How do we know that things are really made of atoms?

Atoms are small. Really, really small. You’ll probably have heard that matter is made of bundles of these tiny things. You’ll likely also know that you can’t see them with the naked eye. We are told to take on trust the idea that atoms are there, interacting with each other and being building blocks for our world.

For most people, though, that’s not good enough. Science prides itself on the way it uses real observations to work out the mysteries of the universe – so how did we come to conclude that atoms exist, and what have we learned about these tiny structures?

It might seem as if there’s a simple way to prove atoms exist: put them under the microscope. But this approach won’t work. In fact, even the most powerful light-focusing microscopes can’t visualise single atoms. What makes an object visible is the way it deflects visible light waves. Atoms are so much smaller than the wavelength of visible light that the two don’t really interact. To put it another way, atoms are invisible to light itself. However, atoms do have observable effects on some of the things we can see.

Visible light cannot reveal individual atoms (Credit: Yevgen Lyashko/Alamy Stock Photo)

Hundreds of years ago in 1785 Dutch scientist Jan Ingenhousz was studying a strange phenomenon that he couldn’t quite make sense of. Minute particles of coal dust were darting about on the surface of some alcohol in his lab.

Even the most powerful light-focusing microscopes can’t visualise single atoms

About 50 years later, in 1827, the Scottish botanist Robert Brown described something curiously similar. He had his microscope trained on some pollen grains. Brown noticed that some of the grains released tiny particles – which would then move away from the pollen grain in a random jittery dance.

At first, Brown wondered if the particles were really some sort of unknown organism. He repeated the experiment with other substances like rock dust, which he knew wasn’t alive, and saw the same strange motion again.

It would take almost another century for science to offer an explanation. Einstein came along and developed a mathematical formula that would predict this very particular type of movement – by then called Brownian motion, after Robert Brown.

Einstein’s theory was that that the particles from the pollen grains were being moved around because they were constantly crashing into millions of tinier molecules of water – molecules that were made of atoms.

It might come as a surprise that atoms can be broken down – particularly since “atomos” means “indivisible”

“He explains this jiggling motion that you see as actually being caused by the impact of individual water molecules on the particles of dust or whatever it is that you’ve got on your liquid,” explains Harry Cliff at the University of Cambridge, who is also a curator at London’s Science Museum.

By 1908, observations backed with calculations had confirmed that atoms were real. Within about a decade, physicists would be able to go further. By pulling apart individual atoms they began to get a sense of their internal structure.

It might come as a surprise that atoms can be broken down – particularly since the very name atom derives from a Greek term “atomos”, which means “indivisible”. But physicists now know that atoms are not solid little balls. It’s better to think of them as tiny electrical, “planetary” systems. They’re typically made up of three main parts: protons, neutrons and electrons. Think of the protons and neutrons as together forming a “sun”, or nucleus, at the centre of the system. The electrons orbit this nucleus, like planets.

Atoms are made up of smaller particles (Credit: Science Photo Library/Alamy Stock Photo)

If atoms are impossibly small, these subatomic particles are even more so. Funnily enough, the first particle that was discovered was actually the smallest of the three – the electron.

To get an idea of the size difference here, protons in the nucleus are actually around 1,830 times as large as electrons. Picture a small marble orbiting a hot air balloon – that’s the kind of discrepancy we’re talking about here.

It’s one of the first particle accelerators in a way

But how do we know those particles are there? The answer is because, although tiny, they can have a big impact. The British physicist who discovered electrons, JJ Thomson, used a particularly eye-catching method to prove their existence in 1897.

His special device was called a Crookes tube – a funny shaped piece of glass out of which nearly all the air was sucked by a machine. Then, a negative electrical charge was applied to one end of the tube. This charge was enough to strip the remaining gas molecules in the tube of some of their electrons. Electrons are negatively charged, so they flew down the tube towards the other end. Thanks to the partial vacuum, those electrons were able to shoot through the tube without any big atoms getting in their way.

The electrical charge made the electrons move very quickly indeed – around 37,000 miles per second (59,500 kilometres per second) – until they smashed into the glass at the far end, knocking into yet more electrons associated with the atoms there. Amazingly, the collisions between these mind-bogglingly tiny particles generated so much energy that it created a fantastic green-yellow glow.

