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The Evolution of Atomic Theory: From Ancient Greeks to X-Rays

This educational overview explores the historical development of atomic theory, starting from the ancient Greeks who first conceptualized atoms as the smallest units of matter. It highlights key advancements through the late 1700s and early 1800s, including Avogadro's contributions regarding gas volumes and molecules. The section discusses the quantization of charge demonstrated by Michael Faraday and the experiments of the late 19th century that revealed the size of atoms, culminating in the discovery of X-rays by Wilhelm Roentgen, which significantly advanced our understanding of atomic structure.

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The Evolution of Atomic Theory: From Ancient Greeks to X-Rays

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  1. Modern Physics: Introduction Ch 27-1

  2. Greeks first had atomic theory • B.C. • Smallest Unit of “what is” • Atoms explained crystalline shapes • Chemical Reactions always occurred in precise proportions Atoms

  3. Avogadro – Equal volumes of gas contain the same number of molecules • Late 1700s – Early 1800s • Mass varies according to the mass of the molecule • Avogadro’s Number – 6.022 x 1023 • Periodic Table • Regular Spacing of Elements Enlightenment

  4. Early 1800s • Decomposed substances by passing current through them. • Mass decomposed ∝ time and current • Mass transfer ∝ charge • Charge came only in set values • QUANTIZED! Michael Faraday

  5. By the 1880s a number of experiments established the approximate size of the atom • How big is a Copper Atom? • ρcu = 8.96g/cm3 • Atomic mass of Cu = 63.6g/mol • NA = 6.022x1023 atoms/mol • ρ = m/V = mmol/Vmol =mmol/(NA * V1 atom Cu) • V1 atom Cu = mmol/(NA * ρ) = 1.18 x 10-29m3 • If cube V = l*w*h , then 2.3 x 10-10m per side • If sphere V = (4/3)πr3 , then 1.4 x 10-10m radius • 10-10m = 1 Angstrom Size of the Atom

  6. 1895 William Roentgen • Cathode Ray Tube • Rays passed through light objects • Exposed film (like light) • Photographs of the shadows of the bones of the hand • Like light, it was not deflected by electrostatic or magnetic fields (NO Charge) • Rays discharge electrified objects X-Rays

  7. Roentgen believed X-Rays to be a wave • Diffraction – NO • Reflection – NO • Polarization – NO • Problem – d ~= λ for diffraction to be observed • λ = 0.1nm !!!!! • 1912 first partial diffraction • Roentgen was awarded the 1st Nobel Prize in Physics X-Rays

  8. Space between atoms in a crystal is about 0.1nm Crystal Diffraction

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