Grade 12 Chemistry Notes - History and Development
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)
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)