In antiquity, philosophers posited a variety of ways in which the physical world is constructed. One such philosophy is that of Atomism, which proposed that the universe was made of indivisible, granular elements: atoms. An early example of a philosopher proposing atomism is Aruni, a Vedic scholar from 8th century BCE India, who theorized that particles too small to be seen mass together into the substances and objects of experience; Aruni suggested that these particles be referred to as kana (McEvilley, 2002). This concept was further developed amongst Indian philosophers and then reflected in Western philosophy in Greece in the 5th century BCE by Leucippus and Democritus, who proposed their own thinking: that the physical world is constructed of infinite principles and elements that are both minute and indivisible (Pullman, 2001). However, although the concept that atoms exist is older than most written history, modern atomic theory is not philosophically derived, but has, in fact, been experimentally derived over the last 200 years.
Initial research into atomic theory was initially undertaken by John Dalton in the early 19th century. Dalton's notebooks showed that he was initially intending to find reasoning for his law of multiple proportions, announced in 1803, and ideated that chemical bonding was a product of atomic interaction (The Editors of Encyclopaedia Britannica, 2020). The first written example of his consideration of atoms is as follows: Why does water not admit its bulk of every kind of gas alike? This question I have duly considered: and though I am not able to satisfy myself completely, I am nearly persuaded that the circumstance depends upon the weight and number of the ultimate particles of the several gases (Dalton, 1806). Through further research, Dalton continued to refine and develop his own atomic theory, ultimately proposing the following key hypotheses:
- Indivisible particles (atoms) are the foundational building blocks of the physical world,
- All atoms of the same element are identical in shape and mass, though differ from those of other elements,
- Atoms cannot be produced, divided, or destroyed,
- Chemical compounds are produced when atoms of different elements combine in simple ratios,
- And the atom is the smallest unit that can react chemically (Anon, 2020).
Later, the physicist J. J. Thomson conducted experiments using cathode rays in order to ascertain whether they were a form of electromagnetic radiation, like visible light, or were in fact wholly material and that they mark the paths of particles charged with negative electricity (Thomson, 1897). Thomson found that cathode rays are, in fact, made up of particles with a mass of only 11800th of that of a hydrogen atom, what was previously considered to be the lightest particle. As a result, it was concluded that cathode ray particles were subatomic particles that were, at the time referred to as corpuscles but were subsequently renamed electrons. Thomson was also able to prove that electrons were indistinguishable from the particles given off by radioactive and photoelectric materials (Thomson, 1901). This produced the concept that is currently referred to as the 'Plum Pudding' model, that an atom was made up of a cloud of positive charge populated with negatively charged electrons.
Later, one of Thomson's colleagues, Ernest Rutherford, completed an experiment with two other researchers, Hans Geiger and Ernest Marsden, which showed that alpha particles were deflected by atoms when they passed through a thin gold film. This scattering pattern showed that most particles passed through the film with negligible deflection, however a small number were found to have been deflected by over 90Â° (Manners, 2018). From this, Rutherford hypothesised that, rather than an atom being made up of a positively charged cloud, it in fact contained a small, positive nucleus at its centre and that its associated electrons resided in an empty volume around it (Rutherford, 1911).
Subsequently, the physicist Niels Bohr adapted Rutherford's model to propose the first complete physical model of the atom in order to explain the emission spectrum of the hydrogen atom (Anon, 2019). This model proposed that an atom comprised of a small, dense, positively charged nucleus surrounded by empty space and negatively charged, orbiting electrons (like Rutherford's initial model). However, Bohr posited that an atom's given electrons could only occupy a finite set of orbits, and could make ‘quantum leaps’ from one orbit to another causing discrete changes in energy (these are equivalent to either the absorption or radiation of a photon) (Stern, 2005). This model explained the stable nature of electron orbits as well as why different elements' emission spectra vary. Further research in the early 20th century hypothesised that atoms exhibited characteristics of both waves and particles (McEvoy and Zarate, 2014). This concept of wave-particle duality was then used to create a mathematically-derived model of the atom, describing electrons not as particles, but as waveforms (Kozlowski, 2019). As a result of the wave-like properties of electrons, it was posited that Heisenberg's uncertainty principle would now apply, meaning that it would be impossible to calculate both the position and the momentum of a given electron (Pais, 1986). Due to this, electrons were no longer seen as following orbitals, but existing within probability ‘clouds’ in which they are most likely to inhabit within a given instance. Thus, the previous planetary model was abandoned, and the newer quantum model was adopted.
Up until the early 1930s, an atom's nucleus was thought to be comprised of protons and electrons, yet this thinking presented a variety of inconsistencies, both theoretically and when atomic behaviour was experimentally observed (Pais, 1986). As a result, it was posited that a third subatomic particle existed, which was confirmed upon James Chadwick's discovery of the neutron (Chadwick, 1932). This led to a quick uptake of the thinking that the nucleus was not comprised of protons and electrons, but instead protons and neutrons (Miller, 1995). This led to the current model of the atom: a central nucleus containing neutral neutrons and positively charged protons (held together using the nuclear strong force), surrounded by a mainly empty void containing a number of negatively charged electrons, existing within discrete energy levels around the nucleus, all held together by electrostatic forces.