Why does the periodic table show a decimal atomic mass?

Because most elements exist naturally as a mixture of isotopes, the periodic table lists the weighted average mass across all naturally occurring isotopes, not the mass of a single atom.

Do the abundances need to add up to 100%?

For a two-isotope element, yes — the two abundances should sum to 100% (or close to it) since together they represent the entire naturally occurring sample.

Average Atomic Mass Calculator (Two Isotopes)

Calculate an element's standard average atomic mass — the weighted average shown on the periodic table — from the mass and natural abundance of two of its isotopes.

Inputs

Results

Average Atomic Mass: 10.811 amu

Try an example

Formula

Average Mass = (Mass₁ × Abundance₁) + (Mass₂ × Abundance₂)

Learn How This Works

Acid-Base Chemistry and Reaction Calculations, Worked Through

Titration, the Henderson-Hasselbalch equation, reaction rate, gas density, and isotope-weighted atomic mass — five core reaction chemistry calculations.

Frequently Asked Questions

Related Calculators

Mass-to-Moles Calculator

Convert a mass of a substance to moles using its molar mass.

Molarity Calculator

Calculate the molarity of a solution from moles of solute and volume.

Dilution Calculator (C1V1=C2V2)

Calculate the volume needed to dilute a stock solution to a target concentration.

Ideal Gas Law Calculator

Calculate pressure, volume, moles, or temperature using PV = nRT.

Percent Yield Calculator

Calculate the percent yield of a chemical reaction.

Boyle's Law Calculator

Calculate the new pressure or volume of a gas at constant temperature.