Atomic Theory (Unit 8) Introduction Atomic Theory Theories in science are proposed to explain the evidence available at the time. As new evidence is discovered, theories are adapted to explain new data. This is the nature of Science. In the future this will continue. Democritus Plato BC First Greek philosopher to suggest that tiny different pieces of particles exist atoms Atoms make up the world His idea was refuted by Aristotle Aristotle Plato BC Four Elements Teacher of Aristotle Aristotle Air Fire Water Earth dryness wetness coldnesshotness conflictharmony The Four Element Theory PropertiesForces The Four Element Theory lasted for about 2 thousand years because no one tested the theory with scientific experiments. It was not a scientific theory- which is tested by experiment. Science back then was more of a philosophical moralistic field that philosophers rather than experimentation decided. John Dalton Re-visited the idea of Atoms Atomic Theory Daltons Atomic Theory Evidence 1. The Law of Conservation of Mass 2.The Law of Constant Composition- water is 11 % H and 89 % O Theory 1. Each atom is an indestructible and unique spherical particle 2.Atoms combine in simple whole number ratios to form compounds Hydrogen Oxygen Dalton Theory- Water 1 g H 16 g O 18g O 16 g O 1 g H Dalton Theory- Water 18g O = 11% H 16 g O 1 g H Dalton Theory- Water 2 g H 18 g H 2 O x 100%89 % O Still 11 % H 89 % O Having a molecule (atoms combine in simple whole # ratios) explains the law of constant composition. No matter how much water you have its still 11% H 2H 2 + O 2 2H 2 O If the atoms are not destroyed then the mass does not change J. J. Thomson He discovered the electron Plum Pudding model/blueberry muffin model The Thomson Theory of the Atom The atom is made up of positive material with negative particles throughout- like blueberries in a blueberry muffin. We now know this is not trueelectrons are not found inside the nucleus Ernest Rutherford He was J. J Thomsons student The Rutherford Atom 1911 Evidence: Gold Foil Experiment % of alphas are not deflected % of alphas are radically deflected Theory 1.Most of the atom is empty space. 2.There is a small dense nucleus in the center of the atom that makes up most of the mass. Electrons circle the nucleus randomly. The Rutherford Atom 1911 Scale: nucleus is home plate; atom is the baseball field Be Nucleus- 4 protons and 5 neutrons Electrons Nucleus- is small but has most of the mass Niels Bohr Electrons are restricted to having certain specific energies and are restricted to following specific paths called orbits at a fixed distance from the nucleus. Electrons emit energy when they move from one orbit to the other |Niels Bohr Electrons _________ energy/light when they move from a high energy level to a low energy level. Electrons __________ energy/light when they move up from a low energy level to a high energy level. |Niels Bohr Limitations of his model is that . It only worked for the hydrogen atom ( only atoms with one electron) Electrons do not really move in circular motionthe correct description requires quantum mechanics to answer this question Discovery of the Nucleus Size of the Atom Read pages from your textbook. They cover the theories we just talked about. It is your responsibility to know what each scientist did/contributed to the understanding of the atom. You can do this by writing out a summary/list of each scientists accomplishments.