What Is the Lightest Element? Hydrogen and the Runners-Up

The short answer is hydrogen. It’s the lightest element by atomic mass, the least dense element, and the lightest gas. But “lightest” can mean a few different things, and the runners-up are more interesting than you might expect.

Lightest by atomic mass: hydrogen

An atom’s mass comes almost entirely from its nucleus. Hydrogen, element 1, has the simplest nucleus possible: usually a single proton and no neutrons at all. Its standard atomic weight is 1.008.

The next lightest elements, in order of atomic number, are:

Rank	Element	Atomic number	Atomic weight
1	Hydrogen	1	1.008
2	Helium	2	4.0026
3	Lithium	3	6.94
4	Beryllium	4	9.0122
5	Boron	5	10.81
6	Carbon	6	12.011

Atomic mass rises with atomic number for all the light elements. (There are a handful of places further along the table where it doesn’t — argon is heavier than potassium, for example — but not at the light end.)

Helium is almost exactly four times heavier than hydrogen because its nucleus has two protons and two neutrons.

Lightest by density: hydrogen again

Hydrogen gas (H₂) has a density of only about 0.09 g/L at 0 °C and 1 atm. Air is about 14 times denser. The second-least-dense element is helium, at about 0.18 g/L.

That’s why both gases have been used to lift airships and balloons. Hydrogen gives slightly more lift but is extremely flammable — as the 1937 Hindenburg disaster showed. Helium is inert, which is why modern blimps and party balloons use it, even though it’s a finite resource extracted from natural gas.

The lightest metal: lithium

Among metals, the lightest is lithium, with a density of 0.534 g/cm³ — about half the density of water. It floats on water (while reacting with it, fizzing and giving off hydrogen) and even floats on some oils.

The next lightest metals are mostly its fellow alkali metals:

Metal	Density (g/cm³)
Lithium	0.534
Potassium	0.89
Sodium	0.97
Rubidium	1.53
Calcium	1.54
Magnesium	1.74

Notice potassium is less dense than sodium, even though it’s heavier per atom — its atoms are much larger, so they pack fewer per unit volume.

Lithium’s low mass is exactly why it’s the element in lithium-ion batteries: it stores a lot of charge for very little weight, which matters for phones, laptops and electric cars. See the alkali metals for more on this family.

The lightest solid element

At room temperature the lightest solid element is also lithium. The lightest nonmetal solids are white phosphorus (about 1.82 g/cm³) and sulfur (about 2.07 g/cm³); carbon varies with its form, from about 2.2 g/cm³ for graphite to 3.5 g/cm³ for diamond.

Lightest isotope

Even within hydrogen there’s variety. Ordinary hydrogen, protium (¹H), has no neutrons. Deuterium (²H) has one neutron and is about twice as heavy; tritium (³H) has two neutrons and is radioactive. Protium is the lightest atom that exists. You can see all of hydrogen’s isotopes on its element page.

Why hydrogen is so light — and so common

Hydrogen was the first element to form after the Big Bang, and it’s still the most abundant element in the universe: roughly three-quarters of all ordinary matter by mass. Stars, including the Sun, spend most of their lives fusing hydrogen into helium. See the most abundant elements in the universe.

On Earth, though, hydrogen gas is rare in the atmosphere. It’s so light that its molecules move fast enough to escape Earth’s gravity over time. Nearly all Earth’s hydrogen is locked up in compounds, above all in water.

Quick answers

What is the lightest gas? Hydrogen, followed by helium.

What is the lightest noble gas? Helium.

What’s the least dense solid material? Not an element: engineered materials like aerogels and metallic microlattices can be less dense than air-filled foam. Among pure elements, it’s lithium.

Explore the data

Compare every element’s atomic mass and density visually in the atomic mass and density trend maps. For the other end of the scale, see the densest element and the heaviest element.