8Conclusions? The stream of particles was … Negative Overall, atoms are...NeutralTherefore, atoms must also contain charges that are...Positive
9Plum Pudding ModelThomson figured out that atoms contained positive and negative charges but had no idea how they were arranged.He guessed there was a diffuse positive cloud with negative charges randomly distributed
10.Milliken’s oil drop experiment: American scientist Robert Milliken calculated the charge of electron (e) by conducting his famous oil drop experiment.
11Millikan Oil Drop Experiment Robert Millikan (University of Chicago) determined the charge on the electron in 1909.Figure 2.5ANIMATION:
12Robert A. Millikan, 1909 Calculated the charge on the electron. Electrons are present in atoms of all elements.It was inferred that:atoms contain a positive charge to balance the negative electrons.Atoms contain other particles that account for most of the mass.Oil drop experiment: oil drops become charged when exposed to X rays,each drop has several charges, e-, attach to it. Oil drop is surrounded by an electric field and then field is adjusted to balance gravitational force until the drop becomes stationary. Applied electric field was dependent on # of charged particles attached to the oil drop.
13Ernest RutherfordDiscovered that the nucleus was positively charged and very tiny.Found that the atom is mostly empty space.
14Set-up Uranium Gold foil (0.00006 cm thick) Detection screen Positive particlesUraniumGold foil ( cm thick)Detection screen
18HOLY ATOMS! What conclusions can be drawn here? There is something inside an atom that is heavy enough to deflect a particle straight back.That “something” must be positively charged.It must also be very small.
19Conclusions?Heavy, dense, positively charged, small piece of the atom is….The nucleus!!Lets watch this animated
20Niels Bohr Developed the “solar system” model of the atom. The nucleus in the center of the atom is made of protons (+) and neutrons.Electrons (-) orbit the nucleus like the planets around the sun.
21James Chadwick, 1932In the nucleus, there is also a particle without charge, called a neutron, which is about the size of a proton and has a slightly greater mass.Chadwick bombarded Beryllium atoms with alpha particles which produced a strong beam of particles that were not deflected by an electrical field.
23Electron Cloud Model The current model of the atom. Electrons do not orbit the nucleus in orderly paths.Electrons move around the nucleus at near the speed of light in a general area.The electrons float around the nucleus as if in a cloudResearch continues.
24Heisenberg Uncertainty Principle Position and speed of electron in electron cloud CAN NOT be determined at the same timeMoving too fast