The History of the Atomic Model
- Chom Arunrat
- Aug 10, 2023
- 7 min read
Updated: Sep 16, 2023
Chemistry studies subjects ranging from the properties of molecules to thermodynamics and even electricity. However, at its core, chemistry is the study of atoms and how they interact. As such, it is a necessity for such a subject to understand the atom’s structure. In the field of atomic theory, many scientists have created models which illustrate the structure of the atom in order to better understand it. This was a slow process that involved many scientists and new theories, and which culminated in our current understanding of the atom. Exploring the history of how the atomic structure became what it is today provides valuable insight into the evolution of scientific theories and paradigm shifts.
The word ‘atom’ comes from the Greek word of uncuttable which was defined by Democritus. As such, we would think the first atomic model would be from the Greeks but, at the time, there was no concrete model thought of by them. Although Democritus and Aristotle had tried to describe the matter around them, it is debatable whether they had created the first atomic model as they had only tried to define whether the atoms were continuous or discontinuous and that they were invisible and unsplittable. This means the first atomic model in history is the Dalton model of the atom. Created by chemist and physicist John Dalton in 1808, the atomic model he created was that of a sphere. His model was based on the observation that some gases dissolve better in water than others. He deduced that each atom had a different weight. With the addition of pre-established theories like the conservation of matter, proportionality of matter and the ideas made by the Greek philosophers, he created the first atomic model. The law of conservation of mass, created by Antoine Lavoisier stated that matter cannot be created nor destroyed. With hindsight, this law is false as since the discovery of relativity by Sir Albert Einstein, it is revealed that matter can be converted into energy and that energy can do the same inversely. As such, it is possible for mass in a system to be lost or gained by the conversion of energy. The other law, the law of definite proportions, created by Joseph Proust, states that in chemical reactions, the mass ratio is always constant. This law is true in all cases except when there are multiple isotopes of an element or compound in a reaction, which is the basis of stoichiometry. Dalton’s model combined many other theories left behind by his predecessors. He defined that all matter is made up of atoms, atoms of the same element are the same, and that the inverse is true. Although this is the building block many other models would use as a base, it was wrong in that the model assumed that atoms are the smallest thing in the universe, which was incorrect as an atom was made up of other things.
Dalton's Model
After Dalton’s model, the next iteration of the atomic model would be the plum pudding model. The plum pudding model, created by Joseph John Thompson in 1904, added electrons in an atom. He discovered this when observing the movement of charged particles (which were cathode rays) between magnets. He observed that the particle deflected towards the positively charged magnet. This shows that the particle was negatively charged. He concluded that there were subatomic particles that could be found in the atom of elements with each particle weighing a thousandth of a hydrogen atom, the smallest atom in the universe. This theory disproved previous theories that the atom was inseparable. It also led to the model containing electrons, which are negatively charged particles, and the existence of positive particles in the atom as an element is neutral which means it needs a positively charged particle to cancel out the negative charge from the electrons. This model is described as a plum pudding as it looks like that of plum pudding with the electrons being the plums while the pudding represented the positively charged particles. With hindsight, we know that his description of the electrons and positively charged particles is false as it did not explain how the positively charged particles held the electrons.
Plum Pudding Model
(Source: https://www.breakingatom.com/learn-the-periodic-table/the-history-of-the-atomic-model-thomson-and-the-plum-pudding)
The third model introduced was the Rutherford model, proposed by Ernest Rutherford. Also known as the father of nuclear physics, Rutherford added the nucleus and the proton as key figures in the atomic model in 1911. He discovered the nucleus through his famous gold leaf experiment. The gold leaf experiment was an experiment where Rutherford, Hans Geiger, and Ernest Marsden shone an alpha particle ray at thin gold leaf foils. Alpha particles are helium ions (He 2+) with momentum which are radioactive and high in energy but are easily blocked due to their low speed. According to Thompson’s model, all the alpha particles would pass through with almost no change in angle as the only mass in the atom was thought to be electrons which had almost no mass. However, some of the particles deflected at obtuse angles meaning they had collided with something which had enough mass to withstand the speed of the alpha particle. Rutherford deduced that there was a small, charged particle in the middle of the atom which had most of the mass in the atom. This would later be known as the nucleus. He also deduced from the experiment that the atom was mainly made up of space. In 1919, Rutherford continued his experiments with alpha particle rays. This time he shot it at elements that were light like nitrogen. His results showed that a sole hydrogen atom was emitted, and a new isotope of oxygen was emitted. He deduced that the mass of the particle which he hit had a charge and had ¼ the momentum of the alpha particle. He would later name this the proton. His model showed that the atom had a nucleus that was made up of protons, the nucleus was dense, the electrons orbited around the nucleus, and the number of protons was equal to the number of electrons. However, the trajectory of the electrons was contradictory to classical physics at the time as when the electron orbited, work would have to be done on the electron which would mean the electron would eventually collapse into the atom as it ran out of energy to orbit around the nucleus which had an electric field pulling it in.
