•EXAM #2 RESULTS

•THIS WEEK’S SCHEDULE

•LECTURE-

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Announcements

Energy is Quantized

Light also acts like particlesPhoton= “Packet” of energyEnergy is proportional to Frequency (and )Planck’s Equation:

hEphoton

UV-B Light and DNA Damage

Some more examples…

How much energy does a mole of 230nm photons possess?

Can this light break a C-C bond with an energy of 346kJ/mol?

c

chhEphoton

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For one photon :

E photon = h ⋅ν =h ⋅cλ=(6.626x10−34 J ⋅s)(3.0x108m /s)

230x10−9m= 8.64x10−19J

For one mole of photons :

E =8.64x10−19J

photon⋅6.02x1023 photons

1mole= 520285J = 520kJ

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520kJ > 346kJ , ∴ yes 230nm light CAN break a C −C bond.

Does 1 mole of 1200nm photons have enough energy to break a C-C bond?

1 2 3

62%

13%

26%

1. Yes2. No3. Abstain

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For one photon :

E photon = h ⋅ν =h ⋅cλ=(6.626x10−34 J ⋅s)(3.0x108m /s)

1200x10−9m=1.6x10−19J

For one mole of photons :

E =1.6x10−19J

photon⋅6.02x1023 photons

1mole= 99721J = 99.7kJ < 346kJ

Where does light come from?

Excited solids emit a continuous spectrum of light

Excited gas-phase atoms emit only specific wavelengths of light (“lines”)

Light emitted by solids

Light emitted by hydrogen gas

Emission spectra of gaseous atoms

Excited atoms emit light of only certain wavelengths

The wavelengths of emitted light depend on the element.

Absorption and Emission Spectra

Line spectra of atoms

The Bohr Model of Hydrogen Atom

Light absorbed or emitted is from electrons moving between energy levels

Only certain energies are observedTherefore, only certain energy levels exist

Energy levels are Quantized