Starter Question Draw a timeline showing all the important theories/ discoveries in the evolution of Atomic Theory
Today we are: • Looking at the Fundamental Laws of Chemistry • Exploring the history of Atomic Theory • What is the difference between an atom, molecule ion? Important Dates 9/4- Test Corrections due 9/9- Chapter 2 Questions due 9/9- Atoms, Molecules and Ions Quiz 9/10- Nobel Prize Lecture 9/12- Atoms, Molecules and Ions Test
Dalton’s Atomic Theory 1. Each element is made up of small particles called atoms 2. The atoms of a given element are identical. The atoms of different elements are different. John Dalton (1766- 1844) 3. Chemical compounds are formed when atoms of different elements combine with each other 4. Chemical reactions involve the reorganization of the atoms
The Electron Thomson determined the charge to mass ratio of an electron; -1.76x108 Cg-1 Thomson reasoned that since electrons could be produced from electrodes made from various metals that all atoms must contain electrons Since atoms are known to be neutral, Thomson assumed that all atoms must contain a positive charge JJ Thomson (1856- 1940)
In 1911 Rutherford decided to carry out an experiment to test Thomson’s theory of the atom This experiment involved shooting alpha particles at a thin sheet of gold foil. If Thomson’s theory was correct then the massive alpha particles would travel through the foil with very few deflections. Rutherford 1871- 1937 This was not the case… In fact, Rutherford was so surprised by the results he said it was like ‘firing a 15- inch shell at a piece of tissue paper and it coming back to hit you’
This resulted in Rutherford coming up with a new model of the atom. In this model most of the atom was composed of empty space, explaining why most of the α particles passed straight through the foil The positive charge was concentrated in the nucleus, explaining the large deflections of a few α particles
The Neutron Now Rutherford’s model explained almost everything that was known about an atom, however it didn’t explain the discrepancy in mass of atoms It was known that hydrogen contained 1 proton and that helium contained two. Therefore the mass of helium should be twice that of hydrogen, however it was not. It was almost 4 times that of hydrogen, so there must be another particle Proof of this particle was provided by James Chadwick in 1932
Chadwick carried out an experiment, similar to Rutherford's, where he bombarded a thin sheet of metal (this t8ime beryllium) with alpha particles. When he did this high- energy radiation, similar to γ radiation was emitted. Chadwick 1891- 1974 Further experimentation showed that this radiation consisted of a third type of sub atomic particle, named neutrons. Since neutrons had a mass slightly greater than a proton but no electrical charge, the mass ratio mystery could be solved.