The most up-to-date version of the periodic table contains 118 elements. It’s easy to imagine you can find the heaviest by scanning to the last element on the table, but that only gives one of several answers.

What Do We Mean By Heaviest?

You’ve probably heard the old trick question; “Which is heavier, a pound of lead or a pound of feathers?” It’s based on the fact that people confuse weight with density. The answer of course is that since “pound” (or kilogram for most of the world) is a measure of weight, the two are equally heavy. If you doubled the quantity of feathers, they’d be heavier than the lead.

Consequently, any element can be the heaviest if you have enough of it. Hydrogen is the lightest element per atom, but as the most common element in the universe, if you brought it all together it would be heavier than anything else. 

Which Element Has The Heaviest Atoms?

Not many people are probably looking for the answer above, and if they were they’d ask it another way. We’re guessing most of those seeking the heaviest element mean: which element has the heaviest atom?

In that case, the answer is pretty much a draw. Oganesson, that element at the bottom right of the periodic table, has 118 protons. The only isotope of it we have managed to make has 176 neutrons, for a combination of 294 nucleons, the main determinant of an atom’s mass.

However, the next atom to the left of it on the periodic table is tennessine, and we’ve made two isotopes of that. One of those is lighter, but the other also has 294 nucleons – one less proton and one more neutron. When not ionized, it will also have one less electron.

Neutrons are 0.14 percent heavier than protons, so on that count tennessine might seem to win, if only by a hair. Even adding the extra electron to the equation doesn’t fully bridge that tiny gap. However, nuclei also have binding energy which can add to their mass. We haven’t been able to find any estimates of the binding mass of the two nuclei, to see whether that makes any difference, so all we can really say is that oganesson and the heavier isotope of tennessine are in a photo-finish for this prize. 

What If Short-Lived Atoms Don’t Count?

Neither oganesson nor tennessine stick around long enough to have practical applications – they both have half-lives measured in milliseconds. Many people might not consider that to count, since you wouldn’t even notice them before they are gone.

The heaviest stable isotope (that is one that does not radioactively decay at all) is lead-208. However, there are also some much heavier isotopes that easily last long enough to study. We only discovered fairly recently that bismuth-209 is faintly radioactive – with a half-life a billion times longer than the age of the universe these things are hard to notice. 

You can set arbitrary limits on how long an atom needs to stick around in order to count. Depending on whether the half-life needs to be more than a minute, more than a day or more than a year you will get different answers.

But What About The Densest?

Getting back to the trick question referred to at the start, sometimes when people say heaviest they’re using (incorrect) shorthand for density not weight/mass.

You might think that the answer to the densest element question would be the same. In general, materials made from atoms with heavy nuclei also tend to be denser at a particular temperature than those with light nuclei. However, the correlation is a long way from perfect. The density of oganesson and tennessine, while high by the standards of the materials we encounter most often, are thought to be a long way off the maximum. That’s because their outer electrons are probably relatively dispersed compared to some other heavy elements, keeping the nuclei further apart. We don’t know this for certain, because of that annoying millisecond half-life thing, but there is plenty of reason to expect it, based on elements with longer half-lives.

It is thought hassium and meitnerium are each about four times as dense as oganesson and tennessine, but again we have no reliable measurements, and in particular can’t tell which of these two is denser,

The two densest elements that have ever been measured are iridium and osmium. We know these have a density of 22.6 g/cm², but there are conflicting reports of which of the two is slightly heavier. 

If anyone really wants to know the answer, they are welcome to send a substantial stock of each to IFLScience and we promise to measure the weight and volume of each precisely. It would need to be quite a large stock though, for accuracy, and at a price of thousands of dollars an ounce for iridium we don’t guarantee to send it back.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.