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.Dalton's Billiard Ball Model

John Dalton (1766 – 1844) proposed a basic model of the atom that helped establish many scientific concepts and also created the foundation for more modern models. His model suggested that atoms are the smallest particle of an element, that atoms of different elements have different masses, and that they are solid, indestructible units - much like a billiard ball.

Additionally, Dalton’s Atomic Theory included five main points. Atoms:
  1. Make up all matter.
  2. Cannot be subdivided, created, nor can they be destroyed. They can simply be rearranged into different groups.
  3. Of a given element are identical, while atoms of a different elements vary and can be distinguished from one another using their respective weights.
  4. Combine with or separate from other atoms to form compounds in chemical reactions.
  5. Of different elements form compounds in wholenumber ratios.

Dalton’s Theory also supported three physics concepts:
His theory stated that in chemical reactions, atoms are neither created nor are they destroyed, which clearly enforces the Law of Conservation of Matter. Dalton also provided evidence that a pure compound is always composed of identical elements and always has the same mass. This corresponds to the Law of Definite Composition. Lastly, Dalton's theory explains the Law of Multiple Proportions: Atoms combine in whole number ratios to form compounds and if these proportions differ, so will the compound.

However, the modern Atomic Theory provides evidence that disproves some of Dalton's Theory:
  1. Atoms of the same element can differ (isotopes and ions).
  2. Atoms are divisible.

While Dalton's Theory was not without fault, it provided the basis for other scientists to form their ideas on. Dalton's initial model lead to the evolution of the model of the atom:
  • 1808: Dalton's original Billiard Ball Model surfaces.
  • 1897: J.J. Thomson invents the Plum Pudding Model. He believed that a positively charged substance filled the atom. And because he had discovered the electron, he also believed that electrons were arranged within it.
  • 1898: Rutherford performs the gold foil experiment and discovers the nucleus. His model includes a large central nucleus with electrons orbiting around it.
  • 1911: The inconsistencies in the classical model of the atom pushed Borh create a new model. There had previously been no explanation as to how the electron continues to move in a circular pattern without losing energy. Borh therefore clarified that the electron either emits or absorbs energy in order to move to different orbits.

Evolution of the Atomic Theory
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A summation of Dalton's Billiard Ball Model and an explanation as how he came to realize it can be found in this video: http://www.youtube.com/watch?v=L2KmCTst2o0