The particle theory of matter was not so much discovered as it was formulated, and that formulation began in ancient Greece.
The person who is credited with having conceived of the idea that the world is composed of tiny, indivisible particles is the philosopher Democritus, who lived from 460 to 370 BCE. He devised an experiment to prove his idea, and while the Democritus experiment may seem overly simplistic today, it helped give birth to the concept of the atom, which is central to the modern understanding of matter.
In the centuries that followed the experiment, the Democritus particle theory didn't make much progress, but at the turn of the nineteenth century, it was taken up by English chemist and physicist John Dalton (1766 - 1844).
Dalton's work remained virtually unchanged for the better part of a century until a crew of modern physicists that included such names as Thompson, Rutherford, Bohr, Planck and Einstein got involved. That's when sparks began to fly, and the world entered the nuclear age.
The Democritus Particle Theory
It sounds as if the word "democracy" may have been derived from his name, but Democritus was not a political philosopher. The word actually comes from the Greek words demos, which means "the people," and kratein, which means "to rule."
Known as the "laughing philosopher" because of the great importance he placed on cheerfulness, Democritus did coin another important word: atom. He referred to the small particles that make up everything in the universe as atomos, which means uncuttable or indivisible.
This was not his only pioneering contribution to science. Democritus was also the first to posit that the light we see from the Milky Way is the combined light of a multitude of individual stars. He also proposed the existence of other planets and even postulated the existence of multiple universes, an idea which is on the cutting edge of science today.
According to Aristotle (384 - 322 BCE), Democritus believed that the human soul was composed of fire atoms and the body of earth atoms. This was contrary to Aristotle's belief that the world consists of the four elements of air, fire, earth and water, and that the ratio of the elements determined the characteristics of the matter.
Aristotle even believed that the elements could be transformed into one another, an idea that fueled the search for the Philosopher's Stone throughout the Middle Ages.
The Democritus Experiment to Prove the Existence of Atoms
Neither Aristotle nor the equally influential Plato (circa 429 - 347 BCE) subscribed to the Democritus particle theory, and it would take 2,000 years for the "laughing philosopher" to be taken seriously. That could have something to do with the experiment that Democritus devised to prove his theory, which was less than convincing.
Democritus reasoned that if you take a stone or some other object and continue to divide it in half, you eventually come to a piece that is so small that it can't be divided any more. It is said that he performed this experiment with a seashell, and when he reduced the shell to a fine powder that he could no longer cut into smaller pieces, he considered that proof of his theorem.
Democritus was a materialist, unlike Plato and Aristotle, who believed that the purposes of events were more important than their causes. He was a pioneer in mathematics and geometry, and he was among few people at the time who believed the earth was spherical. Even if he couldn't prove it convincingly, his conception of atoms existing mostly in empty space, each having a little velcro-style hook that allowed it to connect with other atoms, isn't that far removed from the modern scientific model of the atom.
John Dalton and Modern Atomic Theory
Was Democritus' theory correct? The answer is a qualified yes, but it wasn't even considered as a possibility until 1800. That's when John Dalton revisited it while he was working on the Law of Constant Composition advanced by French chemist Joseph Proust. Proust's law followed directly from the Law of Conservation of Mass, which had been discovered by another French Chemist, Antoine Lavoisier.
The Law of Constant Composition states that a sample of a pure compound, no matter how it is obtained, always contains the same elements in the same mass proportions. Dalton realized this could only be true if matter consisted of indivisible particles, which he called atoms (with a nod of the head to Democritus). Dalton made four statements about matter that together constitute his atomic theory:
- All matter is composed of indestructible and indivisible particles called atoms.
- Atoms of a specific element are identical in mass and properties.
- Atoms can combine to form compounds.
- When a chemical reaction occurs, it is due to a rearrangement of atoms.
Dalton's atomic theory remained virtually unchanged for most of the nineteenth century.
Particle Theory Meets the Quantum
Throughout the nineteenth century, a debate had been raging about the nature of light – whether it propagated as a wave or a particle. Many experiments confirmed the wave hypothesis, and many more confirmed the corpuscular one. In 1887, German physicist Heinrich Hertz discovered the photoelectric effect when he was doing experiments with a spark gap generator. This discovery proved to be far more important than Hertz realized.
Around that time, English physicist J.J. Thompson discovered the first subatomic particle, the electron, by examining the behavior of cathode rays. His discovery helped explain what constituted the electrical discharge from a conducting plate when you shine light on it – which is the photoelectric effect – but not what causes the discharge nor why the strength of the electrical impulse is related to the light frequency. The solution had to wait until 1914.
None other than Albert Einstein explained the photoelectric effect in terms of little packets of energy called quanta. These had been proposed by German physicist Max Planck in 1900. Einstein's explanation proved quantum theory, and he was awarded the Nobel Prize for it.
Quanta, as Planck conceived them, were both particles and waves at the same time. According to Planck, light was composed of quanta called photons, each of which had a particular energy defined by its frequency. In 1913, Danish physicist Neils Bohr used Planck's theory to give the planetary model of the atom, which had been proposed by New Zealand physicist Ernest Rutherford in 1911, a quantum do-over.
The Modern Atom
In Bohr's model of the atom, electrons can change orbits by emitting or absorbing a photon, but since photons are discrete packages, the electrons can only change orbits in discrete amounts. Two experimenters, James Franck and Gustav Hertz, devised an experiment that confirmed Bohr's hypothesis by bombarding mercury atoms with electrons, and they did it without even knowing about Bohr's work.
With two modifications, Bohr's model has survived to the present time, although most modern physicists consider it an approximation. The first modification was the discovery of the proton by Rutherford in 1920, and the second was the discovery of the neutron by British physicist James Chadwick in 1932.
The modern atom is a confirmation of the Democritus particle theory, but it's also something of a repudiation. Atoms turn out to not be indivisible, and that's also true for the elementary particles that comprise them. You can subdivide electrons, protons and neutrons into smaller particles called quarks, and it may even be possible to subdivide a quark. The journey down the rabbit hole is far from over.
- Encyclopedia Brittanica: Democritus
- Ancient History Encyclopedia: Democritus
- Chemistry Libre Texts: Proust's Law of Constant Proportion
- Encyclopedia Britannica: John Dalton
- Indiana University Northwest: Dalton's Atomic Theory
- Rochester Institute of Technology: The Franck-Hertz Experiment Supports Bohr's Model
- The Physics Hyper Textbook: Photoelectric Effect