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Unit 3 Light, Electrons & The Periodic Table

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3.1 Light & Electromagnetic Spectrum

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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 10 8 m/s; abbreviated “c” All electromagnetic radiation travels at this same rate when measured in a vacuum Light is a Wave

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The Electromagnetic Spectrum Visible light is a very small portion of the entire spectrum

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Waves 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

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Wavelength 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

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Wavelength 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 10 8 m/s) (lambda) = wavelength, in meters (nu) = frequency, in units of hertz (Hz or s -1 )

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Light Problem Solving 1.Calculate the wavelength of yellow light emitted by a sodium lamp if the frequency of radiation is 5.10 x 10 14 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

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White Light White light is made up of all the colors of the visible spectrum Passing it through a prism separates it If light is not white… By heating a gas with electricity, we can get it to give off colors Passing this light through a prism does something different

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Atomic Spectra These are called atomic emission spectra They are unique to each element, like a fingerprint Very useful for identifying elements

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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)

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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. m 2 /s 2 ) h (Planck’s constant) = 6.626x10 -34 J. s (nu) = frequency, in units of hertz (Hz or s -1 )

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More Light Problem Solving 1. What is the wavelength of blue light with a frequency of 8.3 x 10 15 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

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Explanation 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 1 st EL (n=1)

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Explanation 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.

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Lyman series Ultraviolet region Transitions from higher EL’s to lowest EL (n=1) Balmer series Visible region Transitions from higher EL’s to n=2 Paschen series IR region Transitions from higher EL’s to n=3 Explanation of Atomic Spectra Atomic Emission Spectrum of H

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Wave-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=mc 2, c=, E = ½ mv 2 and E = h, then we can get: = h/mv (from Louis de Broglie) called de Broglie’s equation Calculates the wavelength of a particle

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Heisenberg 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!

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BeforeAfter Moving Electron Photon Electron velocity changes Photon wavelength changes Heisenberg 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.

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