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The Quantum-Mechanical Model I

Chapter 7


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Questions

Why are some elements metals while others are

nonmetals?

Why some elements gain 1 electron when forming

an anion, while others gain 2

Why are some elements much more reactive than

others

Why are there periodic properties of the elements?


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Waves and Particles

Before the 20th century all quantities were

expected to behave either as a particle or a wave

Particles, such as a baseballs, have mass and

have a definite position.

Waves, such as light), have properties such as

frequency and wavelength. Generally it is not

possible to describe their position.Interference, refraction and diffraction are wave

properties.


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Electromagnetic Radiation

Electromagnetic radiation consists of perpendicular

oscillating waves, one for the electric field and one

for the magnetic field


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Electromagnetic Radiation

An electric field is a region of space where an

electrically charged particle experiences a force

A magnetic field is a region of space where an

magnetized particle experiences a forceAll electromagnetic waves move through space at

the same, constant speed c = 3.00 x 108 m/s


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Amplitude and Wavelength

Amplitude - height of the wave. It is a measure of

intensity

Wavelength, ( ) is a the distance from one crest tothe next. Units are length units , m, nm, m, etc


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Amplitude & Wavelength


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Speed of Energy Transmission


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Frequency

Frequency, ( ) - the number of waves that pass apoint in a given period of time

Units are Hertz, (Hz) or cycles/s = s-1


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Wavelength, Frequency, Speed

=c

= c

=c

c = speed of light = 3.00 x 108 m/s

If is know can be found from

If is know can be found from


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Find the frequency in Hz (s-1) for light that has

a wavelength of 400 nm


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Find the frequency in Hz (s-1) for light that has

a wavelength of 400 nm

=c

114

918

s105.7

m

nm10

nm400

sm1000.3c

=

=

=


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Electromagnetic Spectrum

ENERGY


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Visible Light


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Thermal Imaging using Infrared Light


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Interference a wave property

The interaction between waves is calledinterference

When waves interact so that they add to make a

larger wave it is called constructive interference

When waves interact so they cancel each other it

is called destructive interference


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Interference


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2-Slit Interference


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Diffraction A wave Property

Diffraction is the bending of light it passes through

an opening that is about the same size as the

wavelength.

Particles do not undergo diffraction


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Diffraction

Wave

Particle


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Wave and Particles

Wave Properties

wavelengthfrequency

interference

diffraction

Particle Properties

massdefinite location


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State of Physics in 1900

0

E

= 0B =

t

BE

=

+=t

EJB

00

Wave behavior could be explained by Maxwells Equations

Particle behavior could be explained by Newtons Laws

F = ma


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Observations that could not be

explained by old theories

Blackbody radiation

Photoelectric effect

Line spectra of atoms


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Blackbody Radiation

Hot objects emit lightthat can be

approximated by a

black body radiator


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The observed behavior of a

black body

The observed

behavior could not

be explained by

classical theory

Not enough UV light

was observed as

object gets hotter


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Planck

Planck was able to explain the blackbody

spectrum by making the radical assumption that

energy is quantized

His theory contained a new constant

h = 6.626 x 10-34 Js

Plancks Constant,


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Quantized Energy Levels

Energ

y

Continuous

Energy Levels

Quantized

Energy Levels

In a system with

quantized energy

levels some amounts

of energy are not

possible


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The Photoelectric Effect

Many metals emit electrons when a light shines on theirsurface


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Energy Threshold for the

Photoelectric Effect

The low energy red photon cannot provide

enough energy to allow the electron to escape


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Classical Theory and The

Photoelectric Effect

According to classical theory more electrons should be

ejected

1. if the wavelength of light is made shorter

2. the intensity is increased

If a dim light is used, there should be a lag time before

electrons were emitted to give the electrons time to

absorb enough energy


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The Photoelectric Effect

Observations

There is a minimum frequency required before

electrons are emitted (threshold frequency)

The intensity does not effect the threshold

frequency

No lag time

Energy of emitted electrons increases with the

frequency of light


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Explanation

Einstein proposed that the light energy was

delivered to the atoms in packets, called quanta or

photons

The energy of a photon of light was directlyproportional to its frequency

Ephoton = hh = 6.626 x 10

-34

Js Plancks Constant


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An Electrical Current Causes

Gaseous Atoms to Emit Light

Hg He H


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Emission Spectra of Atoms

expected

observed


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Emission Spectra


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Atomic Line Spectra

Oxygen spectrum

Neon spectrum


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Emission vs. Absorption

Spectra

Spectra of Mercury


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Quantum Theory

The quantum-mechanical model is first the theory

to correctly predict the behavior of electrons in

atoms

One of the surprising predictions of quantumtheory is that electron in atoms behave as if they

are waves.

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