Why Do Atoms Look Like Little Balls In Microscope Images?

Microscope images of atoms show round bumps because the instrument does not photograph the atom at all; it maps a quantity near the surface, such as the leaking electric current under a sharp probe tip or the push-back the tip feels, and renders that map in false color. Three things make the map round: the electron cloud of an atom with filled electron shells has no preferred direction, the probe's own tip is stamped onto every feature it crosses, and the atom wobbles with heat, so a long measurement averages it into a smooth blob. When the probe is sharp enough and the atom is bonded to its neighbors, atoms stop looking like balls, and one 2009 experiment showed a molecule's rings and bonds like a textbook drawing.

Open a chemistry textbook to the page on the atom and you get two pictures that disagree. One shows a nucleus inside a fuzzy cloud. The caption explains that an electron has no fixed position, so the atom has no real edge. A few pages on comes a photograph from a scanning tunneling microscope. It shows rows of tidy round bumps, as neat as a tray of ball bearings.

Both pictures come from serious physicists. So which one is lying?

Neither, and the gap between them is the whole story. The round bumps are honest measurements. They are just not measurements of what most people assume. Once you know what the microscope records, the balls make sense. So does the strange moment when a better microscope makes them go away.

Can The Human Eye See Atoms, And Why Can't A Camera Photograph One?

Hold a hair up to a window. You can see it because light bounces off it and into your eye. The hair is about a tenth of a millimeter wide, big enough to block light and send some of it your way. An atom cannot do that, and the reason is size.

Light travels as a wave. The waves your eyes detect run from 380 to 700 nanometers from crest to crest, according to NASA. A nanometer is a billionth of a meter. Britannica puts the radius of an atom at 1 to 2 angstroms, or 0.1 to 0.2 nanometers. About 50 million atoms in a row would stretch 1 cm (0.4 inches). That makes a wave of visible light roughly a thousand times wider than an atom. The wave rolls over the atom like an ocean swell over a pebble. Nothing bounces back that carries the pebble's shape.

No lens can fix that. The 1986 Nobel press release put the limit of a light microscope at about 4,000 angstroms, or 400 nanometers. A large virus is about that size. Scientists proved atoms exist long before anyone imaged one. We tell that story in how we know so much about atoms when we can't see them.

Scientists got around the problem by giving up on light. One route is a beam of electrons. Their wavelength is a few picometers, and a picometer is a thousandth of a nanometer. The other is a needle so sharp it ends in a single atom. The needle is where the balls come from.

What Do Atoms Look Like Under A Microscope, And What Is The Image A Picture Of?

The ball-bearing images come from a scanning tunneling microscope, or STM. Gerd Binnig and Heinrich Rohrer built it at IBM in Zurich, and it won them half of the 1986 Nobel Prize in Physics. It is a microscope in name only. There is no lens and no light. It works more like reading Braille with a needle.

The needle is a metal tip that ends, in the Nobel committee's words, in "one single atom". It hovers about a nanometer above a conducting surface and never touches it. The release calls that gap "about two atom diameters". A small voltage sits between tip and surface. By everyday physics, no current should cross that gap. One does, because electrons can leak through a thin barrier they lack the energy to climb. The effect is quantum tunneling, and it gives the microscope its name.

The leak depends ferociously on distance. The Nobel release calls the current "strongly dependent on the distance". That dependence is what makes the machine work. As the tip sweeps across the surface, a feedback loop raises and lowers it to keep the current steady. Record that up-and-down motion line by line and you have a map. In 1986 the sideways resolution was about 2 angstroms, or 0.2 nanometers. The vertical resolution was 0.1 angstroms. Both are fine enough to pick out single atoms.

But a map of what? In 1985, Jerry Tersoff and Donald Hamann worked that out. The tunneling current is "proportional to the local density of states of the surface, at the position of the tip". In plain terms, the map shows where the surface has electrons ready to leak into the tip. The voltage picks which electrons count. That is usually close to where the atoms are. It is not a picture of the atoms, and it is not always centered on them.

The glowing orange colors are a choice, too. The data is a grid of heights. Someone picks a color scale, usually gold, for reasons physics cannot defend. Why microscope images have no colors of their own is its own story.

Are Atoms Actually Spheres?

The honest answer has two halves. A lone atom with full electron shells is round, on average. An atom bonded to its neighbors usually is not.

