by Tim James
By taking hundreds of photographs in different positions and playing them at high speed, the IBM researchers were able to tell the story of an atomic stick-figure who plays with a pet atom. This wasn’t an easy thing to do, because as we saw in the previous chapter atoms are too small to be seen.
The trick to their film is that each photograph of the atomic models is not really a photograph. They are images obtained from a scanning tunneling microscope (STM), a device that allows us to peer at distances smaller than visible light can access.
Imagine standing beside a dark hole and dropping a rock over the edge. By timing how long it took to reach the bottom, you could calculate how deep the hole was without being able to see it. STMs work on a similar principle.
The business end of an STM is not a lens but a thin nozzle with tiny particles clinging to the tip. These particles are bound loosely so when you apply an electric current they fall off and land on the surface beneath. As they fall they lose a certain amount of energy, which the STM can measure, calculating how far away the surface is.
By scanning the tip back and forth across an object, any bumps and blips will correspond to a different amount of energy being lost, and the STM can indirectly create a map of what the object must look like.
The filming of A Boy and His Atom was carried out by creating a flat sheet of copper and bonding carbon monoxide particles to it in specific positions. As the microscope scanned across the copper, it picked up these carbon monoxides like dots on a Braille picture and created the corresponding image in the computer.1
It’s a novel idea but how can it be possible? In order to detect the outline of an atom, our STM would need to be dropping particles even smaller than atoms. Where can we find particles that small?
CALL ME “J. J.”
At the turn of the twentieth century, the main pursuit of any serious physicist was trying to understand electricity. There were two leading nineteenth-century hypotheses under consideration, each supported by some of the biggest names in science. In one corner was the legendary Hermann von Helmholtz, a staunch believer in particles. He argued that since arcs of electricity cast shadows, for example, it had to be made from matter—electrical atoms.
Leading the opposition was his student, Heinrich Hertz, who preferred to explain things with invisible force fields. Having recently shown that magnetic fields could be used to bend the path of electric current, Hertz argued that electricity also had to be a disturbance in some sort of electric field.2
Their disagreement was passionate, although Helmholtz and Hertz remained good friends until the end. Sadly they both died in 1894, shortly before the matter was finally settled by a brilliant British physicist named Joseph John “J. J.” Thomson. It was Helmholtz who had been right.
J. J. Thomson was, by anyone’s account, a wunderkind of science. He was admitted to the University of Manchester at the age of fourteen and was later appointed to the most prestigious physics post in Britain, taking over from Lord Raleigh as the Cavendish Professor of Experimental Physics at Cambridge University.
The precise details of Thomson’s electricity experiments are very mathematical but the premise is straightforward. Fill a small chamber with gas and connect two ends of a circuit to the front and back. At a high voltage it is possible to generate streams of electricity through the gas and, if you place magnets at certain points, you can manipulate their behavior.
By carrying out a variety of studies on this theme, Thomson made several crucial observations. Most important was the fact that electricity moved slowly. Hertz’s field hypothesis predicted electricity should move at the speed of light but Thomson’s measurements clocked it as practically sluggish by comparison. This meant electricity had mass and was therefore made from particles.
The Irish scientist George Stoney called these particles electrons, from the Greek electron, meaning amber (which could be rubbed to create shocks of static electricity), and it caught on. Except electrons were notably different from other particles.
For one thing, the atoms discovered by Dalton and Einstein were two thousand times bigger. In fact, it was possible to shoot a stream of electrons through a plate of solid iron because they could apparently fit through the gaps.
Normal atoms are also happy to approach each other whereas electrons actively repel. This repulsive property was named charge and, in all honesty, it’s still a mystery. We can measure its influence and describe the mechanism that causes one electron to repel another, but why electrons have charge is not yet understood.
More pressing for Thomson was the issue of where electrons were coming from. Batteries are composed of regular atoms (bigger and chunkier) so electrons had to be somehow hidden within them. Apparently, atoms weren’t the smallest things after all—they contained electrons.
So how come atoms didn’t have this property of charge? If the electrons within them were repelling, how were two atoms able to approach each other and even bond?
Thomson concluded that atoms had to contain some additional substance with an anti-charge, canceling the electron charge and giving atoms the appearance of being neutral overall.
Thomson proposed that electrons were nestled within a kind of atomic sponge. Slice away a segment of an atom and you’d see the electrons arranged like plums in a traditional British Christmas pudding. A bit like this:
The electrons and dough had opposite and attractive charges, which was why it took so much effort to pull electricity from an atom—you had to rip electrons away from their complementary dough.
Thomson’s model of the atom was given the rather catchy name of “The Plum-Pudding Hypothesis.”
THE IMPORTANCE OF BEING ERNEST
The name atom had stuck by the time Thomson published his work, which is a shame as it’s notoriously misleading. What we call atoms are not uncuttable at all, and neither are they the smallest things. They’re just stable structures that prefer not to be pulled apart.