A Crookes tube with Maltese cross-shaped metal (Credit: sciencephotos/Alamy Stock Photo)

“It’s one of the first particle accelerators in a way,” says Cliff. “It’s accelerating electrons from one end of the tube to the other and they strike the screen at the other end and give this phosphorescent glow.”

The discovery of the electron suggested there was more to learn about atoms

Because Thomson found that he could actually steer the beams of electrons with magnets and electrical fields, he knew they weren’t just weird rays of light – they had to be charged particles.

And if you’re wondering how these electrons could go flying around independently of their atoms, that’s because of a process called ionisation, in which – in this case – an electrical charge changes the atom’s structure by pushing those electrons off into the space around.

In fact, it’s because electrons are so easily manipulated and moved around that electrical circuits are possible. Electrons in a copper wire travel in a train-like motion from one atom of copper to the next – and it’s that which carries the charge through the wire to the other end. Atoms, it’s worth noting again, are not solid little pieces of matter, but systems that may be modified or undergo structural changes.

Light bulbs glow because of the flow of electrons (Credit: Feng Yu/Alamy Stock Photo)

But the discovery of the electron suggested there was more to learn about atoms. Thomson’s work revealed that electrons are negatively charged – but he knew that atoms themselves had no overall charge. He reasoned they must contain mysterious positively charged particles to cancel out the negatively charged electrons.

He had demonstrated the existence of a dense nucleus within the atom

Experiments at the beginning of the 20th Century identified those positively charged particles and at the same time revealed the atom’s solar system-like internal structure.

Ernest Rutherford and his colleagues took very thin metal foil and put it under a beam of positively charged radiation – a stream of small particles. Most of the powerful radiation sailed right through, just as Rutherford thought it would, given how thin the foil was. But surprisingly, some of it bounced back.

Rutherford reasoned that the atoms in the metal foil must contain small, dense areas with a positive charge – nothing else would have the potential to reflect the radiation to such a strong degree. He had found the positive charges in the atom – and simultaneously proved they were all bunched together in a tight mass in a way that electrons aren’t. In other words, he had demonstrated the existence of a dense nucleus within the atom.

Cambridge physicist James Chadwick was desperate to discover the neutron

However, there was still a problem. By now, the mass of atoms could be estimated. But given what was known about how heavy a particle in the nucleus should be, the idea that they were all positively charged didn’t make sense.

“Carbon has six electrons and therefore six protons in the nucleus – six positive charges and six negative charges,” explains Cliff. “But the nucleus of carbon doesn’t weigh six protons, it weighs [the equivalent of] 12 protons.”

Early on it was thought the other six nuclear particles would have the same mass as protons but be neutrally charged: neutrons. But no-one could prove this. In fact, neutrons weren’t actually discovered until the 1930s.

Everything around us is made of atoms (Credit: Magictorch/Alamy Stock Photo)

Cambridge physicist James Chadwick was desperate to discover the neutron. He’d been working on the theory for years. In 1932, he made a breakthrough.

In the 1930s we had figured out a lot about atoms, but no-one had produced a direct image of one

A few years earlier, other physicists had been experimenting with radiation. They fired positively charged radiation – the same sort that Rutherford had used to discover the nucleus – at beryllium atoms. The beryllium kicked out radiation of its own: radiation that was neither positively nor negatively charged, and that could penetrate far through material.

By this time, others had already worked out that gamma radiation was neutral and deeply penetrating, so the physicists assumed this is what the beryllium atoms were releasing. But Chadwick wasn’t convinced.

He generated some of the new radiation himself and aimed it at a substance which he knew was rich in protons. Unexpectedly, the protons were knocked into the air away from the material as though they had been hit by particles with the same mass – like snooker balls being hit by other snooker balls.

Gamma radiation can’t deflect protons this way, so Chadwick realised the particles in question here must have the same mass as the proton but lack its electrical charge: they were neutrons.

All the key bits of the atom had been figured out, but the story doesn’t stop there.

You can even work out what atoms look like by poking at them

Although we had figured out a lot more about atoms than we had before, they were still difficult to visualise. And back in the 1930s, no-one had produced a direct image of one – which is what many people would want to see in order to really accept that they are there.

Importantly, though, the techniques that had been used by scientists like Thomson, Rutherford and Chadwick, would pave the way for new equipment that would eventually help us produce those images. The beams of electrons Thomson generated in his Crookes tube experiment proved particularly useful.

