1. Unit 3
Light, Electrons
&
The Periodic Table

2. 3.1 Light & Electromagnetic Spectrum

3. 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 = x 108 m/s; abbreviated “c”
All electromagnetic radiation travels at this same rate when measured in a vacuum

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

6. 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

7. 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

8. 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)

9. 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

10. 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

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

12. 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)

13. 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)

14. 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

15. 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)

16. 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.

17. 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

18. 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

19. 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!

20. 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