By the time physicists (including Marie Curie, Ernest Rutherford and Albert Einstein) gathered for the 1911 Solvay Conference, they had a pressing new question to answer: What’s inside the atom? CLICK TO ENLARGE
J.J. Thomson of Cambridge University discovered the electron in 1897, setting off a race to find the rest of the atom’s pieces.
Thomson’s discovery of the electron came from his study of the Crookes tube, an early version of the cathode ray tubes that were later common in television sets. An electrical current gave off a glow as it passed from the cathode at one end to the anode at the other. Thomson found that even when he used different metals for the cathode, the resulting electrons were always the same – a sign that the electron was a piece of every atom.
Marie Curie’s study of the curious rays emanating from uranium led to the discovery of radioactivity and two new elements: polonium and radium. Photo: ACJC
Marie and Pierre Curie’s discovery that radium glows in the dark led to the realization that radioactivity was atoms falling apart and spitting out energy. That meant there must be smaller pieces inside, still awaiting discovery. Photo: ACJC
In 1910, Rutherford and two of his graduate students used radioactive alpha particles to make an astonishing discovery about the structure of the atom.
Rutherford’s vision of the atom: a dense core called the nucleus containing the atom’s positive charge and most of its mass, surrounded by tiny, negatively charged electrons, orbiting at a much greater distance from the nucleus than this drawing suggests.
In 1913, physicist Harry Moseley discovered there was a simple relationship between an element’s X-ray spectrum and its atomic number. This led to the discovery of the positively charged particle called the proton – and the realization that each element is defined by the number of protons in its nucleus: its atomic number.
Rutherford’s Manchester physics laboratory included one of the finest groups of scientists ever assembled in one place. Along with Harry Moseley, they included Hans Geiger (inventor of the Geiger counter), Charles G. Darwin (grandson of the great biologist), and James Chadwick, who would win a Nobel Prize for his discovery of the neutron. CLICK TO ENLARGE
The discoveries of Moseley and Rutherford raised a puzzling question about the atom: Why did the atomic weights of the elements rise so much faster than their atomic numbers? The puzzle was solved by James Chadwick’s 1932 discovery of the neutron.
Using neutrons to bombard uranium atoms, Italian physicist Enrico Fermi discovered what he thought were the first elements beyond uranium. For this he won the Nobel Prize in 1938.
Four German scientists, including longtime collaborators Otto Hahn and Lise Meitner, discovered that Fermi had actually split the uranium in half, into much lighter elements – an outcome almost no one had thought possible.
Ironically, in one sense the discovery of atomic structure proved the ancient Greeks correct: Matter really is made up of just a few things in combination – not air, water, earth and fire, but protons, neutrons and electrons.
The Unchangeable, Unsplittable Atom
One of the most profound shifts in our understanding of matter occurred over a 40-year period that began on the cusp of the 20th century. Up to that point, nearly all scientists subscribed to two unshakeable beliefs about matter:
- The first was that atoms were the smallest units of matter. Each element in the Periodic Table had its own unique kind of atom – that’s what distinguished one element from another. And most chemists believed there was no way to cut things any finer. Indeed, the very word atom comes from the Greek atomos, meaning “uncuttable.”
- The second was that these atoms were stable and permanent. The alchemist’s ancient dream of turning one element into another had been banished once and for all. Chemists were now confident that the elements had been, and always would be, the same – forever fixed.
By the end of that 40-year period, however, a rapid-fire series of discoveries by scientists across Europe had shattered both these bedrock principles. The unchangeable, unsplitabble atom had been debunked. In its place was a radically new view in which matter was more malleable, more unstable – and more potent.
Thomson Discovers the Electron
The first hint that matter might still hold some surprises came in 1897, when physicist J.J. Thomson of England’s Cambridge University discovered what seemed to be an unthinkably small particle. Thomson had set out to study the electrical rays that passed through a glass device called a Crookes tube, lighting up the far end in a way that thrilled crowds still mystified by electricity. When Thomson moved a magnet near the tube, he saw that it bent the path of the beam. Electricity, he realized, must be made up of negatively-charged particles – what soon came to be called “electrons.” But Thomson realized that the electron was not just the unit of electricity, because even when he used different metals to generate the rays, the resulting electrons were always the same. His bold conclusion was that the electron must be a tiny piece of every atom – nearly two thousand times smaller than the smallest atom.
Thomson’s electron was so outlandish that many scientists initially refused to believe anything so tiny could exist. But the very next year, the findings that started pouring out of the laboratories of Marie and Pierre Curie began to turn the tide.
