A history of atomic theory


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Podcast Transcript

The great Nobel Prize-winning physicist Richard Feynman was once asked to convey, in a single sentence, the most important scientific knowledge that humans possessed.

His answer was short and simple: “Everything is made of atoms.

Believe it or not, this was believed to be the case over 2000 years ago in ancient Greece and India. However, it is only in the modern age that we have been able to prove this.

Learn more about atoms and how we discovered their existence in this episode of Everything Everywhere Daily.


Most of what the ancients knew about the word physical was wrong. Simply wrong. Aristotle, for example, believed that everything in the world was made up of only four elements: earth, air, fire and water.

According to him, everything consisted of a combination of these four elements.

Almost every ancient culture had ideas about how the world worked, which wouldn’t really stand up to modern science.

That being said, given the state of knowledge hundreds or thousands of years ago, you can’t really blame them. Most of their ideas about the world were based on philosophy more than science. They had ideas that made sense to them but were not based on experimentation or observation.

However, there were a few instances where they basically succeeded, even though they didn’t really know it at the time.

This is the case of the theory of atoms.

What we call science was part of philosophy. The branch of philosophy that dealt with the natural world was known as natural philosophy.

A school of ancient natural philosophy was called atomism.

There are two ancient civilizations that each independently developed theories on atomism: Greece and India.

Around 600 BC. AD, one of the Vedic schools of thought was known as Vaisheshika. It was developed by the Indian philosopher Kanada and postulated that the world could be reduced to indivisible pieces called stopped ??.

According to the Vaisheshika school, everything was composed of stopped ?? you, and everything in the world could be considered a combination of stopped ?? and the interactions between them.

About 200 years later, the same idea was independently developed in Greece. A philosopher named Democritus believed, like Kanada in India, that the world was made up of indivisible particles, which he called “atoms”, meaning uncuttable.

According to Democritus, there were an infinite number of atoms of different sizes and shapes. They were always in motion and the nature of the material reflected the nature of the atoms.

For example, water atoms would have been slippery. Iron atoms would be heavy with hooks that linked them together.

Sweet things were smooth and bitter things were jagged.

Also, according to Democritus, between the atoms there was nothing. Just a void. Atoms could not be created or destroyed.

Democritus and Kanada simply used deductive reasoning to arrive at these conclusions.

Aristotle did not believe in atomism. He believed that his four elements could be continuously divided forever. Moreover, he did not believe in a vacuum, which was necessary if one believed in atoms.

Although he wrote 80 different works, none of Democritus’ writings have survived. All we know of him is what has been written about him by others.

Aristotle’s works, however, have survived. His ideas eventually caught on and were revived in the Middle Ages.

However, there was a rediscovery of atomism in the 13th century, and there were scientists who believed in atomism, including Isaac Newton. At this point, as modern science developed, believing in atoms was still a matter of philosophy.

Our modern understanding of atoms began to develop in the late 18th and early 19th centuries. There were discoveries made on a regular basis on the fundamentals.

One of the first big steps towards determining atoms came from the French chemist Antoine Lavoisier, who postulated the law of conservation of mass. He realized that in any chemical reaction, the total mass before and after the reaction was the same.

However, the big step in atomic theory came from the English chemist John Dalton.

Dalton developed what is known as the Law of Multiple Proportions, which states that the masses in any compound were actually ratios of small whole numbers. He felt that each element, and there were new elements being discovered all the time, were their own unique atoms.

This theory made perfect sense. He explained the chemical equations and he explained the discovery of the elements.

Throughout the 19th century, more and more evidence accumulated in agreement with the atomic theory of matter. Avogadro’s law, the ideal gas law, the discovery of Brownian motion and everything that was discovered during this period supported the idea that everything is made of atoms.

This idea that atoms were the smallest unit of matter actually came crashing down in 1897 with the discovery of the electron by British physicist JJ Thompson.

He discovered that there must be a negatively charged particle that had a mass 1,800 times less than that of hydrogen, which was the lightest known element.

Thompson thought his new particle was smaller than an atom and was actually part of an atom’s makeup.

With this new discovery, Thompson created a new theory of the atom which has been dubbed the plum pudding theory. This theory held that there was a positively charged substance into which negatively charged electrons were embedded, much like how raisins were embedded in plum pudding.

Thompson’s plum pudding model did not last and was not disproved until a few years later.

New Zealand physicist Ernst Rutherford put an end to the plum pudding model when he discovered that atoms had very small and dense nuclei. Rutherford had already won the Nobel Prize in 1908 for his work on radioactivity.

In a famous 1911 experiment known as the Gold Leaf Experiment, he actually set out to prove Thompson’s atomic model.

He fired particles of alpha radiation at a thin piece of gold foil. He expected all alpha particles to pass through. Instead, what happened was that some particles were reflected, often at very odd angles, including backwards.

This result was unexpected and should not have happened if the plum pudding model were true.

Rutherford realized this could only have happened if two things were true. The first was an extremely dense and positively charged nucleus relative to the atom which reflected some of the alpha particles. The second was a bunch of nothing everywhere, letting the particles through.

Rutherford proposed a new model of the atom where there was a small, dense, positively charged nucleus with small negatively charged electrons that revolved around it like the planets revolve around the sun.

Although this model fits the data better than the plum pudding model, it raises just as many questions. What held the electrons in orbit and what was the nucleus made of?

Just two years later, Danish physicist Niels Bohr developed the model by explaining that electrons would have been parked at different levels depending on their energy.

That same year, 1913, another discovery demanded explanation. British radiochemist Frederick Soddy discovered that some atoms have different weights. These became known as isotopes.

In 1919, Ernst Rutherford discovered that there were positively charged particles in the nucleus, which he dubbed a proton.

He assumed that there were neutral and positive particles in the nucleus. The existence of neutral particles, called neutrons, was confirmed by English physicist James Chadwick in 1932.

With this new model, protons and neutrons had about the same weight, with protons having a positive charge and neutrons having zero charge.

With that, everything seemed pretty complete… except, of course, it wasn’t. In fact, it was going to get even more complicated.

Researchers began to find a multitude of particles inside the nucleus, which were discovered by colliding atoms in a particle accelerator.

I’m going to skip a lot of ground here, but there were a bunch of particles that were even smaller than protons and neutrons. These particles, known as elementary particles,

These included particles such as quarks, leptons, antiquarks, antileptons, bosons, photons, gluons and neutrinos.

Of the ones I’ve listed, there are many different types and flavors.

The person who started to make sense of it all was Caltech physicist Murray Gell-Mann.

He created a model that made sense of everything, called the standard model. The Standard Model is probably worth its own episode at some point, but suffice it to say that it largely explained how atomic particles are put together.

Even the standard model, as far as it is explained, still needs updating as more and more discoveries are made that do not fit the model perfectly.

Our understanding of the atom is not yet complete.

Understanding how the atom works has been one of mankind’s most important projects. The workings of the atom are the foundation of all other sciences, including chemistry and biology.

What began as a philosophy in ancient India and Greece is still being developed today in the world’s largest particle accelerators, 2,600 years after its beginnings.

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