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 Crest Wavelength Amplitude Origin 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 l (Greek letter lambda)

Frequency The number of waves that pass a given point per second. Units are cycles/sec or hertz (Hz) Abbreviated n (the Greek letter nu) c = ln

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

Electromagnetic Spectrum Low energy High energy Radiowaves Microwaves Infrared Ultra-violet X-Rays GammaRays Low Frequency High Frequency Long Wavelength Short Wavelength Visible Light

Energy and Frequency E = (h)(n) E is the energy of the photon n 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 = ln E = hn 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

White Light 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)

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 also a particle  it comes in chunks called photons 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.

After Before Photon changes wavelength Photon Electron changes velocity Moving Electron