Grade 12 Chemistry Notes - History and Development

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3.1 & 3.2 Chemistry Notes
Dalton
* In 1805, John Dalton reintroduced the idea to explain 3 fundamental principles Experimental Work
* Atoms of different elements have different properties
* Law of definite proportion and multiple compositions: atoms of 2 or more elements can combine in a fixed ratio to form new substances depending on their combining capacities (eg. H2O vs H2O2) * Law of conservation of mass: atoms cannot be created or destroyed during a chemical reaction Conclusions

* All matter is composed of atoms
* Atoms are the smallest pieces of matter and cannot be broken down further * All atoms of one element have identical properties
Problem
* Development of a cathode ray tube (by William Crookes)
Thompson (1897)
Experimental Work
* Used a cathode ray tube a vacuum tube with electrodes at both ends * Found that there were charged particles that were travelling from one end of the tube to the other (from negative end to positive end) Conclusion

* Proposed that an atom was a positively charged empty sphere containing negatively charged electrons raisin scone analogy What Thompson left us with?
* Atoms consist of negative electrons embedded within a positively charged sphere * Analogy of raisin bun often used
Milikin’s Famous Oil Drop Experiment
* Determined size and charge on electron
* Discovered charge on single electron was 1.6 x 10^19 C
How it worked?
* Knew mass of single drop of oil, calculated gravity on one drop * Charge was applied to falling drops by illuminating bottom chamber with x-rays, exciting electrons, causing them to attach to oil. * Using a battery, electric voltage was applied to the plates. When just right, the electromagnetic force would balance out the force of gravity, suspending particles in midair. * Noticed charge was always multiples of 1.6 x 10^19

* Q= mg/E
Gold Foil Experiment
* Radioactive particles (alpha radiation) were fired at thin gold sheets * Screens coated with zinc sulfide detected the presence of the alpha radiation * Vast majority of alpha particles passed straight through gold sheet, however, approximately 1 in 8000 particles were deflected Chadwick and the Neutron

* When calculating the mass of specific nuclei, the calculated mass did not correlate with the associated charge of the nucleus * Chadwick proposed that neutral particles must be present to make up for the missing mass * Chadwick proposed a positive nucleus containing neutral particles Isotopes

* Mass spectrometers were used to discover that all atoms of the same element were not the same * Elements contained several different forms of isotopes (atoms with the same number of protons, but different numbers of neutrons) Problem with the Rutherford Model

* Physics – bodies are accelerating when they change speed and/or direction * And electron travelling in a circular orbit is constantly changing its direction and therefore accelerating * This acceleration would result in the electrons emitting electromagnetic radiation, lose electrons, and collapsing the atom as it continuously spirals inward because it is losing electrons Enter Max Plank

* Her was studying the emission of light from hot objects * What is visible light?
* When objects are heated, they emit various colors of light depending on how hot the object is * Ex. “white hot” objects are emitting the whole range of the visible spectrum * “red hot” objects emit light with wavelength of the infrared – longest wavelength * “blue hot” objects are the hottest as they emit light of shortest wavelength * Hot objects emit radiation. The hotter they are, the more energetic the radiation emitted is. The electromagnetic radiation changes as the object gets hotter. * The color of light emitted reveals temperature

Explaining Intensity vs. Energy – The staircase which changed physics * Planck suggested that...
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