Chapter 2 (CIC) and Chapter 6 (CTCS)

• Read in CTCS Chapter 6.1-3

• Problems in CTCS: 1, 3, 5, 7, 9, 13, 15, 19, 23, 25, 27, 29

Light• How does O3 interact with UV light?

• What is UV light?

http://www.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html

http://astrosun.tn.cornell.edu/courses/astro201/emspectrum.htm

• Prism breaks visible light into separated colors• Light is a wave• Visible light has wavelengths () from 400 – 700

nm (VIBGYOR)• Radio waves ~ football field• X-rays ~ atom

Waves of Light• Wavelength is distance between successive

peaks and has units of m• Frequency is the number of waves passing a

fixed point in one second () and has units of Hz or 1/s

= c/ where c is the speed of light (3.00 x 108 m/s

Q: Calculate the frequency of red light

A: 7.5 x 1014 s-1

Q: If the radio in front of you is off, are you being hit by radio waves?

Q: Arrange Visible, UV, and IR light from shortest to longest wavelength

Q: Arrange Visible, UV, and IR light from shortest to longest frequency

What does the sun emit?

*Stanitski, D.L.; Eubanks, L.P.; Middlecamp, C.H.; Stratton, W.J. Chemistry in Context: Applying Chemistry to Society, 3rd Edition, McGraw-Hill, Boston, MA, 2000, pg 58.

Why is UV light most harmful when it consists of only 8% of the light coming from the sun?

Dual Nature of Light• None of the following could be explained by

classical physics• All solids emit electromagnetic radiation

– Usually infrared (night vision goggles)

• Black-body radiation Any object when heated releases light (750º-red, 1200º-white)

• Photoelectric Effect - when a beam of light shines on metal, a beam of electrons is producedAll could be explained by Quantization – energy can only occur in small discreet units called quanta, i.e., energy is not continuous

Quantum Energy• Radiation considered as consisting of small

discreet bundles of energy called photons

• Energy is particle like without the mass

• E = h = hc/• Planck’s constant, h = 6.63 x 10-34 J.s

Q:Compare the energies of a radiowave (100 MHz FM) vs. UV light (300 nm)

A: 6.63 x 10-26 J/photon vs. 6.63 x 10-19 J/photon

UV Light• UV light has more energy than visible or IR light• 7 x 10-19 J are required to increase the content of

melanin causing tanning to occurQ: Can all the radio particles be added or stored to

add up to 7 x 10-19 J?A: A photon either has or doesn’t have the energy

to make this happen• With 400 nm light and chlorophyll,

CO2 + H2O C6H12O6

• UV has enough E to eject e- and in some cases break bonds which can cause cancer

Q: An O-O bond requires 2.4 x 10-19 J to break. O=O is 8.2 x 10-19 J. What wavelength of light would this require?

A: 830 nm compared to 240 nm

More Evidence of Quanta• Line Spectra• http://home.achilles.net/~jtalbot/data/elements/• Niels Bohr developed the Planetary Model for

an atom (can’t be)• Electrons can jump from one orbit to the next

(quantum leaps only)

• En is the energy of the electron in the nth orbit

• RH is the Rydberg constant (2.18 x 10-18 J)• n is the orbit that the electron is in

2

1( )n HE R n = 1, 2, 3, 4, ...

n

• When an electron gains enough energy to escape an orbit and then relaxes, it must give off energy

Q: Calculate the wavelength of the red line in H (n=3 to n=2)

A: 657 nm (actual = 656 nm)• Model doesn’t work for species other than 1 e-

2 2

1 1( )H

i f

E R

h h n n