Chemistry Study notes

Chemistry Study notes - Development of the Atomic Model...

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Development of the Atomic Model “Because of the indirectness of physical science, scientists must be constantly looking for new theories and models to explain phenomena” Democritus (c.300 B.C.): observed phenomena of evaporation and diffusion and accounted for them by ‘atomic theory’, which stated that matter was made up of tiny particles called ‘atoms’ (singular – ‘atomus’), which were indivisible. Lavoisier (1789): established Law of Conservation of Mass, which stated that a chemical reaction does not result in a gain or a loss of mass. There had to be a complete accounting for all matter involved in chemical reactions. Proust (1792): established Law of Constant Composition, which stated that the percentage composition by mass of a particular compound never varies. Dalton (1803): developed an atomic theory of matter (the ‘billiard ball’ theory), which made sense of the above two Laws by proposing that elements were made up of similar atoms, that compounds were made up of combinations of different types of atoms and that during chemical reactions these atoms were rearranged into new compounds. Dalton’s Atomic Theory 1. Matter is made of tiny indivisible particles called atoms. 2. All atoms of a particular element are identical in mass, size and other properties. 3. Atoms of different elements have different characteristics, such as mass and size. 4. Atoms of different elements combine in small whole number ratios to form compounds; for example, 1:1, 1:2, 3:2. The combination of atoms forms a molecule. 5. In chemical reactions, atoms are not destroyed, they simply joined together or separate from each other. (1.) – derived from Democritus (4.) – derived from Law of Constant Composition (Proust) (5.) – derived from Law of Conservation of Mass (Lavoisier) Crookes (1879): invented the Crookes’ tube or Cathode ray tube or gas discharge tube. - When a very high voltage is applied to the electrodes of a discharge tube at low pressure, electricity flowed through the tube and the air remaining in the tube glowed in pink. - Different gases in the tube produced different colours. - These rays were composed of particles with mass when they were found to be able to turn a wheel. - These rays originates from the cathode and were negatively charged when they were found to be deflected by a positively charged electrode. - The cathode rays consisted of tiny, negatively charged particles known as electrons. Goldstein (1886): experiment with a modified gas discharge tube. - A hole was created in the centre of the cathode metal plate. When the tube was in operation, a faint glow appeared in the part of the tube behind the cathode. - Concluded that the glow in the tube was caused by charged particles moving through the hole in the cathode, opposite in direction to the cathode rays.
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Chemistry Study notes - Development of the Atomic Model...

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