Electrons are the truly uncuttable particles and, as far as Thomson could figure, they were suspended in an oppositely charged dough. But science makes progress by disproving a hypothesis, not by proving it, and the plum-pudding idea was eventually torn to pieces by Thomson’s student Ernest Rutherford.
Raised on a New Zealand sheep farm, Rutherford was known for rejecting expensive equipment and carrying out ludicrous experiments because nobody else was doing them. His unorthodox approach earned him the 1908 Nobel Prize in Chemistry, though, so people tended to let him get on with it.
He won the prize for discovering that larger atoms could spit out tiny pieces, which he called alpha particles, that are much heavier than electrons and carry the opposite charge.
Rutherford assumed this happened because atomic dough repelled itself and when the atoms were large there was more chance of self-repulsive instability, leading ultimately to an explosion. The alpha particles he discovered were believed to be bits of atomic dough spat out by the micro-blasts.3
Most people would have accepted the Nobel Prize and moved on, but Rutherford was a scientist to the core. He wanted to put his own hypothesis on the chopping block and see if he could disprove it. So he hired the world’s best experimentalist, Hans Geiger, and together they developed a method for probing the interior of an atom.
They discovered that alpha particles would produce tiny flashes of light when they hit a piece of zinc sulfide (ZnS), so they spent countless hours sitting in dark rooms firing alpha particles at ZnS, looking for flashes through a lens.
The boredom was unbearable, so Geiger invented an electronic counter that would detect the impacts automatically. His invention was the crackling Geiger counter used in a thousand spy movies ever since.
One morning in 1909, Geiger went to see Rutherford to talk about one of their promising undergraduates, Ernest Marsden. Marsden was only twenty but was gaining a reputation for exceptional lab prowess.
Geiger wanted to give him a new project so Rutherford, in his typical oddball style, came up wi
th something peculiar: “Why not let him see if any alpha particles can be scattered through a large angle in the gold foil experiments?”4
The gold-foil experiments had been designed a few years earlier. By taking a piece of radium (a highly alpha-spitting metal) and pointing it at a thin foil you could shoot alpha particles right through the foil. Placing a detector on the other side would let you measure how much the particles were affected by the foil and gave clues about the density of the atomic dough. The best metal was gold because it could be stretched into a leaf only a few atoms thick. The setup looked like this:
For some reason, Rutherford wanted Geiger and Marsden to put the detector at huge angles to the foil rather than directly on the other side. Geiger must have been puzzled since surely the detector would read nothing but, given Rutherford’s reputation (the Nobel Prize medal on his desk probably helped too), he just shrugged and set Marsden to work. The very next day, Rutherford’s eccentricity paid off.
The detector began picking up scattered alpha particles even when the detector was moved to the same side as the alpha source. This couldn’t be explained with the plum-pudding hypothesis because how could an alpha particle get bounced back by dough? It would be like setting up a machine gun to fire at an actual plum pudding and have the bullets bounce back and shoot you in the face. You would expect them to cut right through the pudding and hit the opposing wall, so why are you now in hospital? And what explanation are you going to give the admissions nurse?
Rutherford described it in similar terms: “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you.”5
The results were published in February 1910 and, by the following year, Rutherford had run the math. There was only one possible explanation for the result. The atom wasn’t a soft sponge all the way through but had hard lumps for the bullets to bounce off. The plum pudding apparently contained nuts.
These nuts were most likely small and clustered in one place within the atom, since only a few bounces were detected for every thousand bullets. It would also make sense for them to have the same charge as alpha particles in order to scatter them when impacted, not to mention holding the electrons in place.
Rutherford called this clump of particles the nucleus, from the Latin for nut, and proposed that electrons orbited it like planets around the Sun. Thomson’s plum-pudding idea had to be abandoned. It was ingenious, but it had no evidence and in science no evidence means no theory.
YOU WANNA GET NUTS? LET’S GET NUTS!
Did Rutherford have a hunch about the nucleus or was he just fooling around when he suggested moving the detector? Was he trying to think of some task for Marsden and that was the only thing he could come up with at short notice?
Personally, I like to imagine Marsden putting the detector on the wrong side as a sulky middle finger to Rutherford. Here was this great man giving him a stupid task to perform. Oh, you want wide angles? How does the wrong side of the foil sound to you? That wide enough for you, Rutherford?
We’ll probably never know but whatever happened in that lab, and whatever went through the minds of the three men, the results have become a part of science lore.
There was still a niggling question that needed answering, though. Rutherford’s idea was that the nucleus contained particles with a charge opposite to an electron, but if this was so, why wasn’t it ripping itself apart? Particles with the same charge repel each other so the nucleus shouldn’t exist at all. The answer was discovered by another of Rutherford’s students, James Chadwick, in 1932.
Using a piece of polonium, known to eject alpha particles, Chadwick bombarded a lump of beryllium metal and set up a piece of wax on the other side to cushion any impacts.
Every time there was an emission from the polonium, something inside the beryllium came flying out the other side as if a pool-ball collision was taking place within the nuclei. These ejected particles were obviously heavy but they didn’t repel each other, meaning they had to be neutrally charged.