Marie Curie and Radioactivity
As a woman, Maria Sklodowska was barred from attending university in her native Poland, so she had come to study science in one of the few places in the world where she could: Paris. Soon after graduating from the Sorbonne, she met and married a French physicist named Pierre Curie. After giving birth to their first child, she set out to become the first female scientist to earn a doctorate in science in France. Needing a topic for her dissertation, she considered X-rays, which had just been discovered and were the talk of scientific world. But Marie chose instead to focus on another newly discovered ray that was given off by the element uranium.
Like X-rays, “uranic rays” had the power to penetrate thick black paper and make an image on a photographic plate; they also “ionized” the air around them, making the air a better conductor of electricity. Using delicate instruments devised by Pierre, Marie painstakingly tested all the other known elements to see if any of them had similar powers. In early 1898, she discovered that the element thorium behaved in the same way. Since this property was not limited to uranium, it needed a new name. Marie called it “radioactivity.” She went on to use radioactivity to identify two new elements: polonium, named for her homeland, and radium.
Once they had isolated enough of it, the Curies discovered that radium had another mysterious property: It glowed in the dark, seemingly forever. Where was that energy coming from? The Curies couldn’t decide, but their discoveries prompted others around the world to conclude that the energy was a result of the disintegration of atoms. The radium atom was breaking down, spitting out pieces of itself, and generating energy in the process. This theory had a profound implication: If radioactivity was atoms falling apart, then atoms must have parts. There must be smaller pieces inside, still awaiting discovery. Thanks to the Curies, scientists had a pressing new question to answer: What’s inside the atom?
In light of the Curies’ discoveries, Thomson’s impossibly tiny electron suddenly made sense: It was one piece of the atom. Now the race was on to discover the rest of the atom’s pieces and understand how they fit together.
Rutherford Discovers the Nucleus
By 1910, it was generally accepted that J.J. Thomson’s tiny, negatively charged electron was one piece of the atom. But that left two big unanswered questions: Since most atoms were electrically neutral, where in the atom were the positive charges needed to offset those negative electrons? And since electrons accounted for only a tiny part of it, where was the rest of the atom’s mass?
One of the scientists working to answer these questions was physicist Ernest Rutherford at the University of Manchester in England. Rutherford had already won the Nobel Prize for his discovery that radium atoms spit out different kinds of particles (alpha and beta) as they decay. Now he asked two of his graduate students, Ernest Marsden and Hans Geiger, to use alpha particles to probe the structure of the atom. They aimed a beam of alpha particles at an ultrathin sheet of gold foil. Most of the time, the alpha particles would sail right through. But every so often, one of the projectiles would bounce practically right back in their faces. “It was the most incredible thing that has ever happened to me,” Rutherford recalled. “It was almost as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you!”
In late 1910, Rutherford came into the lab one day and announced he knew what this surprising result meant: The atom must be mostly empty space but have an incredibly dense, hard center. What emerged from Rutherford’s work was a brand new vision of the atom. Its positive charge and almost all of its mass were concentrated in a tiny central core – the nucleus. And whirring around the nucleus, at a great distance on the scale of the atom, were the negatively charged electrons.
“One of the most remarkable things about the atom is that it’s mostly made of nothing!” says University of Maryland physicist Jim Gates.
“I think the feeling in those hallways, the laboratories of Manchester, must have been one of great excitement,” says MIT historian David Kaiser. “They could sense that Rutherford and his team had literally cracked open a new view of matter.”
Moseley and Atomic Number
The next critical discovery about the atom came from another member of Rutherford’s Manchester lab, physicist Harry Moseley. In 1912, German scientists had discovered that X-rays could be split up, or “diffracted,” into different wavelengths, just as light can be split into different colors. The next year, Moseley showed that each chemical element has its own unique X-ray spectrum, analogous to the light spectra that had been so helpful in identifying new elements since the mid-1800s. More surprisingly, Moseley found there was a remarkably simple relationship between an element’s X-ray spectrum and its “atomic number.”
Up to then, atomic number had simply referred to the number of an element’s box in the Periodic Table. But Moseley’s results showed that atomic number was more than a convenient label. It represented the positive charge on an atom’s nucleus, and it invariably increased by a single number from one element to the next. The implication of Moseley’s findings was that the nucleus is not one big positive blob, but a collection of positively charged particles that increase in number with each heavier element.
