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.