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Waves & ParticlesCh. 4 - Electrons in Atoms
Properties of Light
• Different types of electromagnetic radiation (x-rays, radio waves, microwaves, etc…) SEEM to be very different from one another. Yet they share certain fundamental characteristics.
• 3.0 x 108 m/sec is the speed of light!
A. Wavelength
• Wavelength – distance between identical points on successive waves
• Usually measured in nanometers
B. Frequency
• Frequency – the number of complete wave cycles that pass a given point in one second: the unit is cycles/second but is written as sec-1, or Hertz.
• One wave in one second equals one Hertz!
Amplitude
• Amplitude is the height of a wave. It is also measured in length units like nanometers, angstroms, etc.
A. Waves
A
greater amplitude
(intensity)
greater frequency
(color)
crest
origin
trough
A
EM Spectrum
• Frequency & wavelength are inversely proportional
c = f c: speed of light (3.00 108 m/s): wavelength (m, nm, etc.)f: frequency (Hz)
EM Spectrum
c = f
•Longer Wavelength means Lower Frequency
•Shorter Wavelength means Higher Frequency
•Higher Frequency means Higher Energy
•Lower Frequency means Lower Energy
Range
• The range of visible light is from 400 to 700 nanometers. Low energy is colored red while high energy is colored violet.
• Violet: 400 - 420 nm • Indigo: 420 - 440 nm • Blue: 440 - 490 nm • Green: 490 - 570 nm • Yellow: 570 - 585 nm • Orange: 585 - 620 nm • Red: 620 - 780 nm
•Ephoton = hf
E : engery of a photon (J)
h: Plank’s constant (6.63 x 10-34 J . sec)
f: frequency (Hz or sec-1)
EM Spectrum
LOW
ENERGY
HIGH
ENERGY
EM Spectrum
LOW
ENERGY
HIGH
ENERGY
R O Y G. B I V
red orange yellow green blue indigo violet
Examples
• 2. What is the wavelength of radiation whose frequency is 6.24 x l013 sec-1?
Given: Work:
f = 6.24 x l013 Hz
c = 3.00 108 m/s
λ = ?
λ = c/f
λ = 3.00 108
6.24 x l013
Answer
• 4.81 x 10-6 m
Examples• 3. What is the frequency of radiation whose
wavelength is 2.20 x l0-6 nm? (1 m = 1,000,000,000 nm)
Given: Work:
c = 3.00 108 m/s
λ = 2.20 x 10-6 nm(you have to change nm to m)
f = ?
f = c/ λ
f = 3.00 108
2.20 x 10-15
Answer
• 1.36 x 1023 Hz
Hydrogen Atom Line Emission Spectra
• When investigators passed electric current through a vacuum tube containing hydrogen gas at low pressure, they observed the emission of a characteristic pinkish glow. When a narrow beam of the emitted light was shined through a prism, it was separated into a series of specific frequencies (and therefore specific wavelengths, c =) of visible light. The bands of light were part of what is known as hydrogen’s LINE-EMISSION SPECTRUM. (page 95)
• The lowest energy state of an atom is its ground state.
• A state in which an atom has a higher amount of energy is an excited state. When an excited atom returns to its ground state, it gives off energy.
B. Bohr Model
• e- exist only in orbits with specific amounts of energy called energy levels
• Therefore…
– e- can only gain or lose certain amounts of energy
– only certain photons are produced
B. Bohr Model
1
23
456 • Energy of photon depends on the difference in energy levels
• Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom
C. Other Elements
• Each element has a unique bright-line emission spectrum.
– “Atomic Fingerprint”
Helium
Bohr’s calculations only worked for hydrogen!
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