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The Development of Atomic Theory is an interesting thing. In its history it has 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.

1. All matter is made up of undividable particles called atoms
2. There is a void, which is empty space between atoms
3. Atoms are completely solid
4. Atoms are homogeneous with no internal structure
5. 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 labelled with different symbols) and atoms. He based his theory on five different ideas:

1. Chemical elements are made of atom
2. The atoms of an element are identical in their masses
3. Atoms of different elements have different masses
4. Atoms only combine in small whole number ratios such as 1:1, 1:2, 2:3 and so on.
5. Atoms cannot be created or destroyed