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“Heaviest element” sounds like a question with one answer. It actually has three, depending on whether you mean the heaviest atom, the heaviest atom found in nature, or the heaviest material to hold in your hand.
Heaviest atom: oganesson (element 118)
The element with the most massive atoms is oganesson, element 118 — the last entry on the current periodic table. Its most studied isotope, oganesson-294, has 118 protons and 176 neutrons, giving an atomic mass of roughly 294.
Oganesson is entirely synthetic. It was first made at the Joint Institute for Nuclear Research in Dubna, Russia, in a collaboration with Lawrence Livermore National Laboratory in the USA, by firing calcium-48 ions at a californium-249 target. Only a handful of atoms have ever been detected, and each one survived for less than a millisecond before decaying. It was officially named in 2016 after the Russian physicist Yuri Oganessian — only the second element named after a living person.
Because so few atoms have existed, almost nothing about oganesson has been measured directly. It sits in the noble gas column, but calculations suggest it may not behave like a noble gas at all, and might even be a solid at room temperature because of relativistic effects on its electrons. We can’t check: you can’t collect enough atoms to see what state they’re in.
See how new elements are made and the newest elements for more.
Heaviest natural element: uranium
The heaviest element found in significant amounts in nature is uranium, element 92, with an atomic weight of about 238.03. Most natural uranium is uranium-238, whose half-life of about 4.5 billion years is similar to the age of the Earth — so roughly half of the uranium-238 the Earth formed with is still here.
Strictly, trace amounts of plutonium (element 94) and neptunium (93) also occur naturally, produced in uranium ores when uranium nuclei capture neutrons. But the quantities are minuscule, so uranium is usually named as the heaviest naturally occurring element in any meaningful amount.
Every element after uranium on the table is called transuranium, and practically all of them are made in reactors or particle accelerators.
Heaviest material to hold: osmium
If “heaviest” means which element would feel heaviest for its size, the answer is density, and the winner is osmium, at about 22.57 g/cm³, closely followed by iridium at 22.42 g/cm³.
The measured values are so close that the ranking was debated for decades. Calculations from X-ray crystallography data now put osmium very slightly ahead. Either way, a cube of osmium 10 cm on a side would weigh about 22.6 kg — roughly twice as much as the same cube of lead.
Here are the densest elements, using the values from our element data:
| Element | Density (g/cm³) |
|---|---|
| Osmium | 22.57 |
| Iridium | 22.42 |
| Platinum | 21.46 |
| Rhenium | 20.8 |
| Neptunium | 20.25 |
| Plutonium | 19.84 |
| Tungsten | 19.3 |
| Gold | 19.28 |
| Uranium | 18.95 |
Lead, often thought of as the classic “heavy” metal, is only 11.34 g/cm³ — it doesn’t make the top twenty. Gold is nearly 70% denser than lead.
Some of the superheavy synthetic elements are predicted to be denser still — hassium (element 108) perhaps around 40 g/cm³ — but those predictions can never be tested on a visible sample.
We cover the density rankings in detail in what is the densest element.
Why the heaviest atoms aren’t the densest
Oganesson has far more mass per atom than osmium, so why isn’t it denser? Density depends on both mass and how tightly atoms pack. The heaviest atoms have enormous electron clouds and, in practice, can’t form a bulk solid at all because they decay almost instantly. Osmium hits a sweet spot: heavy nuclei and a compact electron arrangement that lets atoms pack very tightly.
Quick answers
What is the heaviest metal? By density, osmium. By atomic mass among metals that exist in bulk, plutonium or uranium; among all known metals, the superheavy synthetic elements.
What is the heaviest gas? Among elements that exist in bulk, radon (about 9.7 g/L). Among compounds, tungsten hexafluoride (WF₆) is one of the densest gases known.
Is there an element heavier than oganesson? Not yet. Laboratories are attempting to make elements 119 and 120; none has been confirmed so far.
Explore the data
The atomic mass trend map shades every element by mass, and the density map shows where the dense metals cluster in the middle of period 6.
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