3Ancient Philosophy Who: Aristotle, Democritus When: More than 2000 years ago 460 BCWhere: GreeceWhat: Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”.Why: Aristotle and Democritus used observation and inference to explain the existence of everything.
8Particle Theory Who: John Dalton When: 1766-1844 Where: England, school teacherWhat: Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom.Why: Building on the ideas of Democritus in ancient Greece.
9Does this part hold true today? 4 Part Atomic Theory1. Elements are composed of tiny indivisibleparticles (atoms).Does this part hold true today?
10Does this part hold true today? 2. Atoms of the same element are identicaland different from other atoms.Does this part hold true today?
11Does this part hold true today? 3. Atoms can physically mix and chemically combine in whole number ratios.Does this part hold true today?
12Does this part hold true today? In chemical reactions, atoms separate, join and rearrange.Atoms cannot be changed into other atoms.Does this part hold true today?
13Did Dalton have experimental support for his theory?
15Discovery of Electrons Who: J. J. ThompsonWhen: 1897Where: England, physicistWhat: Thompson discovered that electrons were smaller particles of an atom and were negatively charged.Why: Thompson knew atoms were neutrally charged, but couldn’t find the positive particle.
19Who: Robert MillikenWhen:Where: United States, physicistWhat: Conducted an experiment to determine the quantity of charge on an electron, 1-, in his famous oil drop experimentWhy: He knew that atoms had electrons, wanted to determine charge
21Atomic Structure I Who: Eugene Goldstein When: 1850-1930 Where: EnglandWhat: Conducted an experiment with a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He concluded that they were composed of positive particles- protons.Why: He concluded that they were composed of positive particles- protons
23Atomic Structure I Who: Ernest Rutherford When: 1911 Where: England What: Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core.Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.
25Who: James ChadwickWhen: 1932Where: England, physicistWhat: confirmed existence of the neutronNeutrons have no charge and have amass nearly equal to that of a proton
26Atomic Structure II Who: Niels Bohr When: 1913 Where: England What: Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus.Why: Bohr was trying to show why the negative electrons were not sucked into the nucleus of the atom.
28Electron Cloud ModelElectrons travel around the nucleus in random orbits.Scientists cannot predict where they will be at any given moment.Electrons travel so fast, they appear to form a “cloud” around the nucleus.