Physics and the Quantum Mechanical Model

Section 5.3

Physics and the Quantum Mechanical Model

Neon advertising signs are formed from glass tubes bent in various shapes. An electric current passing through the gas in each glass tube makes the gas glow with its own characteristic color. You will learn why each gas glows with a specific color of light.

5.3

Electrons and Light

Electrons and light both exhibit particle and wave properties at the same time! The “particle” of light is known as a

photon

What is a wave? Electromagnetic radiation includes radio waves, microwaves,

infrared waves, visible light, ultraviolet waves, X-rays, and gamma rays.

All electromagnetic waves travel in a vacuum at a speed of 2.998 108 m/s.

Electromagnetic Radiation The wavelength and frequency of

light are inversely proportional to each other.

Terms associated with light The amplitude of a wave is the wave’s

height from zero to the crest. The wavelength, represented by (the

Greek letter lambda), is the distance between the crests.

The frequency, represented by (the Greek letter nu), is the number of wave cycles to pass a given point per unit of time. The SI unit of cycles per second is called a hertz

(Hz). 1 Hz = 1/second

Equation for relating wavelength and frequency

This equation means: Speed of light = wavelength in meters x

frequency in Hertz If you are given wavelength in nanometers you

will need to convert: 1 m = 1 x 109 nm

c = 2.998 x 108 m/s

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5.1

Separating Light A prism separates light into the colors it contains. When white light passes through a prism, it

produces a rainbow of colors. The rainbow is called a spectrum

Atomic Emission Spectra▪ Unlike white light, light from elements

produces only a few colored lines instead of a full rainbow: called atomic emission spectrum▪ http://jersey.uoregon.edu/vlab/elements/Eleme

nts.html

Separating light from elements Atomic emission spectrum- the discrete lines

formed when atoms absorb energy, forcing electrons into higher energy levels and then lose energy by emitting light as the electrons fall to lower energy levels

The light is made up of only a few specific frequencies, depending on the element Each frequency is a different color The light is emitted as electrons fall from one

energy level to another, like from n=4 to n=1 They are like atomic fingerprints- every

element is unique

Energy changes of electrons When the electron has its lowest possible

energy level , the atom is in its ground state.

Excitation of the electron by absorbing energy raises the atom from the ground state to an excited state. (not the orbital predicted by aufbau chart)

A quantum of energy in the form of light is emitted when the electron drops back to a lower energy level. This energy is directly proportional to

frequency, which determines the light’s color.

Relationship between frequency and energy of light Light emitted by an electron moving

from a higher to lower energy level has a frequency directly proportional to the energy change of the electron

Equation describing energy change of the electron E = h x ν (h = 6.626 x 10-34J*s) So different energy level drops result in

different frequencies (and colors) of light

Why does it matter that an electron behaves as both particle and wave?

The fact that electrons behave as waves leads to some odd observations, like:

Heisenberg’s uncertainty principle it is impossible to know exactly both the

velocity and the position of a particle at the same time.

This limitation is critical in dealing with small particles such as electrons.

This limitation does not matter for ordinary-sized object such as cars or airplanes.

The Heisenberg Uncertainty Principle

Electron microscope

When photons hit metals, they can result in the ejection of electrons- This is the photoelectric effect We use this property to create electron

microscopes, which allow for clearer images since electrons have smaller wavelengths than light

Other odd implications of quantum mechanics

▪ http://www.teachersdomain.org/resources/phy03/sci/phys/fund/quantum/index.html ▪ http://www.teachersdomain.org/resources/phy

03/sci/phys/matter/quantumcafe/index.html - Quantum Cafe