Total abundance is " + totalAbundance.toFixed(2) + "%. Results may be inaccurate unless they total 100%.";
} else {
warn.style.display = 'none';
}
var avgMass = ((m1 * a1) + (m2 * a2)) / 100;
resVal.innerText = avgMass.toFixed(4);
resDiv.style.display = 'block';
}
How Do You Calculate the Mass of an Atom?
Calculating the mass of an atom is a fundamental skill in chemistry. Since atoms are incredibly small, scientists don't measure them in grams for daily work; instead, they use Atomic Mass Units (amu) or Daltons. The calculation depends on whether you are looking for the mass of a single atom (Isotope) or the average mass of an element as found in nature.
1. Calculating the Mass Number (A)
The mass of an individual atom is primarily determined by its nucleus, which contains protons and neutrons. Electrons are so light that their mass is usually ignored in basic calculations.
The Formula:
Mass Number (A) = Number of Protons + Number of Neutrons
Example: Carbon-14 has 6 protons and 8 neutrons. Calculation: 6 + 8 = 14 amu.
2. Calculating Average Atomic Mass
In nature, elements exist as mixtures of isotopes (atoms of the same element with different numbers of neutrons). The atomic mass you see on the periodic table is a "weighted average" of all these isotopes.
The Steps:
Convert the percentage abundance of each isotope into a decimal (divide by 100).
Multiply the mass of each isotope by its decimal abundance.
Add these values together.
Isotope
Atomic Mass (amu)
Natural Abundance
Chlorine-35
34.969
75.78%
Chlorine-37
36.966
24.22%
3. Converting amu to Grams
If you need the actual physical mass of a single atom in grams, you use the conversion factor for 1 amu. One atomic mass unit is defined as 1/12th the mass of a Carbon-12 atom.
1 amu = 1.660539 x 10-24 grams
To find the mass in grams, simply multiply the mass number (amu) by this constant. For example, a single Hydrogen atom (~1 amu) weighs approximately 1.66 x 10-24 grams.
Why is Atomic Mass Important?
Understanding atomic mass allows chemists to perform stoichiometry, which is the calculation of reactants and products in chemical reactions. By knowing the mass of atoms, we can determine how many grams of a substance represent a specific number of atoms (using Avogadro's Number and the concept of the Mole).