They also had to have some glue-like property that held charged particles together more powerfully than they could repel themselves.
The nut of the atom was apparently made of two types of particle. Neutrons (the neutral ones), which had the glue property, and charged protons (from the Greek word for first), which held the electrons in place. Further research from Niels Bohr, Werner Heisenberg, and Oskar Klein elaborated on Rutherford’s findings, and the popular view of the atom was eventually established.
Atoms were like solar systems. Protons and neutrons formed the central nucleus with oppositely charged electrons whizzing around the edge with apparently nothing in between.
If you imagine expanding an atom to the size of a football stadium, an electron would become the size of a dust mote while protons and neutrons would be huddled together in a nucleus, roughly the size of a golf-ball hovering in the center.
The strangest conclusion from this is that most of an atom is empty space. Even something like osmium, the densest element, is apparently 99 percent nothing. As are you.
HIDDEN ELEMENTS
In the Superman movie Man of Steel, the spaceship that brings Kal-El to Earth is analyzed by chemists and found to be made of elements that don’t fit on the periodic table.6 The periodic table is a list of all known elements so the Kryptonians obviously have different elements on their planet to ours.
The idea of hidden elements is a tantalizing one and it’s been thrown around in fiction for decades. In the H. P. Lovecraft story “The Dreams in the Witch House,” the protagonist discovers a small statue made of an element that cannot be identified by any scientist.7 Lovecraft was inspired by a physics lecture he attended in the same year neutrons were discovered, but could such things really exist? Could there be exotic elements tucked away in unknown corners of the Universe?
Not that I want to destroy these fictional stories, but the answer is no. You can’t have hidden elements for a straightforward reason: atoms aren’t atomic. Yeah, I know, right: what the hell?
The word atom was obviously supposed to mean uncuttable but it’s really the electrons, protons, and neutrons that fit this description. The word atom had stuck, however, so we still call them that even though “electron-proton-neutron-superstructures” would be a more accurate term.
The smallest possible atom would logically contain one proton (and one electron since the charges always cancel). This would be element number 1, which turned out to be Cavendish’s exploding gas—hydrogen.
The next element would have two protons (plus some neutrons to glue them together). That turned out to be helium. It wouldn’t be possible to have element 1.5 in between because there is no such thing as half a proton (see Appendix II).
Once you’ve got a list of all the elements, you can be sure you haven’t missed anything because nature is only able to make atoms in whole numbers. The elements you find on Earth are the same elements you find everywhere in the Universe. Which is where we’re going in the next chapter.
So I’m sorry, Superman, your spaceship isn’t possible. Interestingly, though, kryptonite is real. The chemical formula for kryptonite is LiNaSiB3O7(OH)F2, a mineral which was discovered at a Serbian mine in 2007.8
CHAPTER FOUR
Where Do Atoms Come From?
THE COLDEST PLACE IN THE UNIVERSE
The temperature scale we use for our everyday lives was invented in 1742 by Anders Celsius. He took the freezing and boiling temperatures of fresh water, divided the scale into a hundred chunks, and called them “centigrades” from the Latin for hundred steps.
Celsius’s original thermometer defined 100°C as freezing and 0°C as boiling, but this was reversed after his death and the scale was renamed the Celsius scale in his honor. The Fahrenheit scale, used more widely in the United States, was invented by Daniel Fahrenheit, who used salted ice and created a scale going up to human
body temperature.
Whichever scale you’re using, the behavior of particles is the same: as you heat up something, the average speed of its particles increases. Because higher temperatures cause particles to fly around more, this also tells us that when gases get hotter they take up more space. Conversely, if particles become colder they occupy a smaller volume because they move around less. Hotter gas = bigger. Colder gas = smaller.
This simple relationship between temperature and volume is called Charles’s law after Jacques Charles, the physicist who discovered it. But obviously this relationship can’t go on forever. If you keep cooling things down further and further then the volume shrinks with it, so eventually you should reach a temperature where the volume drops to zero.
Charles’s law implies that there’s a temperature so cold the particles would take up no space, effectively winking themselves out of existence. This hypothetical temperature is clearly impossible so we call it “absolute zero,” calculated to be –273.15°C. It’s a temperature so cold you would have to break the laws of physics to reach it.
The coldest place on Earth is usually reported as a point in Antarctica near Dome Argus, which drops to –93.2°C during the winter season.1 The emptiness of deep space has an average temperature of –270°C while the Boomerang Nebula stoops to –272°C, one degree above what’s physically possible.2
But the all-time record for coldest place in the Universe is right here on Earth, at the lab of Martin Zwierlein in Massachusetts, where his team have been able to synthesize a chemical called sodium-potassium, the coldest chemical ever created.
Usually when two atoms bond (see Chapter 8), we attach the suffix ide to whichever element isn’t a metal, e.g. iron oxide. A bond between two metal atoms is so rare, however, that we haven’t invented a naming system for it, hence the rather unusual-sounding sodium-potassium.
