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Unit 3
Light, Electrons &
The Periodic Table
3.1 Light & Electromagnetic Spectrum
• The study of light led to the development of the quantum mechanical model
• Light is a type of electromagnetic radiation (energy)
• Electromagnetic radiation includes many types: gamma rays, x-rays, ultraviolet light, visible light, infrared, microwaves, and radio waves
• Speed of light = 2.998 x 108 m/s; abbreviated “c”• All electromagnetic radiation travels at this same
rate when measured in a vacuum
Light is a WaveLight is a Wave
The Electromagnetic SpectrumThe Electromagnetic Spectrum
• Visible light is a very small portion of the entire spectrum
WavesWaves
• Electromagnetic radiation travels in waves (analogous to water waves)– Wavelength = distance from peak to peak– Amplitude = height of the peak (distance
from axis to crest or trough)– Frequency = the number of wave peaks that
pass in a given time; usually measured per second (1/s or s-1 or Hertz (Hz))
– Speed = rate the waves travel
Wavelength and FrequencyWavelength and Frequency
• Are inversely related• As one goes up the other goes down
• Different frequencies of visible light are different colors (ROYGBIV mnemonic)
• There is a wide variety of frequencies• The whole range is called a spectrum
Wavelength and FrequencyWavelength and Frequency
• Electromagnetic radiation travels through space as a wave moving at the speed of light
Equation: c =
c = speed of light, a constant (2.998 x 108 m/s)
(lambda) = wavelength, in meters
(nu) = frequency, in units of hertz (Hz or s-1)
Light Problem SolvingLight Problem Solving
1. Calculate the wavelength of yellow light emitted by a sodium lamp if the frequency of radiation is 5.10 x 1014 Hz.
2. How far does starlight travel in 10. minutes?
3. During a flame test, strontium emits a strong band at 6.63 x 10-7 m. What frequency is this light? What is its color?
5-Step Problem Solving/SF’s/Dimensional Analysis
White LightWhite Light
Atomic SpectraAtomic Spectra
•These are called atomic emission spectra
•They are unique to each element, like a fingerprint
•Very useful for identifying elements
Light is a particle?Light is a particle?
• Energy is quantized.• Light is a form of energy.• Therefore, light must be quantized.• These smallest pieces of light are called
photons- particles of electromagnetic energy• Photoelectric Effect-Albert Einstein (see article)
Light is a particle?Light is a particle?
• The energy of electromagnetic radiation is directly proportional to the frequency of the radiation.
Equation: E = h
E = Energy, units of Joules (kg.m2/s2)
h (Planck’s constant) = 6.626x10-34J.s
(nu) = frequency, in units of hertz (Hz or s-1)
More Light Problem SolvingMore Light Problem Solving
1. What is the wavelength of blue light with a frequency of 8.3 x 1015 Hz?
2. What is the frequency of red light with a wavelength of 4.2 x 10-5 m?
3. What is the energy of a photon of each of the above?
5-Step Problem Solving/SF’s/Dimensional Analysis
Explanation of Atomic SpectraExplanation of Atomic Spectra
• When we write electron configurations, we are writing the lowest energy configuration.
• The energy level, and where the electron starts from, is called its ground state - the lowest energy level.
• Let’s look at hydrogen, with only 1 electron in its 1st EL (n=1)
Explanation of Atomic SpectraExplanation of Atomic Spectra
Changing the Energy…•By absorbing energy (heat, electricity, or light), the electron can move from the ground state (n=1) to a higher EL (n=2, 3, 4, 5…)•The electron is now said to be in an “excited state”•A quantum of energy is emitted when the electron drops back to a lower energy level• The further they fall, the more energy is
released and the higher the frequency (E = h)•Each transition produces a line of a specific frequency in the spectrum.
Lyman series• Ultraviolet region• Transitions from
higher EL’s to lowest EL (n=1)
Balmer seriesVisible regionTransitions from
higher EL’s to n=2
Paschen seriesIR regionTransitions from
higher EL’s to n=3
Explanation of Atomic SpectraExplanation of Atomic Spectra
Atomic Emission Spectrum of H
Wave-Particle DualityWave-Particle Duality
• Light is a particle - it comes in “chunks”• Light is a wave - we can measure its and it
behaves as a wave
• If we combine E=mc2 , c=, E = ½ mv2 and E = h, then we can get:
= h/mv (from Louis de Broglie)
called de Broglie’s equation • Calculates the wavelength of a particle
Heisenberg Uncertainty PrincipleHeisenberg Uncertainty Principle
It is impossible to know exactly the location and velocity of a particle.•The better we know one, the less we know the other; measuring changes the properties
You can find out where the electron is, but not where it is going.
-OR-You can find out where the electron is going, but not where it is!
Before After
Moving Electron
Photon
Electron velocity changes
Photon wavelengthchanges
Heisenberg Uncertainty PrincipleHeisenberg Uncertainty Principle
• To measure where a electron is, we use light, but the light energy moves the electron
• And hitting the electron changes the frequency of the light.