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Philosophers Leucippus (left) and Democritus (right)

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Early Ideas Concerning The Atomic Theory

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Early Ideas Concerning The Atomic Theory

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Around 2500 years ago, two Greek philosophers by the names Leucippus and Democritus made their ideas known. They proposed a concept in which 'matter is composed of tiny particles'. They named these particles atomos which translates to "indivisible".
Although they were taking steps in the right direction, some of their logic was still flawed as it was still in early development. For example, they believed that iron was made of iron particles, clay was made of clay particles, and cheese was made of cheese particles, etc.

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In the Late 1800's, physicist J.J. Thomson used discharge tubes to help show that there are both negatively and positively charged parts within matter. The negatively charged particles were named electrons.
At the time, the hydrogen atom was the smallest thing known to man, however, Thomson discovered that the electron is around 1000 times smaller than a hydrogen atom, proving that there is something smaller than a hydrogen atom

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The Plum Pudding Model pictured next to a real Plum Pudding to show the resemblance 

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Before the historical physicist J.J. Thomson, close to nothing was known about the structure of the atom. Thomson, knowing that atoms have an overall neutral charge theorized that the negative electrons filled a positively charged inner area within the atom.
He later compared his theory to the famous plum pudding as the raisins act as electrons within the atom pudding.

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Pictured above is Ernest Rutherford, also known as the father of nuclear physics

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In 1909 the Plum Pudding Model was bettered by a physicist by the name Ernst Rutherford, who shot positively charged alpha particles at a thin piece of gold foil. In this experiment, he found that some of the particles did not go through the gold foil and instead deflected at large angles or right back where it came from. This helped him come to the conclusion that the entire positive charge in an atom must be concentrated in a very small area which he ended up calling the nucleus. And to further add on to his theory, he stated that atoms are mostly made up of empty space due to the fact that most of the alpha particles went through the foil.

Not only did Rutherford discover the nucleus but he also discovered the proton through similar experiments involving alpha particles. Protons are positively charged fundamental particles. Rutherford contributed significantly to the theory of the structure of the atom over top of his conclusion that atoms were mostly empty space.

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Here is Bohr alongside his famous Planetary Model

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In 1911, physicist Niels Bohr travelled to Great Britain to study alongside Rutherford. Bohr analyzed the results of Rutherford's gold foil experiment and was able to come up with the most likely positions of electrons within an atom. The resulting model is now known as the planetary model and it represents the electrons in orbit around a central nucleus.

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Pictured above is Theoretical Physicist Werner Heisenberg

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Theoretical physicist Werner Heisenberg has given us one of the most important contributions to the modern understanding of the structure of an atom. In 1927 Heisenberg mathematically determined that it is impossible to know with full certainty the location of an electron at an exact time as well as its momentum. This meant that past research concluding that electrons are in a neatly aligned orbit around the nucleus were false.
His research lead to other chemists and physicists proposing that electrons are not particles, nor are they waves, but they do have properties of both. This meant that the arrangement of electrons around the nucleus could only ever be described in terms of probability. This lead to the regions where these electrons are found to be named orbitals.