chemistry: the atomic structure

8/11/2019 Chemistry: The atomic structure

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Electromagnetic Radiation07_94

10 -12 10 -10 10 -8 4 x 10 -7

Gammarays

X rays Ultraviolet Infrared Microwaves Radio waves

FM Shortwave AM

4 x 10 -7

Wavelength in meters

5 x 10 -7 6 x 10 -7 7 x 10 -7

7 x 10 -7 10 -4 10 -2 1 10 2 10 4

V i s i b l e


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Waves

Waves have 3 primary characteristics:1. Wavelength : distance between two peaks in

a wave.2. Frequency : number of waves per second that

pass a given point in space.

3. Speed c : speed of light is 2.9979 10 8 m/s.


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07_93 1 second

= 4 cycles/second = 4 hertz



(m) (s -1 ) = c (m/s)1Hz = cycles/s = s -1

Relation between wavelength and Frequency:Inverse Proportionality

1 nm = 10-9

m; 1 = 10-10

m; 1 pm = 10-12

m1nm = 10


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As the amplitude increases, the intensity (the brightness)increases

Intensity is proportional to A 2:

I A2


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Nature of Matter?End of 19 th Century: It was believed that MATTER and ENERGY are different

Particles:Mass; momentum;Localized (position specified)

Waves:

Massless; delocalized

BUT WAS THIS REALLY TRUE?

The Answer turns out to be No

No Intermingling between the two forms


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?!! UV-Catastrophe

Planck: E = nh

Planck constanth = 6.626 10 -34 j.sn = 1, 2, 3, 4

frequency s -1

Meaning:

Energy is q uant ized

heat

UV-Catastrophe: significant historical importance

blackbody


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Planck ContributionQuantization of Energy--

Max Planck: ( sum merof 1900)

E = n h Planck constanth = 6.626 10 -34 j.s

n = 1, 2, 3, 4 frequency s -1 Energy transfer in matter is quantizedEnergy is absorbed and emittedin discrete quanta

Each has an energy = h = h c


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The Photoelectric Effect

Increasing the intensity of light increased the number of photoelectrons, butnot their maximum kinetic energy. Red light will not cause the ejection of electrons from Na, however much the

intensity is increased. A weak violet light will eject only a few electrons, but their maximum kinetic

energy is greater than those ejected by intense light of longer wavelengths.

Explained byEins te in


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Summary of experimental results from thephotoelectric effect

Electrons are not ejected when light shined isbelow a minimal frequency, called thethreshold frequency o

The kinetic energy increased as the frequencyof light increased above o

The magnitude of current was independent of but increased as the intensity of lightincreases

The kinetic energy of the e was independent ofthe light intensity


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E = h =E o +K.E.= h o + mv2

Kinetic energy of the electronThreshold frequency

(620 nm)


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Comptons experiment (1922) Collisions betweenX-rays and electronsshowed that photonsactually have

momentum ascalculated by formula:

mc = h/

Evidence of the particle nature of light:


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Summary: what do we know so far?

1. Energy quantization: energy can beabsorbed or emitted by matter indiscrete units called quanta (quantum)

2. Light has particulate and wave nature,known as the dual nature of light

chemistry: the atomic structure

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