Ch. 6 The Structure of Atoms 6.1 Electromagnetic Radiation Electromagnetic Radiation or “Light” is composed of two orthogonal vectors: An electric wave and a magnetic wave. 1 Vibrating electric and magnetic fields at right angles to each other.• The wavelength of EM radiation has the symbol . • Wavelength is the distance from the top (crest) of one wave to the top of the next wave. – Measured in units of distance such as m, cm, nm, Å, etc. – 1 Å = 1 x 10-10 m = 1 x 10-8 cm 23 • The frequency of electromagnetic radiation has the symbol .• Frequency is the number of crests or troughs that pass a given point per second (like water waves hitting the pier). – Measured in units of 1/time – 1/s - s-1 - Hertz  =speed of propagation of the wave or =c (for light) c = speed of light (3.00 x 108m/s) [in a vacuum, speed of light for all λ is the same] Wavelength and frequency are inversely proportional to each other, for the same wave, shorter wavelengths correspond to higher frequency.4 Electromagnetic Radiation • The relationship between wavelength and frequency for any wave is velocity = . • For electromagnetic radiation the velocity is 3.00 x 108 m/s and has the symbol c. • Thus c =  forelectromagnetic radiation.UV: shorter λ, higher ν IR: longer λ, low ν High Energy; High Frequency; Short λ or Low Energy; Low Frequency; Long λClicker Q: Which has the highest frequency? A. γ-rays B. x-rays C. IR D. long radio waves Clicker Q: Which has the lowest energy? 56 Clicker Q: What is the frequency of green light of wavelength 5200 Å? =c (for light) c = 3.00 x 108 m/s 1 Å = 1.00 x 10-10 m Press log button then enter that number. No e notation.6.2 Quantization: Planck, Einstein, Energy, and Photons Planck’s equation E: energy, in Joules/photon h: Planck’s constant, 6.626 x 10-34 J∙s ν: frequency (units of Hertz, 1/s or s-1) E = hν = hc/λ Light with long λ (low ν) has low energy (E). Light with short λ (high ν) has high energy (E). As the frequency of light increases, the energy of the photon increases As the wavelength of light increases, the energy of the photon decreases 7UV light has a ν = 2.73 x 1016 1/s, what is its energy? Clicker Q: Yellow light has a ν = 5.27 x 1014 1/s Energy? Enter as #.##e#A laser having a wavelength of 508 nm emits 4.28 x 1017 photons of light per second. How much energy, in Joules, does this laser emit in 5.00 seconds?The energy of a particular color of red light is 2.92E-22 kJ. What is the wavelength of this light in nanometers. 10• Certain metals will release (eject) electrons when light strikes the metal surface. • The energy of the light must exceed a minimum or “threshold energy” for this to occur. • Any excess energy beyond this minimum goes into the kinetic energy of the ejected electron. (They fly away with greater velocity) A. Einstein (1879-1955) Einstein and the Photoelectric Effect• Classical theory suggests that energy of an ejected electron should increase with an increase in light intensity. • This however is not experimentally observed! • No ejected electrons were observed until light of a certain minimum energy is applied. • Number of electrons ejected depends on light intensity so long as the light is above a minimum energy. (This “minimum energy” is also the ionization energy of the metal.) Photoelectric EffectExperiment demonstrates the particle like nature of light. Photoelectric Effect• Conclusion: There is a one-to-one correspondence between ejected electrons and light waves. • This can only occur if light consists of individual units called “PHOTONS” . • A Photon is a packet of light of discrete energy. • http://www.youtube.com/watch?v=N7BywkIretM&feature=related Photoelectric Effect15 6.3 Atomic Line Spectra and Niels Bohr An emission spectrum is formed by an electric current passing through a gas in a vacuum tube (at very low pressure) which causes the gas to emit light. – Sometimes called a bright line spectrum.16 When the light from a discharge tube containing a pure element (hydrogen in this case) is passed through the same prism, only certain colors (lines) are observed. Recall that color (wavelength) is related to energy via Planck’s law. Light emitted by H2. Emission spectrum of hydrogen.17 • An absorption spectrum is formed by shining a beam of white light through a sample of gas. – Absorption spectra indicate the wavelengths of light that have been absorbed.18 • Every element has a unique spectrum. • Thus we can use spectra to identify elements. -This can be done in the lab, stars, fireworks, etc. • Spectra can serve as fingerprints that allow us to identify different elements present in a sample, even in trace amounts.19 Balmer equation • The Balmer equation is an empirical equation that relates the wavelengths of the lines in the hydrogen spectrum. Derived from numerous observations, not from theory. hydrogen of spectrumemission in the levelsenergy theof numbers therefer to sn’ n n m 10 1.097 Rconstant Rydberg theis R n1n1R 121 1-72221==20 What is the wavelength of light emitted when the hydrogen atom’s energy changes from n = 4 to n = 2?21 Solution: What is the wavelength of light emitted when the hydrogen atom’s energy changes from n = 4 to n = 2?   2122127 -1 227 -1 7 -1 7 -1 6 -1 n 4 and n 21 1 1 R nn1 1 1 1.097 10 m241 1 1 1.097 10 m4 161 1.097 10 m 0.250 0.06251 1.097 10 m 0.18751 2.057 10 m====  =  =  ==22 Notice that the wavelength calculated from the Balmer equation matches the wavelength of the green colored line in the H spectrum.When an electron is excited from a lower energy level to a higher one, it absorbs a definite (or quantized) amount of energy. When the electron falls back to the original energy level, it emits exactly the same amount of energy it absorbed in moving from the lower to the higher energy level.Draw energy levels. Clicker Q: Going from n= 2 to n= 4 is A. Absorption B. emission Clicker Q: Which is closer to the nucleus? A. n= 1 B. n= 2 C. n=3 D. n=4 E. n=∞ 24Fig. 6-10, p. 277Long wavelength = low frequency = low energy transition = short line on the diagram. Short