Unit 3 Light, Electrons & The Periodic Table

3.1 Light & Electromagnetic Spectrum

Light is a Wave 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

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

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

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

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 108 m/s)  (lambda) = wavelength, in meters n (nu) = frequency, in units of hertz (Hz or s-1)

5-Step Problem Solving/SF’s/Dimensional Analysis Light Problem Solving 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 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

Atomic 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? 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? Equation: E = h 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 n (nu) = frequency, in units of hertz (Hz or s-1)

More Light Problem Solving What is the wavelength of blue light with a frequency of 8.3 x 1015 Hz? What is the frequency of red light with a wavelength of 4.2 x 10-5 m? What is the energy of a photon of each of the above? 5-Step Problem Solving/SF’s/Dimensional Analysis

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 1st EL (n=1)

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.

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

Wave-Particle Duality Light is a particle - it comes in “chunks” Light is a wave - we can measure its l 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 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!

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. Moving Electron Photon Electron velocity changes Photon wavelength changes Before After