Today similar beams are generated by electron microscopes, and the most powerful of these microscopes can actually create images of individual atoms. This is because an electron beam can have a wavelength thousands of times shorter than a light beam – so short, in fact, that electron waves can be deflected by tiny atoms to generate an image in a way that light beams can’t.

Neal Skipper at University College London says such images are useful for people who want to study the atomic structure of special substances – those used to make batteries for electric cars, for example. The more we know about their atomic structure, the better we can design them to be efficient and reliable.

Atomic force microscopes can show us individual atoms (Credit: Flirt/Alamy Stock Photo)

You can even work out what atoms look like by poking at them. This is essentially how atomic force microscopy works.

In a liquid, as you heat it up, you can see the atoms have more disordered configurations

The idea is to bring the tip of an extremely small probe close to the surface of a molecule or a material’s surface. At such close quarters, the probe will be sensitive to the chemical structure of whatever it’s pointed at, and the change in resistance as it moves across it allows scientists to produces images of what, for example, an individual molecule looks like.

Recently, researchers published wonderful images of a molecule before and after a chemical reaction using this method.

Skipper adds that a lot of atomic research today explores how the structure of things changes when a high pressure, or extreme temperature, is applied. Most people know that when a material is heated, it often expands. It’s now possible to detect the atomic changes that occur which makes this possible.

“In a liquid, as you heat it up, you can see the atoms have more disordered configurations,” says Skipper. “You can see that from the structural map directly.”

Skipper and other physicists can also work on atoms using the neutron beams first identified by Chadwick in the 1930s.

You can identify atoms by detecting the energy of gamma rays alone

“What we do a lot is to fire beams of neutrons at lumps of materials and from the scattering pattern that emerges you can figure out that you were scattering neutrons from the nucleus,” he says. “You can work out the mass and the rough size of the object that was doing the scattering.”

But atoms aren’t always just sitting there, calmly stable, waiting to be examined. Sometimes they are decaying – which means they are radioactive.

There are lots of naturally occurring radioactive elements. The process generates energy, which forms the basis of nuclear power – and nuclear bombs. Nuclear physicists’ research generally involves trying to better understand reactions in which the nucleus undergoes fundamental changes like these.

Uranium atoms can split into two (Credit: Peter Hermes Furian/Alamy Stock Photo)

Laura Harkness-Brennan at the University of Liverpool specialises in the study of gamma rays – a type of radiation emitted by decaying atoms. A radioactive atom of a given type generates a specific form of gamma ray. That means you can identify atoms by detecting the energy of gamma rays alone – and this is exactly what Harkness-Brennan does in her lab.

We haven’t just worked out what atoms are, we’ve realised that they are marvellously complex structures

“The types of detectors that you would use are detectors that allow you to measure both the presence of the radiation but also the energy of the radiation that’s being deposited,” she says, “and that’s because the nuclei all have a characteristic fingerprint.”

Because there might be all sorts of atoms present in an area where radiation is detected, especially after a large nuclear reaction of some kind, it’s important to know precisely which radioactive isotopes are present. This sort of detection is commonly done in nuclear power plants, or areas where there have been nuclear disasters.

Harkness-Brennan and her colleagues are now working on detection systems that can be set up in such places to show, in three dimensions, where radiation might be present in a particular room. “What you want to do is to have techniques and tools that allow you to image a three dimensional space and tell you in that room, in that pipe, that’s where the radiation is,” she says.

Given how small the atom is it’s amazing how much physics we can get out of it

It’s also possible to visualise radiation in a “cloud chamber”. This is a special experiment in which alcohol vapour, cooled to -40C, drifts in a cloud around a radioactive source. Charged particles of radiation flying away from the source remove the electrons from alcohol molecules. This makes the alcohol condense into liquid around the path of the emitted particle. The results of this type of detection are really rather stunning.

We haven’t just worked out what atoms are, we’ve realised that they are marvellously complex structures that can undergo amazing changes – many of which occur naturally. And by studying atoms this way, we’ve been able to improve our technologies, harness the energy of nuclear reactions and better understand the natural world around us. We’ve also been able to better protect ourselves from radiation and discover how materials change when placed under extreme conditions.

Harkness-Brennan puts it well: “Given how small the atom is it’s amazing how much physics we can get out of it.”

Everything we can see around us is made of these little things. It’s good to know they’re down there, making it all possible.