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Atomic Theories

Atomic Theories. Early Greek Theories. Aristotle (350 B.C.) 4 Elements. Democritus (400 B.C) Matter: atoms and a void (empty space) Atoms are indivisible. Dalton’s Billiard Ball Model (1805). All matter is made of atoms. Atoms of an element are identical.

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Atomic Theories

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  1. Atomic Theories

  2. Early Greek Theories Aristotle (350 B.C.) • 4 Elements • Democritus (400 B.C) • Matter: atoms and a void (empty space) • Atoms are indivisible

  3. Dalton’s Billiard Ball Model (1805) • All matter is made of atoms. • Atoms of an element are identical. • Each element has different atoms. • Atoms of different elements combine in constant ratios to form compounds. • Atoms are rearranged in reactions, but are not created nor destroyed.

  4. Thomson’s Raisin Bun Model (1897) • Cathode Ray experiment: Sent electricity through a gas, producing negatively charged particles “cathode rays” • Discovered the electron: a negatively charged particle with a small mass • Atoms consisted of a positively charged sphere embedded with negative electrons

  5. Rutherford’s Gold Foil Experiment (1910) • Alpha particles (positively charged helium ions) were fired toward a very thin gold foil.

  6. Most alpha particles passed through. • Some positive -particles deflected or bounced back! • Conclusion: Atoms are mostly empty space. Most of the mass of an atom is concentrated in a positive core, called the atomic nucleus.

  7. Shortfalls of Rutherford’s Model • Orbiting electrons should emit light, losing energy in the process • This energy loss should cause the electrons to collapse into the nucleus • However, matter is very stable, this does not happen

  8. Bohr’s Planetary Model (1913) • Electrons orbit the nucleus in energy “shells” • Electrons in these shells have a specific and constant amount of energy, without losing energy in the shell • The greater the distance between the nucleus and the energy level, the greater the energy level • An electron changes energy levels by emitting or absorbing a specific quantity (quantum) of energy in the form of light (photons)

  9. Electrons at their lowest possible energy state are in their ground state • If light or heat is directed at an atom, electrons can absorb the energy, and move to a higher energy shell • When the electron returns to a lower energy state the energy is released 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

  10. 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)

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