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Inside the atom

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## Inside the atom

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**Inside the atom**A beginners guide to atomic physics**Brief History**JJ Thomson • Discovered the electron • Where is it? • Plum pudding Model • Rutherford • Gold foil experiment • Positive nuclear core**Problems with classical model**• Coulomb force could make electrons orbit nucleus like planets, BUT… • Accelerated charge is supposed to emit EM waves…why is atom stable? • Also, why do atoms emit and absorb only discrete frequencies of radiation?**BOHR ATOM**• Circular electron orbits • Only “stationary” orbits allowed • The energy of each orbit is given by: En= (-13.6 eV) 1/n2(where n is an integer) • Photons of the correct energy are absorbed, “exciting” the atom into a higher energy orbit • Photons are emitted when the electron makes a transition to a lower energy orbit • The energy of the photons corresponds to energy difference of the two orbits DE=hf**Bohr atom (continued)**Bohr atom...java applet • What wavelength of radiation would excite the hydrogen atom from the ground state (E1) to the third energy level? E1 = -13.6 eV/1 = -13.6 eV E2 = -13.6/9 = -1.5 Ev hc/l = 12.1 eV l = 1243 eV-nm / 12.1 eV = 100 nm**What possible wavelengths of light could be emitted when a**hydrogen atom “de-excites” from the 4th energy level? -0.82 eV -1.51 eV -3.4 eV -13.6 eV**Stationary orbits**• Conservation of energy: KE + PE = Etot ½ mv2 – ke2/r = Etotal • Circular motion: F=ma ke2/r2 = mv2/r ke2/r = mv2 • Combined: Etotal= mv2 – ½ mv2 = -½ mv2 OR Etotal = ½ ke2/r – ke2/r = -½ ke2/r**Stationary orbits (continued)**• Which orbits are allowed? • One way of getting the answer is to look at deBroglie wavelength of electrons. • l=h/mv • To “fit” in a particular orbit, 2pr=nl