PowerPoint to accompany Chapter 5 Electronic Structure of Atoms Part 1

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Waves: Chemistry’s best tool  To understand the electronic structure of atoms, one must understand the nature of electromagnetic radiation.  The distance between corresponding points on adjacent waves is the wavelength ( ). Figure 5.3

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Waves  The number of waves passing a given point per unit of time is the frequency ( ).  For waves travelling at the same velocity, the longer the wavelength, the smaller the frequency.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Which wave above has the highest energy? How can you tell?

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia b) Highest amplitude for greatest frequency. Energy ~ frequency Intensity ~ Amplitude 2

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Electromagnetic Radiation  All electromagnetic radiation travels at the same velocity in vacuum.  The speed of light (c) is 3.00  10 8 m/s and  = c. Figure 5.4

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Black Body Radiation & Optical Pyrometry

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Quantized Energy and Photons  The wave nature of light does not explain how an object can glow when its temperature increases.  Max Planck explained the statistical distribution of light, by assuming that energy comes in packets called quanta.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Quantized Energy and Photons  Einstein used this assumption to explain the photoelectric effect.  He concluded that energy is proportional to frequency: E = h where h is Planck’s constant, 6.63  10 −34 Js. To know frequency is to know energy!

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Potential Energy = mgh continuous h (ramp), quantized h (steps)

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia photoelectric effect

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Quantized Energy and Photons  Therefore, if one knows the wavelength of light, one can calculate the energy in one photon, or packet, of that light: c =  = h  = (hc)/ Quantized Energy and Photons

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Quantized Energy and Photons Another mystery involved the emission spectra observed from energy emitted by atoms and molecules. Different gases have different bright line emission spectra (or dark line absorbtion spectra) Figure 5.9 Quantized Energy and Photons

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia  One does not observe a continuous spectrum as one gets from a white light source.  Only a line spectrum of discrete wavelengths is observed. Figure 5.8 Figure 5.10: Sodium versus Hydrogen Quantized Energy and Photons: For Matter….

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Quantized Energy and Photons: Hydrogen green is visible  Niels Bohr adopted Planck’s assumption and explained these phenomena in this way: 1. Electrons in an atom can only occupy certain orbits (corresponding to certain energies). 2. Electrons in permitted orbits have specific, “allowed” energies; these energies will not be radiated from the atom. 3. Energy is only absorbed or emitted in such a way as to move an electron from one “allowed” energy state to another; the energy is defined by: E = h Figure 5.11

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Quantized Energy and Photons The energy absorbed or emitted from the process of electron promotion or demotion can be calculated by the equation: where R H is the Rydberg constant, 2.18  10 −18 J, and n i and n f are the initial and final energy levels of the electron.  E = −R H ( ) 1nf21nf2 1ni21ni2 - Figure 5.11

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia The Wave Nature of Matter  Louis de Broglie posited that if light can have material properties, matter should exhibit wave properties.  He demonstrated that the relationship between mass and wavelength is: = h mv

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

End of Chapter 5 part 1