The development of atomic theory is an interesting thing. In its history it has had four major developments. The developments were made by the ancient Greeks/John Dalton, J.J Thomson, Neils Bohr/Ernest Rutherford and Erwin Schrödinger/Werner Heisenberg.
Ancient Greeks/John Dalton Model of the Atom.
I will begin with the ancient Greeks and John Dalton. The word "Atom" derives from Ancient Greece, the prefix "a" means "not and "tomos' means "cut". Ergo the word atom or atomos means "undividable". The whole atomic concept was conceived in the 5th Century BC by Leucippus of Miletus. However it was his pupil, Democritus of Abdera, who made the five major points on which they based their theory on.
- All matter is made up of undividable particles called atoms
- There is a void, which is empty space between atoms
- Atoms are completely solid
- Atoms are homogeneous with no internal structure
- Atoms vary in size, shape and weight
However, not all people who heard about this theory were so confident. In the 4th Century BC Aristotle, a famous philosopher, disagreed with Democritus and Leucippus that atoms existed. Because of this for a very long time it was dismissed by scientists. John Dalton who came a long time after the Greeks in the 1800's went further with the atomic theory. In 1803 he wrote down his first set of atomic weights in his daily logbook. Much later he wrote a book talking about elements (Dalton being the first to propose that the different elements be labeled with different symbols) and atoms. He based his theory on five different ideas:
- Chemical elements are made of atom
- The atoms of an element are identical in their masses
- Atoms of different elements have different masses
- Atoms only combine in small whole number ratios such as 1:1, 1:2, 2:3 and so on
- Atoms cannot be created or destroyed
Democritus worked on the bases of an atom and defined what atoms are and how they might work, but Dalton worked on their different atomic weights and the way that they group. All together both groups were vital to our modern understanding of the atom and we probably wouldn't know they existed without them.
J.J Thomson Model of the Atom.
Now onto the Thomson's plum pudding model of the atom. In 1897 Thomson produced his version of the atom. Every previous atomic theorist said that atoms were the very last thing, which couldn't get any smaller. However Thomson made the first discovery of sub-atomic particles with the electron. He proved this with his “cathode ray” experiment, which was a glass tube with a vacuum separating two electrodes. When he ran a voltage through these electrodes cathode rays are created. What Thomson did was (through experimentation) found that if you put an electric field there the rays were deflected.
Thomson concluded that instead of being rays, the were negatively charged particles that he called “corpuscles” which were later renamed electrons by other scientists. For his second experiment he measured the charge-to-mass ratio by measuring how much the cathode ray was deflected by the electric field. He found that the charge-to-mass ratio was very high, as such it was either very highly charged or it was very light. So he concluded that the cathode rays were made up of electrons, but he also went further and said how the atom was arranged. He thought that the atom was totally made up of electrons spread about evenly and that they were kept there by a positively charged blob. Or as Thomson called it, plums in a pudding, hence it was called the plum pudding or the raisin bun model of the atom (below).
Although later disproved Thomson's discoveries related to the atom were very significant in that, he discovered electrons and that he proved the existence of sub-atomic particles. Without Thomson, the concept of the sub-atomic particle probably would've been discovered much later and hence our modern world wouldn't be as developed as it is. Of all of the different atomic theorists J.J Thomson is my favorite because I think he was important to the development of the atom.
Ernest Rutherford/Neils Bohr Model of the Atom
I will now start with the Rutherford model of the atom. Ernest Rutherford was famous for his “Geiger-Marsden Experiment”. In this experiment he shot alpha particles at a sheet of gold foil two atoms thick. His hypothesis was that all of the particles would go straight through or be deflected slightly. He was correct in proving that atoms have a small center which contained most of its mass and is positively charged (nucleus). This also proved that the majority of an atom is space. So much in fact that if you saw a fly in a cathedral, that fly would be the size of the nucleus and the cathedral, the atom. The Rutherford model of the atom therefore is made up of a small nucleus, mostly space and a cloud of electrons covering the atom. See the example below.
Although Rutherford's experiment proved that the majority of an atom consists of space it was Neils Bohr who disproved Rutherford's view for the electron. Rutherford thought that electrons covered the atom in a planetary motion. This couldn't work because the electrons would produce electromagnetic radiation as they orbited the nucleus. Gradually the electrons would lose energy and spiral down and collapse on the nucleus. Neils Bohr overcame with his model of how electrons orbited the nucleus. His two main ideas were:
- Not every orbit is possible but only certain specific ones, at certain specific distances from the nucleus
- The electrons will not slowly lose energy as they travel, and hence will remain in stable, non-decaying orbits
Bohr's theories were later proved wrong, but at the time his discoveries were very influential. Both Rutherford and Bohr presented an excellent version of the atom and without either of them we wouldn't have the understanding we do, especially in the field of quantum physics.
Werner Heisenberg/Erwin Schrödinger “Wave Mechanical” Model of the Atom
After Bohr's explanation of how electrons move in orbits Erwin Schrödinger was fascinated and explored if the movement of the atom could be better explained as a wave. Schrödinger's equation explains the electron like a wave rather than a particle; it also explained more technically what different phenomena that Bohr failed to explain himself. Heisenberg said that the position and the momentum of an electron can't be simultaneously precisely calculated, this is known as the uncertainty principle. Heisenberg and Schrödinger's model of the atom is fundamental to our modern understanding of the atom as it is exactly what we use.
From the Ancient Greeks until Heisenberg and Schrödinger, scientists have contributed to our understanding of the atom which is vital if society is to progress, to things like machines that teleport you. It wouldn't be able to be done without us understanding atoms.
