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# Modern Physics: Introduction - PowerPoint PPT Presentation

Modern Physics: Introduction. Ch 27-1. Greeks first had atomic theory B.C. Smallest Unit of “what is” Atoms explained crystalline shapes Chemical Reactions always occurred in precise proportions. Atoms. Avogadro – Equal volumes of gas contain the same number of molecules

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### Modern Physics: Introduction

Ch 27-1

• Greeks first had atomic theory

• B.C.

• Smallest Unit of “what is”

• Atoms explained crystalline shapes

• Chemical Reactions always occurred in precise proportions

Atoms

Enlightenment

• Early 1800s molecules

• Decomposed substances by passing

current through them.

• Mass decomposed ∝ time and current

• Mass transfer ∝ charge

• Charge came only in set values

• QUANTIZED!

• 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

• 1895 William Roentgen approximate size of the atom

• 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

• Roentgen believed X-Rays to be a wave approximate size of the atom

• 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

Crystal Diffraction