Atomic Structure Timeline -

Atomic Structure Timeline -

Atomic Structure Timeline Use the following information to complete the lecture handout. On tomorrows quiz, you will be expected to draw the atomic models match scientists to their experiments and discoveries place the models in chronological order

Democritus (400 B.C.) Proposed that matter was composed of tiny indivisible particles Not based on experimental data Greek: atomos

Alchemy (next 2000 years) Mixture of science and mysticism. Lab procedures were developed, but alchemists did not perform controlled experiments like true scientists. John Dalton (1807) British Schoolteacher based his theory on others experimental data

Billiard Ball Model atom is a uniform, solid sphere John Dalton Daltons Four Postulates 1. Elements are composed of small indivisible

particles called atoms. 2. Atoms of the same element are identical. Atoms of different elements are different. 3. Atoms of different elements combine together in simple proportions to create a compound. 4. In a chemical reaction, atoms are rearranged, but not changed. Henri Becquerel (1896)

Discovered radioactivity spontaneous emission of radiation from the nucleus Three types: alpha () - positive beta () - negative gamma () - neutral

J. J. Thomson (1903) Cathode Ray Tube Experiments beam of negative particles Discovered Electrons negative particles within the atom

Plum-pudding Model J. J. Thomson (1903) Plum-pudding Model positive sphere (pudding) with negative electrons (plums) dispersed

throughout Ernest Rutherford (1911) Gold Foil Experiment Discovered the nucleus dense, positive charge in the center of the atom Nuclear Model

Ernest Rutherford (1911) Nuclear Model dense, positive nucleus surrounded by negative electrons Niels Bohr (1913) Bright-Line Spectrum

tried to explain presence of specific colors in hydrogens spectrum Energy Levels electrons can only exist in specific energy states Planetary Model

Niels Bohr (1913) Bright-line spectrum Planetary Model electrons move in circular orbits within specific energy levels Erwin Schrdinger (1926)

Quantum mechanics electrons can only exist in specified energy states Electron cloud model orbital: region around the nucleus where e- are likely to be found

Erwin Schrdinger (1926) Electron Cloud Model (orbital) dots represent probability of finding an enot actual electrons James Chadwick (1932) Discovered neutrons

neutral particles in the nucleus of an atom Joliot-Curie Experiments based his theory on their experimental evidence James Chadwick (1932)

Neutron Model revision of Rutherfords Nuclear Model

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