History of the Atomic Model

History of the Atomic Model

Greeks (300 B.C) • Hypothesized that matter cut into smaller and smaller pieces would eventually reach the atom (literally meaning indivisible) • Thought there were 4 elements: Air, Fire, Earth, Water • Findings based on reason, not empirical evidence (experimentation)

Dalton’s “Billiard Ball” Model (1808) • Atoms are small indestructible, indivisible spheres • All matter is made of atoms or combinations of atoms (in specific proportions/ratios) • All atoms of one element are identical • Matter cannot be created or destroyed, only changed in form

Dalton’s Laws • Law of definite composition: elements combine in a characteristic mass ratio • Law of multiple proportions: there may be more than one mass ratio • Law of conservation of mass: total mass remains constant

Dalton’s Shortfall • Dalton’s model did not explain the behaviour of substances (why atoms combined in ratios)

Thomson’s “Raisin Bun” or “Plum Pudding” Model (1897) • Discovered a negatively charged particle, the electron using cathode ray experiments • Atoms consisted of a positively charged sphere embedded with negatively charged electrons • Overall charge of the atom is neutral • Charged particles could exist if electrons were added or removed http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::100%::100%::/sites/dl/free/0072512644/117354/01_Cathode_Ray_Tube.swf::Cathode%20Ray%20Tube

Rutherford’s “Electron Cloud” Model (1911) • Discovered the proton by using cathode ray tubes and found the smallest possible positive charge from ionized hydrogen • Gold Foil Experiment: • Aimed alpha particles at thin metal foil • 1/8000 alpha particles were deflected significantly • Inconsistent with Thomson’s Model • Conclusion: • Atoms are made up mainly of empty space, with small concentrated charge around the center

Gold Foil Experiment • http://www.mhhe.com/physsci/chemistry/animations/chang_2e/rutherfords_experiment.swf

Rutherford’s Limitations • A nucleus made of entirely positive charges should fly apart • Did not explain the total mass of the atom • Solution: In 1932 the neutron was discovered by Chadwick by bombarding nuclei with alpha particles and noticing a difference in their mass and that of the number of protons they contained • Orbiting electrons should emit light and constantly lose energy and then collapse • However, matter is very stable and this does not happen

Bohr’s “Planetary” Model • Electrons have specific, allowable energy levels • Electrons moving within an energy level (shell) do not lose energy • An electron changes energy levels by emitting or absorbing a specific quantity (quantum) of energy in the form of light (photons) • When electrons in an atom are in the lowest possible energy levels, they are in its ground state

Bohr’s “Planetary” Model (cont.) • When an electron absorbs energy in the form of photons, the atom becomes excited • If enough energy is absorbed, the electron can leap to the next energy level • When the electron returns back to it’s original state, it releases energy in the form of a photon, which we see as visible light • The energy of the photon determines its wavelength or colour • Each element has its own frequencies of color

http://www.visionlearning.com/library/flash_viewer.php?oid=1347&mid=51http://www.visionlearning.com/library/flash_viewer.php?oid=1347&mid=51

Summary of Atomic Models • Dalton’s “Billiard ball” model (1800-1900) Atoms are solid and indivisible. • Thomson’s “Raisin bun” model (1900) • Negative electrons in a positive framework. • Rutherford’s “Nuclear” model (~1910) • Atoms are mostly empty space. • Negative electrons orbit a positive nucleus. 4) Bohr’s “Planetary” model (~1920) Negative electrons orbit a positive nucleus. Quantized energy shells 5) Quantum Mechanical model (~1930) Electron probabilities (orbitals) Song: http://www.youtube.com/watch?v=vUzTQWn-wfE

History of the Atomic Model