Element Records: Densest, Lightest, Highest Melting Point, Most Reactive and Rarest

Which element is densest, lightest, melts at the highest temperature or is rarest? A quick-answer table, plus how to tell a real record from a prediction.

Osmium is the densest element, tungsten has the highest melting point of any metal, hydrogen is the lightest and astatine is usually named as the rarest one found in nature. "Most reactive" needs a split, because the answer is fluorine among the non-metals and caesium among the metals.

Records are easy to quote and easy to get slightly wrong, because some depend on the method and some belong to elements nobody has ever held. Table first, small print after.

The quick-answer table

Temperatures are in kelvin (K), where zero is the coldest anything can be. Subtract 273 for degrees Celsius. Density is mass packed into a space: g/cm³ is grams per cubic centimetre, g/L grams per litre, and kJ/mol kilojoules per mole, a unit of energy. The electron terms in the table are explained under the electron records below.

How to read a record honestly

The rule for this page: a record counts only if it was measured, on a real sample or, for single-atom properties such as ionisation energy (explained below), on real atoms. Predictions are out, which removes the superheavy elements, the laboratory-made ones from element 104 (rutherfordium) upwards that exist a few atoms at a time, and in practice fermium to lawrencium (elements 100 to 103). Rarity is the exception: nobody can weigh all the astatine on Earth, so the astatine row is an estimate and says so.

Open the bubbles and choose Size: density, where denser elements get bigger bubbles. The eight biggest are hassium, meitnerium and six other superheavy elements, and osmium only comes ninth. Hassium looks like the champion at 40.7 g/cm³, about 1.8 times osmium, but the hassium page says why it is not: that figure is calculated from a prediction of how its atoms would pack, because so few short-lived atoms have ever been made that nobody has weighed a lump. Osmium's figure is calculated too, but from the measured spacing of atoms in a real crystal.

Three checks catch most bad records.

• Has anyone held a sample? Elements from 104 upwards are made a few atoms at a time (see how new elements are made), so their bulk figures are predictions, and fermium to lawrencium exist only as specks, so theirs are estimates.
• Is the bubble grey elsewhere? In the numeric Colour modes (melting point, density and so on), grey means no data. Huge in density but grey for melting and boiling point suggests calculation. It is a clue, not proof: rutherfordium has estimated melting and boiling points.
• Does the question have an answer? Carbon is grey for melting point, but not for missing data: at normal pressure it has no melting point to give.

What is the densest element?

Picture a cube one centimetre along each edge. Filled with water, its mass is 1 gram. Made of osmium, its mass is 22.59 grams, roughly an AA battery, and a one-litre bottle of osmium would have a mass of over 22 kilograms.

Iridium is a whisker behind at 22.56 g/cm³, about 0.13 per cent less. Both numbers are worked out from X-ray measurements of how far apart the atoms sit. That avoids the tiny gaps in real lumps, which make them seem lighter. Different methods can still swap the order, and reference books have listed the pair both ways, so osmium wins, but only by a hair. See osmium vs iridium.

What is the lightest element?

Hydrogen. Its atoms have the least mass and it is the least dense element: a litre of the gas has a mass of 0.08988 grams at 0 °C and normal air pressure, about a fourteenth of the mass of a litre of air.

Watch the units. Gases are listed in grams per litre, solids in grams per cubic centimetre, and a litre is 1,000 cubic centimetres. Converted, hydrogen is 0.00009 g/cm³, so lithium, the lightest solid metal at 0.534 g/cm³, is nearly six thousand times denser than hydrogen gas. Never compare a gas figure with a solid one until the units match. Helium is second lightest (hydrogen vs helium).

Which element has the highest melting point?

The melting point is the temperature at which a solid turns to liquid, 0 °C for ice. Among metals, tungsten wins at 3,695 K, about 3,422 °C, which is why it made good light bulb filaments. Molybdenum, in the same column, melts nearly 800 degrees lower (tungsten vs molybdenum).

At normal pressure carbon does not melt at all. It sublimes, turning straight from solid to gas, at about 3,600 °C, a little above tungsten's melting point. Melting it needs around a hundred times atmospheric pressure. So "highest melting point of any element" has no clean answer. Likewise avoid "the hardest element": diamond and graphite are both pure carbon, one the hardest natural material and the other soft enough to write with.

Boiling points are murkier: tungsten and rhenium both boil near 5,800 to 6,200 K, and published tables disagree about which is higher.

Which element has the lowest melting point?

Helium, at 0.95 K, about minus 272 °C, but only under about 25 times normal atmospheric pressure. At ordinary pressure helium never freezes, so the lowest melting point at normal pressure belongs to hydrogen: 13.99 K, about minus 259 °C.

