A Quick Overview on the Evolution of the Atom

Throughout history, there have been many scientists who have contributed to previous theories which have resulted in revolutionizing the way humankind see the world. An example would be how Isaac Newton contributed to Galileo’s already existent theories on forces and gravity. Because of the modifications made by Newton to Galileo’s already existent theory, the way people understood movement was revolutionized, and ever since, the world has never been the same. Another such series of great contributions can be seen in the evolution of the structure of the atom. Just like forces, the theories surrounding the atom evolved mainly in the BCE era by Greek philosophers, and was took up by European and North American chemists in the 18^th, 19^th, and 20^th, centuries.

Perhaps the first contributor to what had been dubbed ‘Atomism’ was the Greek Philosopher, Leucippus. His very existence has been questioned, but if he did exist, he did during the 5^th Century BCE. Leucippus claimed that besides empty space there was body, which was occupied by what Parmenides described as ‘real’, but these real could not be infinitely divided because of their smallness.¹

Democritus (460-370 BCE), who was a disciple of Leucippus, and also expanded on his atomic theory by confirming that objects could not be divided into infinity. Democritus described atoms to be originally similar, impenetrable, and have a density equal to their volume. Democritus based many of the properties of atoms on the Law of Necessity, and claimed that the properties of all the things in the world, including humans were the result of ‘the endless multiplicity of falling atoms’. Democritus also explained that all atomic motion was the result of previous atomic collisions, plus the inertia of atoms.²

Epicurus (341-271 BCE), one of the major philosophers of the Hellenistic period, taught his students that the basic constituents of the world are composed of atoms, and atoms are uncuttable bits of matter flying through empty space and could be of any size. He argued against his critics that we see that there are bodies in motion, and that nothing comes into existence from what does not exist. Epicurus also believed that, because of the Principle for Sufficient Reason, the universe always existed, and therefore, atoms always existed. To account for Democritus’ theory of collisions, Epicurus stated that when falling, atoms may ‘swerve’, thus causing collisions.³

Lucretius (99-55 BCE) only really made one contribution to the existent atomic theory, and that was a correction of Epicurus’ statement that atoms could be of any size, he argued that if they were to be of any size, then some would be visible to the naked eye and some very large in size.⁴

For a long time, there was no real progress concerning atoms until 1805 when John Dalton introduced his atomic theory. He stated that the atom differed from element to element and was in the shape of a sphere, similar to a billiard ball, and that the smallest piece of matter was an atom, which was indivisible. This theory was deemed successful because it satisfied the Law of Conservation of Mass, the Law of Definite Composition and the Law of Multiple Proportions.⁵

Michael Faraday’s experiments with Electrolysis and electricity as a whole suggested that electric charges were components of matter, and as a result of this discovery by Faraday, J.J. Thomson was able to enhance the existent atomic theory. J.J. Thomson, using cathode rays, discovered the electron in an atom, leading to his own modification of the atomic theory. Thomson created a model of the atom suggesting it was a positively charged sphere containing negatively charged electrons scattered about like raisons in a bun.⁶

Ernest Rutherford, a student of J.J. Thomson, devised an experiment, in 1911, with some of his own students to test the Thomson Theory of Atomic Structure. Rutherford used radium as a source of alpha particles and fired them at a thin sheet of gold. Rutherford predicted, based on the Thomson model, that the alpha particles would barely deflect, if at all. When they did shoot the alpha particles, some went through, and others bounced around. This led Rutherford to the conclusion that the positive charge had to be in small volume compared to the atom itself, and that it was in the centre of the atom, in a nucleus, and had the negatively charged electrons circle it in the surroundings.⁷

In 1932, James Chadwick was bombarding elements with alpha particles to calculate the masses of the nuclei. The results he was expecting was that the sum of the masses of the protons and the masses of the nuclei would be equal, since it was predicted that the nucleus was composed of positively charged electrons. When he saw his actual results, they did not agree with the prediction, but the atoms were still neutral. This discovery led Chadwick to believe that the nucleus contained neutral charges, which he dubbed ‘neutrons’. Now, isotopes could be accurately explained as an atom containing a different number of protons and neutrons.⁸

When Niels Bohr began studying the atom in 1913, he discovered that Rutherford’s model of the atom was incorrect, structurally. He determined that the if Rutherford’s explanation were to be true, then, according to the classical laws of physics, the atom would collapse, since as the electron circles the nucleus, it would accelerate, causing it to lose energy, and eventually spiral into the nucleus. This does not happen in real life though, and as a result, Bohr concluded that the atom does not follow the classical laws of physics, but rather, has its own set of laws. Bohr stated that the electrons were actually orbiting the nucleus without losing any energy because they were orbiting in energy levels. According to Bohr’s theory, electrons were able to move to higher energy levels, but only when they acquired more energy, otherwise they would remain in their ground state.⁹

In 1923, French Chemist, Louis De Broglie suggested that if a wave can act like a particle, as is the case with light, then maybe a particle can act like a wave. Later, Erwin Schrödinger and others used this theory to explain the activity of the electron inside the atom. Basing his work on Quantum Mechanics, Schrödinger proposed that electrons can only have certain quantized energies because of the requirement for only whole numbers of wavelengths for the electron wave, thus confirming that the electrons (a particle) act like a wave in an atom.¹⁰

It is easy to see how these different philosophers and chemists and physicists have contributed to such an important aspect of science. Every time one of them has contributed their idea, it has changed the way people may view the world around them. It was through these scientists, that we are now able to portray the atom as accurately as modern technology has allowed us to.