Heaviest Naturally Occurring Elements by Atomic Mass

About This Dataset

This chart ranks the heaviest naturally occurring chemical elements by standard atomic mass, in atomic mass units. All are radioactive and all sit at the far end of the periodic table among the actinides and heavy metals.

✨ AI Insight

Uranium is the heaviest naturally occurring element in significant quantity, with an atomic mass near 238, followed closely by thorium (232) and protactinium (231). Every element on this list is radioactive, and most, including actinium, radium, francium, radon, astatine and polonium, exist in nature only as fleeting decay products of uranium and thorium. Their atomic masses cluster tightly between 209 and 238, reflecting how the periodic table effectively runs out of stable nuclear configurations at this end. Bismuth (209), long regarded as the heaviest stable element, is now known to be very faintly radioactive, with a half-life vastly longer than the age of the universe. Beyond uranium lie only the synthetic transuranium elements such as plutonium and americium, produced in reactors and particle accelerators rather than found in nature.

Source: IUPAC Standard Atomic Weights

Key Findings

Auto-computed highlights from this dataset

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Uranium leads with 238 in All Time.

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Average value in All Time: 223 · median: 225.

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Gap in All Time: Uranium is 1.1× larger than Bismuth.