Light and the

Quantum Mechanical Model of the Atom

Light

• The study of light led to the development of the quantum mechanical model.

• Light is a kind of electromagnetic radiation.

• Electromagnetic radiation includes many kinds of waves

• All move at 3.00x108 m/s or 3.00x1010 cm/s (abbreviated: c)

Parts of a wave

Wavelength

AmplitudeOrigin

Crest

Trough

Parts of Wave

• Origin - the base line of the energy.• Crest - high point on a wave• Trough - Low point on a wave• Amplitude - distance from origin to

crest• Wavelength - distance from crest to

crest

• Wavelength - is abbreviated Greek letter lambda)

Frequency• The number of waves that pass a

given point per second.• Units are cycles/sec or hertz (Hz)• Abbreviated the Greek letter

nu)

c =

Frequency and Wavelength

• Are inversely related• As one goes up the other goes down.

• Different frequencies of light are different colors of light.

• There is a wide variety of frequencies

• The whole range is called a spectrum

Radiowaves

Microwaves

Infrared

Ultra-violet

X-Rays

GammaRays

Low energy

High energy

Low Frequency

High Frequency

Long Wavelength

Short WavelengthVisible Light

Electromagnetic Spectrum

Energy and Frequency

• E = (h)(• E is the energy of the photon• is the frequency• h is Planck’s constant • h = 6.626 x 10 -34 Joules*sec.

The Math You WILL need to Know for this Chapter

Only 2 equations:• c = • E = h

• c is always 3.00 x 108 m/s

• h is always 6.626 x 10-34 J•s

Examples

• What is the frequency of red light with a wavelength of 4.2 x 10-5 cm?

• What is the wavelength of The River 105.9, which broadcasts at a frequency of 640 kHz?

• What is the energy of a photon of each of the above?

Atomic Spectra How light & color tells us about

atoms

• It is made up of all the colors of the visible spectrum.

• Passing it through a prism separates it.

• This is called a continuous spectrum • because you see all the

colors of the visible spectrum blurred together (ROYGBIV)

White Light

If the light is not white• By heating or electrifying a

sample of atoms scientists can get it to give off colors.

• Passing this light through a prism does something different

• You get colors of light that relate to specific frequencies and therefore specific energies

• This is called …

Atomic Emission Spectrum or Line Spectrum

• Each element gives off a unique set of colors.

• This info can be used to help identify atoms in substances• Ex: stars

http://jersey.uoregon.edu/elements/Elements.html

An explanation of Atomic Spectra

How it all begins…• In the atom all

electrons start in the lowest energy level they are normally found in.

• This is called the ground state.

Hydrogen atom

• Let’s take a closer look at a hydrogen atom

Changing the energy• Adding energy can move the electron up

to higher energy levels

• The more energy added, the more energy levels the e- moves

• As the electron falls back to its original ground state it gives the absorbed energy back as light we can see

• The further the e- fall, the more energy, and the higher the frequency of light.

Max Planck

• 1900 – from Germany

• Tried to explain why metals changed color the way they did when they are heated

• They only show certain colors, not all the colors in the visible spectrum. WHY?

• The energy is changed in a specific amount called a quantum

Light as a Particle

• Energy is quantized.

• These smallest pieces of light are called photons.

So…

What is light then??

• Light is a wave we can measure its wave length and determine its frequency and it behaves as a wave

• Light is also a particle it comes in chunks called photons• Albert Einstein came up with this idea

Photoelectric Effect

• Metals eject electrons when light of a specific frequency (called a threshold frequency) shines on them.

• The light has to have enough energy and if the light is the wrong frequency then it won’t work at all (no matter how bright the light)• Uses of this: solar cells, camera flash, &

automatic doors• FYI: Explaining this effect is how Einstein won his

Nobel Prize

More obvious with the very small

• To measure where a electron is, we use light.

• But the light moves the electron• And hitting the electron changes the

frequency of the light.

Matter is also a Wave• BUT…• This does not apply to objects bigger

than an atom b/c their wavelengths are too small• Ex: A baseball has a wavelength of

about 10-32 m when moving 30 m/s

• An electron at the same speed has a wavelength of 10-3 cm

• This length is big enough to measure.

• We called these “matter waves”

The physics of the very small

• Quantum mechanics explains how the very small behaves.

• Quantum mechanics is based on probability because

Heisenberg Uncertainty Principle

• It is impossible to know exactly the speed and position of a particle.

• The better we know one, the less we know the other.

• The act of measuring changes the properties.

Moving Electron

Photon

Before

Electronchanges velocity

Photon changes wavelength

After