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McGill University
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Chemistry
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CHEM 110
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Ariel Fenster
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Chapter 8 Electrons in Atoms
8.1 Electromagnetic Radiation
Electromagnetic Radiation:
A form of energy transmission in which electric and magnetic fields are propagated as
waves through empty space (a vacuum)
One of the four forces Known in Nature
Mediators of electromagnetic force are photons :(massless particles of spin one)
1. Electromagnetic force chemistry infinite range
F= (k) (q1 2/ r ) (r) (r is a unit vector)
F is Faraday (the charge on a mole of electrons)
2. Gravity cosmology infinite range
2
F= (G) (m 1 2 r ) (r) (r is a unit vector)
3. Weak nuclear force nuclear physics within nucleus short range
4. Strong nuclear force nuclear physics - with nucleus short range
A wave is a disturbance that transmits energy through space or a material medium. Waves
of light are always in a SINUSODIAL MOTION.
The maximum height of the wave above the center line or the maximum depth
below is called the amplitude (A)
The distance between the tops of two successive crests is called the wave
lengths
Frequency is the number of crests or troughs that pass through a given point
per unit of time ()
Phases: shifting the sine curve to the left o right along the direction of
propagation ()
Nodes: when the wave hits zero along the sinusoidal motion (nodal line is in two
dimension)
Antinodes: when the waves hit its maximum along the sinusoidal motion
Electromagnetic Radiation: An oscillating charge produces EM radiation which displays a
frequency equal to the frequency of oscillation.
Sound and water waves travel through a medium EM radiation travels through a
vacuum at the speed of light and is mediated (transmitted) by photons
-1
The SI unit for frequency, s is the hertz (Hz), and the basic SI wavelength unit is the meter (m).
The angstrom is not an SI unit.
Speed of light is represented by the symbol c (2.998 x10 ms ), and the relationship
between speed and frequency and wavelengths is:
c= v x monochromatic light: unique frequency/wavelength
v = frequency and = nanometers (convert to m)
The wavelength of electromagnetic radiation is shorter for high frequencies and
longer for low frequencies. Chapter 8 Electrons in Atoms
Interference in two overlapping light waves:
In constructive interference, the troughs and crests are in step (in phase), leading to
addition of the two waves
In destructive interference, the troughs and crests are out of step (out of phase), leading
to cancellation of two waves
The dispersion of different wavelength components of a light beam through the interference
produced by reflection from a grooved surface is called a diffraction (ex. A CD)
Fourier series: any wave shape can be created by a superposition of many waves
Superposition of waves is essential to quantum mechanics/wave mechanics
Refraction of light:
Light is refracted (bent) as it passes from air into the glass prism, and again as it emerges
from the prism into air.
INSERT EM SPECTRUM HERE
Shorter the wavelength = higher the frequency
Visible light = 390 nm to 790 nm
All waves have the same speed in a vacuum
8.2 Atomic Spectra
Continuous spectrum: when the light being diffracted consists of many wavelength
components
Discontinuous spectrum: if the source of a spectrum produces light having only a
relatively small number of wavelength components
These discontinuous spectra are called atomic, or line, spectra
Balmer equation: formula for wavelengths of spectral lines
v = 3.2881 x10 s (1/4 1/n )2
o v = frequency of the spectra line
o n = an integer greater than 2
Line Spectra: emission of electrons in quantum jumps
Rydnerg Formula:
H is Plancks constant
n1and n 2re integers from 1 to infinite
RH= 3.2881 x 10 s5 -1
E= hv
v= Rh (1/n = 11n ) 22Chapter 8 Electrons in Atoms
8.3 Quantum Theory
Intensity does not increase indefinitely: energy, like matter, is discontinuous.
Classical physics places no limitations on the amount of energy a system may
possess, whereas quantum theory limits this energy to discrete set of specific
values
The difference between any two allowed energies of a system also has a specific
value called a quantum of energy (when energy increases, it increases by a tiny
jump or quantum)
Plancks assumption: = nhv
is energy, n is a positive integer, v is the oscillator frequency and h is Plancks
constant (h = 6.62607 x 10 -3J s)
Plancks equation: E = hv (J/photon)
Higher the frequency, the greater the energy
Used to develop relationships among frequencies, wave lengths and energy
Blackbody Radiation: first experiment that classical mechanics could not explain; as a
blackbody is heated, the atoms start to oscillate faster and faster, and as they do, the
frequency of emitted light increases without limit
8.4 Photoelectric Effect
Discovered by Heinrick Hertz in 1988, explained by Einstein in 1905; when light
strikes the surface of certain metals, electrons are ejected.
1. Light intensity depends on the number of ph

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