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Isotope lab

Mix the isotopes yourself and watch the atomic weight appear.

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Explanation mode
  • Cl-3575.76%
  • Cl-3724.24%
Cl-3575.76%
Cl-3724.24%

Abundances total 100.00%

Description of this simulation

The natural mixture of Cl-35 at 75.76%, Cl-37 at 24.24% averages to 35.4529, which is the published atomic weight of 35.45. That is where the number on the periodic table comes from.

Natural mix

35.4529

Weighted atomic mass

35.45

Published atomic weight

An atomic weight is not a whole number because it averages over the isotopes actually found in nature, weighted by how common each one is.

What this model shows — and what it simplifies

Educational Model

Abundances are the values IUPAC publishes for terrestrial samples.

Where do we see this in real life?

Carbon dating reads the ratio of carbon-14 to carbon-12 in a sample. Both are carbon and behave identically in chemistry; only the neutron count, and therefore the radioactivity, differs.