What Are the Largest and Smallest Atoms?

Atoms don’t have hard edges. Their electron clouds just thin out with distance, like the fading edge of a fog bank. So “how big is an atom?” depends on how you decide to measure it. Even so, the answers at the two extremes are clear: helium is the smallest atom, and cesium and francium are the largest.

How atomic size is measured

Because there’s no sharp boundary, chemists define radius by the distance between nuclei in real substances:

• Covalent radius: half the distance between two identical atoms joined by a single covalent bond.
• Metallic radius: half the distance between neighbouring atoms in a metal crystal.
• Van der Waals radius: half the distance between two atoms that are touching but not bonded — the “personal space” of an atom. This is the largest of the three.
• Calculated radius: computed from quantum mechanics for an isolated atom.

Each method gives different numbers, so always compare radii of the same type. Our element pages list the van der Waals radius, and the atomic radius trend map shows it across the whole table.

The smallest atom: helium

Helium is the smallest atom by most measures — calculated radius about 31 picometres (pm). It has just two electrons, both in the first shell, held by a nucleus with two protons.

Hydrogen, with only one electron, might seem like it should be smaller. But it has only one proton pulling on that electron, while helium’s two protons pull its electrons in tighter. So helium edges out hydrogen.

In our van der Waals data, the smallest values are:

Element	Van der Waals radius (pm)
Hydrogen	120
Fluorine	135
Zinc	139
Helium	140
Copper	140
Oxygen	152

Different data sets disagree about hydrogen versus helium on this measure — van der Waals radii are hard to pin down, especially for noble gases that barely interact — which is a good reminder that “size” depends on method.

The largest atoms: cesium and francium

At the other end, the largest atoms are the alkali metals at the bottom of group 1:

Element	Van der Waals radius (pm)
Francium	348
Cesium	343
Rubidium	303
Radium	283
Potassium	275
Barium	268

Francium’s value is a prediction, since there’s never been enough of it to measure. Of the atoms that have actually been measured, cesium is the largest.

A cesium atom is several times wider than a helium atom — yet it’s still so small that about 1.5 million of them lined up side by side would span a single millimetre.

Why size changes across the table

Two trends explain almost everything:

Down a group, atoms get bigger. Each new period adds a new electron shell further from the nucleus. The extra protons are largely offset by the extra inner electrons, which shield the outer electrons from the nucleus.

Across a period (left to right), atoms get smaller. Electrons are added to the same shell, while the nucleus gains protons. More nuclear charge with little extra shielding pulls the whole electron cloud inward.

So the largest atoms sit in the bottom left (francium, cesium) and the smallest in the top right (helium, neon, fluorine). We explain this in detail in the atomic radius trend.

A surprise: the lanthanide contraction

You’d expect atoms in period 6 to be noticeably larger than those directly above them in period 5. But hafnium is almost exactly the same size as zirconium above it. The 14 lanthanides between them add electrons to 4f orbitals, which shield poorly, so the nucleus pulls harder than expected. This lanthanide contraction is why zirconium and hafnium are so chemically alike that they’re very hard to separate.

Ions change size too

Cations are much smaller than their atoms (they lose their outer shell), and anions are larger. See what is an ion.

Quick answers

What is the smallest atom? Helium, by most measures.

What is the largest atom? Cesium among measured atoms; francium is predicted to be about the same or slightly larger.

How big is an atom? Radii range from about 30 to 350 picometres depending on the element and method, so atoms are roughly 0.1 to 0.7 nanometres across.