Among metals, mercury wins at about minus 39 °C, which makes it one of the two elements that are liquid at room temperature. Next come caesium, at about 28.5 °C, and gallium, at about 30 °C. Francium is estimated to melt a touch lower, near 27 °C, but nobody has enough of it to test.

What is the most reactive element?

"Reactive" means how readily and fiercely an element reacts. Metals react by giving away electrons, the negatively charged particles in an atom's outer region. Non-metals react by taking electrons from metals or sharing them with other non-metals. A giver and a taker cannot share one league table, so there are two champions.

Caesium (cesium in the data behind the bubbles) is the most reactive metal anyone can actually test, judged by how violently it reacts. It explodes in cold water and lets go of an electron more easily than any other element. Francium sits beneath it, so the trend says it should be livelier, but no visible sample has ever existed: its longest-lived version loses half its atoms every 22 minutes. Even on paper francium's win is unclear, because its first ionisation energy (the cost of pulling off one electron, explained below) is measured at about 393 kJ/mol, above caesium's 375.7. The bubbles still hold an older figure, 380. See caesium vs francium.

Fluorine is the most reactive non-metal, attacking nearly every other element, even xenon, a gas that hardly reacts with anything else. Asked for one element overall, fluorine is the usual choice. The reasons are in periodic trends explained.

Electron records, and the one that catches pupils out

Electronegativity is how hard an atom pulls on the electrons it shares in a bond, a tug of war with atoms as the teams. Fluorine pulls hardest, at 3.98 on the Pauling scale.

The next two records are energies for a mole of atoms spread out as a gas (a mole is a fixed, enormous number of atoms, the chemist's dozen), hence kJ/mol.

Electron affinity is the energy released when each atom picks up one extra electron. Many chemistry textbooks write it as a negative number, because energy is given out. This page, like the bubbles, quotes the size. Pupils often assume fluorine wins this too, but chlorine does, at about 349 kJ/mol against fluorine's 328. Fluorine's atom is so small that the electrons already there crowd any newcomer. See chlorine vs fluorine.

First ionisation energy is the energy needed to pull the outermost electron off every atom. Helium's is highest at 2,372.3 kJ/mol and caesium's lowest at 375.7, over six times smaller. Exams word both definitions more formally, so use your course's wording.

What is the rarest element on Earth?

Astatine is the usual answer. It appears only briefly in the decay chains of heavy elements such as uranium, where unstable atoms break apart into other elements. Its natural isotopes (versions of the element whose atoms have slightly different weights) last from a fraction of a millisecond to about a minute. The longest, astatine-219, has a half-life of 56 seconds, the time for half of any batch to decay. Astatine-210 has the longest half-life of all, just over eight hours, but only laboratories make it. Atoms vanish almost as fast as they form, so estimates for the Earth's crust, its rocky outer layer, run from under a gram to a few tens of grams.

Francium is the main rival, usually put at 20 to 30 grams. Astatine is more often quoted as the lower of the two, but both are rough guesses, so treat "rarest" as a best estimate (more in which elements are radioactive). Astatine's density and melting point in the bubbles are predictions, so it is here only for being scarce.

A record hunt in the bubbles

Use the size and colour menus to find each record, applying the three checks above.

• Size: density. Osmium is ninth, iridium just behind. Ignore the gases: the bubbles put g/L figures on the same scale as g/cm³, so xenon looks denser than aluminium.
• Colour: melting point, then Colour: boiling point. Tungsten glows reddest in both, then rhenium. Rutherfordium comes third for boiling point, on an estimate, so it does not count. Below element 104, carbon, phosphorus and arsenic are grey in both, because carbon and arsenic sublime at normal pressure and phosphorus has several forms, so there is no single figure. Boiling point also greys fermium to lawrencium (100 to 103), where the figure is simply missing.
• Colour: electronegativity, then Colour: electron affinity. Fluorine is reddest in the first and chlorine in the second.
• Colour: ionisation energy. Helium is reddest. Caesium and francium are the deepest blue, so check their numbers.

Try it on Periodic Bubbles. Open the bubbles and pop osmium, tungsten, helium, fluorine and astatine. The pop-ups show gas densities in g/cm³ too, so use each element page for the right units. Then see what stuck with today's Daily Bubble or a round of the Pop Quiz.

Key takeaways

• Osmium is the densest element anyone has been able to work out from a real sample, by a hair over iridium, and tungsten melts hottest of the metals.
• Reactivity needs a split: fluorine for non-metals, caesium for metals, francium only in theory.
• Predictions such as hassium's 40.7 g/cm³ are not records.