An electron in an atom has no fixed position, only a set of places it is likely to be. The map of those likelihoods is the electron cloud, and it comes in named shapes called orbitals. The simplest, the s orbital, is a sphere. A Lebanon Valley College chemistry text explains why. Its density "depends only on r, the distance from the nucleus, and not on the direction in space". Its contours come out as circles. The next shape, the p orbital, is a dumbbell along one axis. There are three, one along each axis.

A single p electron is about as far from a ball as you can get. Fill all three p orbitals, though, and something tidy happens. A dumbbell along x, plus one along y, plus one along z adds up to a shape with no preferred direction: a sphere. Albrecht Unsöld proved this in 1927. As one textbook states it, the theorem "shows that any atom or ion with closed (occupied) sublevels has a spherically-symmetric charge distribution". Wikipedia's summary of the theorem adds that half-filled shells count too. Neon, argon and the other noble gases are round for this reason. The inner shells of every heavier atom are round as well.

That is the first reason the bumps are round: much of what an atom carries is already round. The other two reasons have nothing to do with the atom.

Why Does Every Atom Come Out As A Smooth Round Blob?

The microscope tip is not a mathematical point. It is a lump of metal that ends in an atom, and that atom has its own electron cloud. Every feature the tip passes over comes out smeared by the shape of the tip. A sharp tip smears a little. A blunt tip smears a lot. Either way the smear is round, because the end of the tip is round.

Tersoff and Hamann wrote the effect down as a formula. The sideways resolution of an STM depends on the tip radius and the gap distance together:

resolution ≈ √(0.2 nm × (R + d))

Here R is the radius of the tip end and d is the gap between tip and surface, both in nanometers. Their paper gives it as (2 Å)(R + d) under a square root, and it matched the gold data of Binnig's team with a tip radius of 9 angstroms, or 0.9 nm. Take the Nobel release's 1 nm gap as d:

1. R + d = 0.9 + 1.0 = 1.9 nm
2. 0.2 × 1.9 = 0.38
3. √0.38 ≈ 0.6 nm

Now blunt the tip to a 5 nm radius. The bracket becomes 6.0, the product 1.2, and the square root about 1.1 nm. The blur is now wider than the spacing between atoms, so the picture turns to mush. The STM's cousin, the atomic force microscope (AFM), has the same problem. It feels the surface with a tip on a flexible arm instead of tunneling into it. A 2020 study in Scientific Reports notes that its sideways resolution "is limited by the area of the tip in contact with the inspected object". The field calls this tip convolution.

Even a perfect tip has a floor. The current falls off so steeply with distance that the brightest spot is always the nearest point of a feature. The nearest point of anything is its rounded top. The microscope sees each atom's summit and little of its slopes.

Do Atoms Sit Still For Their Portraits?

They do not, and they are terrible at sitting for portraits. Every atom in a solid vibrates about its resting place, and the hotter the solid, the wider the wobble. A scan takes time, so what the microscope records is the atom's average position. That average is a round smudge, centered on where the atom spends most of its time.

For decades this did not matter, because the instrument's own blur was far larger than the wobble. That changed in 2021. A Cornell team led by David Muller used a method called electron ptychography. It fires an electron beam through a thin crystal and moves the beam about. A computer then works out the structure from how the scattering pattern shifts. Their paper in Science reported "an instrumental blurring of under 20 picometers". The widths of the atomic columns were "no longer limited by the imaging system, but instead by the thermal fluctuations of the atoms". Cornell's own account of the result was blunter. The crystal was praseodymium orthoscandate, and in Cornell's words: "The resolution is so fine-tuned, the only blurring that remains is the thermal jiggling of the atoms themselves."

Cooling would shrink the smudge, but not to nothing. As the article notes, "even at zero temperature, atoms still have quantum fluctuations, so the improvement would not be very large." The method has since spread. In 2024 a team at the University of Illinois hit 0.44 angstroms on an ordinary commercial microscope. It had no costly aberration correctors. Our explainer on how an electron microscope works covers the lens side of that story.

Even the best image of an atom ever made holds a round blur that no engineering can remove. It is the atom's own motion, exposed for the length of the scan.

When Do Atoms Stop Looking Like Balls?

If the balls come from the measurement, a better instrument should change their shape. It does, and the first clean demonstration came in 1987.

Randall Feenstra and colleagues at IBM imaged gallium arsenide. The crystal has two kinds of atoms in a strict alternating pattern. Their Physical Review Letters paper reports that the images "show either only Ga atoms, or only As atoms, depending on the bias voltage". Flip the sign of the voltage on the tip and half the atoms vanish while the other half appear. The bumps mark the electrons on offer at a chosen energy. The two elements offer theirs at different energies. The atoms did not move; the question put to them changed.