Rutherford Model
(Source: https://www.britannica.com/science/Rutherford-model)
Following Rutherford’s model, the field of quantum mechanics was created by Max Plank and Niels Bohr at the dawn of the 20th century. Bohr used quantum mechanics to derive a model showing the shelling of electrons in the atom. He found this through an experiment where he heated a light bulb filament and hydrogen and then observed the color difference. The filament heated up and shined light of multiple wavelengths while the hydrogen emitted specific wavelengths of light. He then deduced that electrons had specific energy levels which he called electron shells using the equation E=hf. The equation is the equation of the energy of a photon. A photon is an elementary particle that emits electromagnetic radiation. As such, photons were emitted when Bohr heated up the filament and the hydrogen. These shells are different energy levels at which the electron can be. An example would be the first shell which can contain 2 electrons and the second shell capable of holding 8 electrons. These electrons can then move between different energy levels by absorbing energy to increase their energy levels. They then release that energy of a certain wavelength when they fall back to their original energy level. His model demonstrated the nature of electrons and how they can emit electromagnetic radiation of different wavelengths. His model was lacking in that it utilized classical physics rather than quantum mechanics to explain the nature of electrons. This made his model only applicable to hydrogen and hydrogen emission spectra. This caused a problem where the electrons could not have orbited around forever as according to classical physics, they would eventually lose all their energy and collapse into the nucleus, unable to stay in orbit. Another problem was that he could not have known the location of the electron due to Heisenberg’s uncertainty principle. Heisenberg’s uncertainty principle states that you cannot know the exact location and direction of an electron. This is because you need to shoot an electromagnetic wave of high energy to the electron to observe it but that would change the position and nature of the electron. Thus, Bohr pinpointing an exact location for an electron was incorrect as there was no way for him to know that.
Bohr Model
(Source: https://www.britannica.com/science/Bohr-model)
Finally, the most up-to-date atomic model created using quantum mechanics for its reasoning is the Schrodinger model or the electron cloud model. The Schrodinger model builds up on one of the main flaws of the Bohr model, the orbiting of electrons. Before the Bohr model, quantum mechanics discovered the relationship between energy and frequency through the energy wave theory (E=hf) in 1900. Following this discovery, De Broglie suggested that through the energy wave equation, you could relate energy to be inversely proportional to the momentum of said wave in 1923. This meant that all particles had a wavelength and thus a corresponding energy level and frequency. This paradigm shift created a link between classical and quantum physics as momentum is a property present in both fields. Schrodinger used De Broglie’s equation to mathematically calculate the areas in which the electron would most likely be present. This would be known as the Schrodinger equation and would help develop the electron cloud model in 1926. This model succeeded Bohr’s since it solved the problem of orbitals in Bohr’s model which followed classical physics and proved impossible. This would further be supported following the introduction of the uncertainty principle in 1927 which built upon the de Broglie equation in explaining that the more we try to observe the position and momentum of a sub-atomic particle like an electron, the more we change it. The Schrodinger model follows Bohr in the domain of energy levels and the nucleus; however, it introduces electron clouds which are orbitals that are most probable for the electron to be present. Due to the electrons’ location being up to probability, the model stays consistent with quantum mechanics following the Heisenberg uncertainty principle. As such, this is the most modern and up-to-date version of the atomic model.
Electron Cloud Model
(Source: https://scienceready.com.au/pages/schrodingers-atomic-model-spdf-notation)
In conclusion, the atomic model has evolved over the span of a century starting from Dalton’s model to Schrodinger’s model with each iteration improving on a flaw that existed in the previous model. This helped the human population and future chemists understand the atom and build upon the foundation in the field of chemistry.
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