A beginners guide to atomic physics Inside the atom A beginners guide to atomic physics

Brief History JJ Thomson Rutherford 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 En= (-13.6 eV) 1/n2 (where n is an integer) 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