In this blog post, I’ll explore the history of the atomic concept and the significance of modern atomic models, starting with a question that came to mind while grilling pork belly.
Sizzle~ Mmm~ Ah, it’s truly a joy to eat this no matter when (tears of joy). I just finished grilling and eating pork belly, and now that I’m sitting at my desk, the savory scene is still fresh in my mind. While eating the meat, a thought suddenly crossed my mind. Just as we observe meat changing under heat, might Aristotle’s theory of the four elements have originated from repeated chemical discoveries—such as water extinguishing fire or water evaporating? The transformation of matter must have been a fascinating topic even among ancient philosophers. However, curiosity about the phenomenon of matter changing alone wasn’t enough for anyone to offer a coherent explanation. It took a very long time to arrive at the idea that some extremely small particles were involved in all of this—that there exists a smallest unit of matter, and that all phenomena occur through the mixing of these units. Long ago, chemists named these particles “atoms.” Let’s explore the process through which atoms were studied and what significance they hold in modern chemistry.
The first person to mention the existence of atoms was the Greek philosopher Democritus, who lived in the 5th century BCE. He argued that if one were to keep cutting matter in half using a magical knife—one capable of dividing it exactly in half—one would eventually reach a state of solid atoms that could no longer be cut. This was the earliest concept of the atom. However, humanity had to wait until the early 17th century and the emergence of Dalton for this to become a chemically significant issue.
Like Democritus, Dalton defined the atom as the smallest unit of matter. However, he went a step further, believing that there are various types of atoms and that the unique properties of each substance arise from the characteristics of these individual atoms. He also introduced the revolutionary idea that hydrogen and oxygen are composed of hydrogen and oxygen atoms, respectively, and that water—which is made up of these atoms—exhibits new properties because the two atoms combine. He conceived of atoms as hard spheres—indivisible, unchanging, and absolute entities—though it is now known that not all of Dalton’s ideas were correct. Nevertheless, his theory played a crucial role in the history of chemistry by serving as the starting point for the modern experiments and discoveries that will be discussed later.
Following Dalton’s model, people believed that atoms were the indivisible basic units of matter. However, this belief was shattered in 1899 by the British physicist J.J. Thomson. At the time, he was studying a phenomenon known as “light rays,” which were highly intriguing. These rays were influenced by magnetic and electric fields and, remarkably, possessed mass. Thomson hypothesized that these rays were a stream of very small particles. This light was produced when strong electromagnetic waves were directed at a metal, and he discovered that the properties of the light remained unchanged even when he varied the type of metal used in his experiments. Consequently, Thomson concluded that these particles existed in all metals and, furthermore, that they were present in all other atoms as well. In other words, he proposed the “plum pudding model,” in which electrons were thought to be distributed throughout the atom, and this idea dominated the minds of people worldwide until it was later revised.
Thomson’s experiment was a pivotal one that spurred research into atomic models and served as an attractive research topic even for passionate researchers like Rutherford.
While conducting an experiment in which he fired rays (later identified as helium nuclei) at an extremely thin metal foil, he discovered a strange phenomenon that could not be explained by Thomson’s pudding model. It was the phenomenon where the rays collided with the atoms and were deflected in almost the opposite direction. Rutherford was astonished and recalled the experiment as follows: “It was an astonishing phenomenon, like firing a shell at a sheet of paper and having the shell bounce off.” Through this experiment, he concluded that there must be a nucleus at the center of the atom, carrying a positive charge and containing most of the atom’s mass. This was the only way to explain why the particles were being deflected. He also proposed a new model, imagining that electrons orbited around this nucleus. This is Rutherford’s planetary model. Following his theory, research on the atomic nucleus became more active, and the American physicist Chadwick experimentally discovered the existence of the neutron, which carries no charge. This completed the atomic nucleus structure of modern chemistry.
Only after the elements that make up the atom were thoroughly identified did active research on atomic structure begin. One of the most intriguing problems for scientists was the observation of the line spectrum of the hydrogen atom. Scientists were deeply perplexed because, when a hydrogen atom absorbed energy and emitted light, only a few specific energy values were observed. This was because it was impossible to explain this phenomenon even by fully applying Rutherford’s model and traditional classical mechanics. Consequently, people felt the need to discover a new atomic model.
In the 1910s, as research for a new model was in full swing, Danish physicist Niels Bohr solved the problem in one fell swoop with a seemingly absurd assumption: that there were orbits around the nucleus where electrons could move without colliding. He proposed that multiple such orbits existed and that electrons could occupy various energy states as they moved between them. He also explained that light is emitted in amounts corresponding to the energy difference generated during this transition between orbits. In other words, this made it possible to explain spectral lines. This is known as Bohr’s shell model.
Following Bohr’s model, scientists began to analyze atomic structure using a new tool: quantum mechanics. Schrödinger wanted to create a model that reflected Heisenberg’s uncertainty principle, which states that the exact position of an electron cannot be known. Since Einstein’s theory that matter and light could be interconverted had already gained prominence at the time, Schrödinger used waves to express the electron’s position as a probability. He no longer attempted to determine the exact state of an electron. Instead, he believed it was most reasonable to represent the points within the atom where an electron was most likely to be found as dots. The model created by connecting these regions where an electron is most likely to be found is called the orbital model. This marked the completion of the modern atomic model.
So far, we have examined the history of atomic models. To explain the properties of compounds formed by combining specific substances, as well as why they exhibit certain behaviors, an understanding of the atom itself is essential. Even after the orbital model, research on atomic structure has continued steadily, leading to the discovery of various subatomic particles that make up the atomic nucleus and electrons. I find it truly fascinating that there are particles even smaller than electrons, protons, and neutrons. In this way, atomic models are constantly evolving. In the future, these subatomic particles may come to be recognized as the fundamental building blocks of matter, and the era of the atom may fade into the annals of history. However, one thing is certain: the desire to understand the true nature of atomic structure—the ultimate goal that countless scientists have sought to achieve—will never change. Let us take a moment to reflect on their thoughts and once again appreciate their efforts and determination.