Building on Moseley’s work, Rutherford soon discovered this next piece of the atom – the proton – and showed that each element in the Periodic Table is defined by the number of protons in its nucleus: its atomic number. An element’s atomic number is what gives that element its identity: Add one more proton, or take one away, and you have a different element. “Moseley and atomic number – that’s really the crucial moment where we find out what an element really is,” says Johns Hopkins University historian Lawrence Principe.
Chadwick Discovers the Neutron
Harry Moseley’s discovery of the importance of atomic number raised a troubling new question about the atom: As you move across the Periodic Table, the atomic numbers of the elements rise by a single digit from one element to the next. Hydrogen is 1, helium is 2, lithium is 3, and so forth. But the atomic weights of the elements rise much faster. Hydrogen has an atomic weight of 1, accounted for by its lone proton. Helium has two protons but an atomic weight of four; lithium has three protons and an atomic weight of 7. The gap between atomic number and atomic weight keeps getting wider and wider. What could account for this missing mass? Was there another part of the atom, still to be discovered?
The answer would finally come in 1932, when another of “Rutherford’s Boys,” physicist James Chadwick, discovered the final piece of the atom: the neutron. The neutron has almost the same mass as the proton, and they both occupy the nucleus. Except for hydrogen, which contains no neutrons, almost every atom has at least as many neutrons as it does protons. These neutrons account for the rest of each element’s atomic weight – and they solved the riddle of the missing mass. But the neutron, as the name implies, is electrically neutral, and this unusual property would soon give it a central role in events that revealed shocking new things about the atom.
The Atom Splits
As soon as the neutron was discovered, scientists realized it was the perfect projectile for exploring the innards of the atom. Unlike those positively charged alpha particles that Rutherford and his students had been using, the neutron would not be repelled as it approached the nucleus. Because it has no charge, it could go right in.
One of the first to use the neutron in this way was an Italian physicist named Enrico Fermi. In 1934, Fermi began firing neutrons at uranium atoms, creating a shower of fragments he would then analyze. He found that a neutron sometimes chipped off a piece of the uranium nucleus, lowering its atomic number and turning it into a different element, a few spots lower in the Periodic Table. But some of Fermi’s fragments didn't match any of the elements just below uranium. He concluded that sometimes an incoming neutron is absorbed by the uranium nucleus … and then spontaneously changes into a proton, raising the atom’s atomic number and turning it into a different element heavier than uranium.
For his discovery of the first two elements beyond uranium, Fermi won the Nobel Prize in 1938. But just as he was shaking the hand of the King of Sweden, German scientists were making the discovery that would prove him wrong. Chemists Otto Hahn and Fritz Strassman had repeated Fermi’s experiments, but instead of finding new elements heavier than uranium, they found well-known elements that were much lower in the Periodic Table – about half as heavy as uranium. Puzzled, Hahn wrote to his long-time collaborator, Lise Meitner, a Jewish physicist who had fled Germany after the rise of the Nazis. Meitner realized the findings meant the atom had split in half, in a way almost no one had imagined possible. Her colleague Otto Frisch coined the term by which we know this process today: nuclear “fission.”
A New View of Matter
The tremendous energy released when the atom split had profound implications for a world at the brink of war. In just six years, the discovery of nuclear fission would lead to the atomic bomb. It would also deliver the final blow to the old image of the unsplittable, changeable atom.
In the four decades since J.J. Thomson had set out to understand those mysterious rays in his Crookes tube, a series of discoveries by scientists across Europe had shown that atoms were not the smallest units of matter; nor were they fixed for all time. It was now clear that atoms had smaller parts inside them – and that changes to those parts would alter the identities of the atoms themselves.
Ironically, these discoveries vindicated some of the most ancient beliefs about matter:
- The alchemist’s dream of changing lead into gold was still not a practical path to wealth. But it was possible for one element to turn into another. In fact, it’s happening spontaneously all the time in radioactive elements.
- And despite the discovery of many new elements over these four decades, it was now clear that matter really is made of just a few things in combination, just as the alchemists and the ancient Greeks suggested centuries ago. Not air, water, earth and fire ... but protons, neutrons and electrons. All of matter is made up of just these three particles, mixed in different ratios. “It’s amazing how far nature has been able to get with that extraordinary, simple recipe,” says Brandeis University chemist Gregory Petsko.
Just six years after the discovery of nuclear fission, an atomic bomb powered by the fission of plutonium destroyed the Japanese city of Nagasaki, bringing World War II to an end.
The alchemists were wrong in believing they could get rich by turning lead into gold. But they were right in one sense: It is possible for one element to change into another. Radioactive elements do it all the time.