The second demonstration went around the world. In 2009, Leo Gross and colleagues at IBM Zurich imaged a single molecule of pentacene, a flat chain of five carbon rings, with an atomic force microscope. The trick, their Science paper says, was "functionalizing the microscope's tip apex with suitable, atomically well-defined terminations, such as CO molecules". With one carbon monoxide molecule as the probe's end, the image showed the five rings and the bonds between the atoms, the way a textbook draws them. Chemical & Engineering News called it the first time "all the atom positions and bonds of a single molecule, including its hydrogens" had been seen. The carbon atoms sit only 1.4 angstroms apart.

Two details answer two more questions people ask. First, the paper found that "Pauli repulsion is the source of the atomic resolution". The tip felt the molecule through the refusal of two electron clouds to overlap, the same push that stops your hand passing through a table. The AFM touches the molecule exactly as much as you touch anything: electron clouds shoving each other, never solid on solid. Second, pentacene's atoms do not look like balls because they are bonded. Shared electrons pile up between the carbons, and the round shells of a lone atom give way to the ring-and-stick shape of a molecule.

Has Anyone Ever Seen An Atom's Real Shape?

Nobody has seen an atom with their eyes, and the photo that comes closest shows why. In 2018 an Oxford physicist named David Nadlinger won a UK science photo contest run by the EPSRC. His entry was a long-exposure photo of a single strontium ion held between two electrodes. National Geographic's report is careful about what the dot is. A blue-violet laser lit the atom, and "the photo is actually of the laser light being re-emitted, rather than the outline of an atom". Without the long exposure, the atom would not be visible to the naked eye. You are seeing a pinprick of scattered light, not a shape. You still have to squint.

The closest anyone has come to the shape itself is a 2013 experiment by Aneta Stodolna and colleagues. Their Physical Review Letters paper admits that "in atoms the charge distributions described by the wave function are rarely observed". It then reports an experiment in which the ring pattern of a hydrogen electron's orbital "is directly observed". The atom was not in its everyday state. The APS commentary explains the setup. The hydrogen was pumped into highly excited states (principal quantum number 30). It sat in a steady electric field. An electrostatic lens then magnified the outgoing electron wave onto a detector. The rings appeared there. It is a picture of an atom's wave under unusual conditions, and it has rings rather than a ball.

Atoms have also been handled, which is stranger than being seen. In September 1989, Don Eigler at IBM's Almaden lab found he could drag xenon atoms across a nickel surface with an STM tip. The Nature paper with Erhard Schweizer reports positioning "individual xenon atoms on a single-crystal nickel surface with atomic precision" at 4 kelvin. IBM's own history says the pair spent 22 hours arranging the atoms into the letters I, B and M; Wikipedia gives the count as 35. In 2013 the company made a stop-motion film, A Boy and His Atom, which Guinness World Records lists as the smallest stop-motion film: 242 frames, each 45 by 25 nanometers, made by nudging carbon monoxide molecules around a copper surface. The "atom" in the title is a molecule, and the record page says so. We looked at the storage side of that work in how many atoms it takes to store a movie.

So, Why Do Atoms Look Like Little Balls In Microscope Images?

Because three separate things make them round, and none of the three is a camera.

The instrument measures a quantity near the atom, not the atom. An STM maps where electrons are available to tunnel; an AFM maps the push the tip feels. Both peak near the top of each atom and fall away in every direction, so each atom becomes one bump. The probe adds its own round shape to every bump it crosses. A measurement that takes time averages the atom's wobble into a round smudge. Underneath all that, a lone atom with filled shells is round on average. Its dumbbell orbitals add up to a sphere.

Better instruments change the balls, which is the proof that the instrument is partly to blame. Switch the voltage on gallium arsenide and half the balls disappear. Put a carbon monoxide molecule on the tip and pentacene shows its rings and bonds. Push the blur below the atoms' own vibration and a smudge the size of the atom's jiggle remains. No cooling can fully remove it. Chemistry adds its own distortion. Bonded atoms share electrons and lose their lone-atom symmetry. In most molecules that outweighs anything the tip does.

So the ball-bearing pictures are honest data drawn with a crude pen. The fuzzy-cloud drawing is the theory the pen cannot reproduce. Neither picture shows an edge. Even in the sharpest images ever made, the smallest smudge on record is about 20 picometers wide, and it belongs to the atom.