12.6 light and atomic spectra the quantum mechanical model of the atom grew out of the study of...

12.6 Light and Atomic 12.6 Light and Atomic SpectraSpectra

The quantum mechanical model of the The quantum mechanical model of the atom grew out of the study of light.atom grew out of the study of light. Isaac Newton (1642-1727) thought of Isaac Newton (1642-1727) thought of

light as consisting of particles.light as consisting of particles. By the 1900’s scientists thought of light By the 1900’s scientists thought of light

as a wave.as a wave. We now know that light has We now know that light has

properties of both waves and properties of both waves and particles.particles.


In the wave model, light is In the wave model, light is considered to consist of considered to consist of electromagnetic waves that travel at electromagnetic waves that travel at a speed of 3.0 x 10a speed of 3.0 x 1088 m/s (186,000 m/s (186,000 mi/s).mi/s).

electromagnet radiationelectromagnet radiation – includes – includes radio waves, microwaves, visible radio waves, microwaves, visible light, infrared and ultraviolet light, light, infrared and ultraviolet light, X-rays, and gamma rays.X-rays, and gamma rays.


amplitudeamplitude – the height of the wave from the – the height of the wave from the origin to the crest.origin to the crest.

wavelengthwavelength((λλ, lamda) – the distance , lamda) – the distance between the crestsbetween the crests

frequencyfrequency((νν, nu) – the number of wave cycles , nu) – the number of wave cycles to pass a given point per unit of timeto pass a given point per unit of time


The speed of light (c) in a vacuum is constant The speed of light (c) in a vacuum is constant and is equal to frequency (and is equal to frequency (νν) multiplied by ) multiplied by wavelength (wavelength (λλ). Equation: c = ). Equation: c = ν λν λ

This equation can be rewritten as follows:This equation can be rewritten as follows:

νν = or = or λλ = =

Note: Note:

speed of light = c (must be in speed of light = c (must be in m/sm/s))

frequency = frequency = νν (must be in reciprocal seconds ( (must be in reciprocal seconds (ss-1-1) or ) or hertz, hertz, HzHz))

wavelength = wavelength = λλ (must be in (must be in mm))

ccλλ

ccνν


Wavelength and frequency have an Wavelength and frequency have an inverse relationship.inverse relationship. This means when the value of one This means when the value of one

increases the value of the other one increases the value of the other one decreases.decreases. As wavelength increases, frequency decreasesAs wavelength increases, frequency decreases As wavelength decreases, frequency increasesAs wavelength decreases, frequency increases


Light refers to a small part of the Light refers to a small part of the electromagnetic spectrum that our eyes can electromagnetic spectrum that our eyes can seesee Sunlight consists of light with a continuous range Sunlight consists of light with a continuous range

of wavelengths and frequencies of wavelengths and frequencies When sunlight passes through a prism, the light When sunlight passes through a prism, the light

separates into a spectrum of colorsseparates into a spectrum of colors


Each color in the spectrum Each color in the spectrum (ROYGBIV) blends into the next.(ROYGBIV) blends into the next. In the visible spectrum, In the visible spectrum, red light has red light has

the longest wavelength and lowest the longest wavelength and lowest frequency; violet light has the frequency; violet light has the shortest wave length and highest shortest wave length and highest frequency.frequency.


Elements emit light when energy is Elements emit light when energy is passed through them.passed through them. Electrons absorb energy and are excited to Electrons absorb energy and are excited to

higher energy levels; they then release the higher energy levels; they then release the energy in the form of light when they energy in the form of light when they return to their ground state.return to their ground state.

Passing the light emitted by an element Passing the light emitted by an element through a prism gives the through a prism gives the atomic atomic emission spectrumemission spectrum of the element. of the element. The The atomic emission spectrumatomic emission spectrum are are

unique to each element and are a useful unique to each element and are a useful tool for identifying elements.tool for identifying elements.

12.6 Example12.6 Example

Calculate the wavelength of the Calculate the wavelength of the yellow light emitted by a sodium lamp yellow light emitted by a sodium lamp if the frequency of the radiation is if the frequency of the radiation is 5.10 x 105.10 x 10-14-14ss-1-1..

Known: c = 3.0 x 10Known: c = 3.0 x 1088 m/s m/s

νν = 5.10 x 10 = 5.10 x 10-14-14

Solution - Use the equation: Solution - Use the equation: λλ = =

λλ = = = 5.88 = = = 5.88 x 10x 10-7-7 m m

ccνν

3.00 x 103.00 x 108 8 m/sm/s5.10 x 105.10 x 1014 14 ss-1-1

3.00 x 103.00 x 108 8 msms-1-1

5.10 x 105.10 x 1014 14 ss-1-1

12.6 Concept Practice12.6 Concept Practice

11. List the colors of the visible 11. List the colors of the visible spectrum in order of increasing spectrum in order of increasing wavelength.wavelength.

12. What is meant by the term 12. What is meant by the term frequency of a wave? What are the frequency of a wave? What are the units of frequency? Describe the units of frequency? Describe the relationship between frequency and relationship between frequency and wavelength.wavelength.

12.6 Practice12.6 Practice

13. What is the frequency of radiation 13. What is the frequency of radiation whose wavelength is 5.00 x 10whose wavelength is 5.00 x 10-6-6cm? cm? In what region of the In what region of the electromagnetic spectrum is this electromagnetic spectrum is this radiation?radiation?

12.6 Practice12.6 Practice

14. A hydrogen lamp emits several 14. A hydrogen lamp emits several lines in the visible region of the lines in the visible region of the spectrum. One of these lines has a spectrum. One of these lines has a wavelength of 6.56 x10wavelength of 6.56 x10-5 -5 cm. What cm. What are the color and frequency of this are the color and frequency of this radiation?radiation?

12.6 light and atomic spectra the quantum mechanical model of the atom grew out of the study of...

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