University:
Purdue University
Course:
CHM 37200 | Physical Chemistry
Academic year:

2020
Views:

454

Pages:

5
Author:

Jimena Rivera

Electron Configurations & the Periodic Table Chapter 7 Electromagnetic Radiation visible light, ultraviolet, infrared, X-rays, etc.  oscillating perpendicular electric and magnetic fields  travel through space at 2.998 x 108 m/s  described in terms of frequency and wavelength  νλ = c Wavelength Units 1 Electromagnetic Spectrum Planck’s Quantum Theory Observation: heat a solid – emits light with a wavelength distribution that depends on the temperature vibrating atoms in hot wire caused light emission whose color changed with T classical physics could not explain this – the wavelength should be independent of T Planck’s Quantum Theory  Max Planck (1900)  assume energy can be released or absorbed by atoms in packets only  these packets have a minimum size  packet of energy = quantum  energy of a quantum is proportional to the frequency of radiation  (E = hν)  h = Planck’s constant = 6.626 x 10-34 Js  energy is emitted or absorbed in whole # multiples (1 hν, 2 hν, etc.) E= hc ! 2 Example 1 What is the energy (in kilojoules per mole) of photons of radar waves with ν = 3.35 x 108 Hz? E = hν E = (3.35 x 108 /s)(6.626 x 10-34 J s) = 2.22 x 10-25 J E = (2.22 x 10-28 kJ)(6.022 x 1023 photons/mol) E = 1.34 x 10-4 kJ/mol The Photoelectric Effect The Photoelectric Effect  Einstein assumed light striking metal was stream of tiny energy packets  Each energy packet behaves like tiny particle of light (photon)  E = hν radiant energy is quantized  Dilemma – is light a wave or a particle??? 3 Line Spectra   most radiation sources emit light in continuous spectrum (e.g. white light) when voltage applied to gaseous elements – not continuous spectrum but line spectrum – spectrum containing radiation of specific λ Electronic Transitions in H Atom Line spectrum of H atom contains 4 lines (Balmer series) & 1 1# ' = c$ 2 - 2 ! %2 n " n = 3, 4, 5, 6 c = 3.289 x 1015 s-1 How could this very simple formula be explained?? Bohr Model Original picture of atom – e- orbiting nucleus     classical physics – such an e- would continuously lose energy by emitting radiation and ultimately spiral into nucleus (doesn’t happen) Bohr – only orbits of certain radii (and therefore certain energies) are permitted. e- in a permitted orbit has specific energy (“allowed” energy state) Allowed orbits have specific energies: & 1# En = (- R H )$ 2 ! %n " n = 1, 2, 3, 4, . . . RH = 2.18 x 10-18 J 4 Bohr Model (cont’d)  each orbit – different n value  radius increases as n increases (r ∝ n2 )  1st allowed orbit – n = 1, radius = 0.529 Å  Energies of electrons negative for all values of n. n  ∞: & 1 # En = (- 2.18 x 10 -18 J)$ 2 ! = 0 %' " Bohr Model (cont’d)  Bohr - e- “jump” from one energy state to another by absorbing or emitting photons of certain ν  this ν corresponds to E difference between the two states ΔE = h ν (= 'E & R H #&$ 1 1 =$ ! h % h "$% ni2 n 2f # ! ! " 5

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