Chapter 5 Section 5.1 Electromagnetic Radiation Section 5.2 Atomic Spectra and Energy Levels.

Chapter 5Chapter 5

Section 5.1 Section 5.1

Electromagnetic RadiationElectromagnetic Radiation

Section 5.2 Atomic Spectra and Section 5.2 Atomic Spectra and Energy LevelsEnergy Levels

Problems with the Rutherford Problems with the Rutherford Nuclear Model:Nuclear Model:

Where are the electrons?Where are the electrons?Why don’t the electrons get pulled into the Why don’t the electrons get pulled into the

nucleus?nucleus?No explanation for spectral lines.No explanation for spectral lines.

*Light behaves as both a *Light behaves as both a wavewave and a and a particleparticle..

Sometimes light behaves like a Sometimes light behaves like a wave.wave.

Visible light is only a small part of the Visible light is only a small part of the electromagnetic spectrum.electromagnetic spectrum.

Electromagnetic Spectrum:Electromagnetic Spectrum:All the forms of energy that exhibits wavelike behavior as it All the forms of energy that exhibits wavelike behavior as it

travels through space.travels through space.Examples:Examples: Gamma rays (short wavelength, high frequency)Gamma rays (short wavelength, high frequency) X-raysX-rays Ultra violet Ultra violet Visible Visible (from short to long wavelength - Violet,I,B,G,Y,O,Red)(from short to long wavelength - Violet,I,B,G,Y,O,Red)

InfraredInfrared MicrowaveMicrowave TVTV Radio (long wavelength, low frequency)Radio (long wavelength, low frequency)Speed of all electromagnetic radiation (light): c = 3.0 x Speed of all electromagnetic radiation (light): c = 3.0 x

101088m/s)m/s)

Wavelength (Wavelength (λλ):):

The distance between corresponding parts The distance between corresponding parts of adjacent waves, e.g. crest to crest.of adjacent waves, e.g. crest to crest.

Usually in units of nmUsually in units of nm

Frequency (Frequency (νν):):

Number of waves that pass a given point in Number of waves that pass a given point in a given timea given time

Usually in waves/second or Hertz (Hz)Usually in waves/second or Hertz (Hz)

Speed of Light (c):Speed of Light (c):

Speed of light = wavelength x frequencySpeed of light = wavelength x frequency

c c = = λλvvWhat is the relationship between the wavelength What is the relationship between the wavelength

and frequency?and frequency?Inverse relationship! Inverse relationship!

- the shorter the wavelength the higher the - the shorter the wavelength the higher the frequency (as frequency (as λλ ↑ , ↑ , vv ↓) ↓)- the longer the wavelength the lower the - the longer the wavelength the lower the frequency (as frequency (as λλ ↓, ↓, v ↑v ↑))

What is the frequency of a light wave with a What is the frequency of a light wave with a wavelength of 106 nm?wavelength of 106 nm?

Solution:Solution:1. Convert nm to m:1. Convert nm to m:

106 nm x (1m/1,000,000,000nm) =106 nm x (1m/1,000,000,000nm) = 1.06 x 101.06 x 10-7-7mm

2. Solve the light equation for 2. Solve the light equation for vv..c c = = λλvvv v = c / = c / λλ

= 3.0 x 10= 3.0 x 1088 m/sm/s / 1.06 x 10 / 1.06 x 10-7-7mm

= 2.83 x 10= 2.83 x 101515 HzHz

Sometimes light behaves like a Sometimes light behaves like a particle.particle.

Photoelectric effectPhotoelectric effect: the emission of : the emission of electrons from a metal when light shines electrons from a metal when light shines on the metal.on the metal.

http://wps.prenhall.com/wps/media/objects/439/449969/Media_Portfolio/Chapter_05/Photoelectric_Effect.MOV

Max PlanckMax Planck

Light energy exists in specific small Light energy exists in specific small amounts called amounts called quantaquanta (quantum – (quantum – singular)singular)

quantumquantum: the minimum quantity of energy : the minimum quantity of energy that can be lost or gained by an atom.that can be lost or gained by an atom.

E = E = h vh vPlanck’s constant, Planck’s constant, hh = 6.626 x 10 = 6.626 x 10-34-34 JJ••ss

Determine the energy in joules of a photon Determine the energy in joules of a photon whose frequency is 3.55 x 10whose frequency is 3.55 x 1017 17 Hz.Hz.

Solution:Solution:

E = hvE = hv

E = (6.626 x 10E = (6.626 x 10-34-34 JJ••s) (s) (3.55 x 103.55 x 1017 17 Hz)Hz)

= 2.35 x 10= 2.35 x 10-16-16 J J

Einstein:Einstein:

Won the Nobel Prize for explanation of Won the Nobel Prize for explanation of photoelectric effect.photoelectric effect.

Einstein explained the photoelectric effect Einstein explained the photoelectric effect by proposing that electromagnetic by proposing that electromagnetic radiation is absorbed by matter radiation is absorbed by matter only in only in whole numbers of photonswhole numbers of photons..

Photon:Photon:

Massless particle of light with a quantum Massless particle of light with a quantum of energy.of energy.

The energy of the photon depends on the The energy of the photon depends on the frequency of radiation. Only certain frequency of radiation. Only certain frequencies are energetic enough to knock frequencies are energetic enough to knock electrons off the surface of metals.electrons off the surface of metals.

Flame Test:Flame Test:

Metal ions produce characteristic colors in Metal ions produce characteristic colors in a flame.a flame.

NaClNaCl SrClSrCl22 BaSOBaSO44

Line Emission Spectrum:Line Emission Spectrum:

Produced when the light given off by an Produced when the light given off by an element is separated into a set of colored element is separated into a set of colored lines that are a unique “finger print” of the lines that are a unique “finger print” of the element.element.

Bohr Model of the AtomBohr Model of the Atom

Niels Bohr applied quantum theory to the Niels Bohr applied quantum theory to the Rutherford model.Rutherford model.

1.1. Electrons exist only in definite Electrons exist only in definite orbitsorbits. . Each orbit has a definite fixed energy. Each orbit has a definite fixed energy. Labeled n = 1,2,3,…etc. Lowest energy Labeled n = 1,2,3,…etc. Lowest energy level is called the level is called the ground stateground state, closest to , closest to the nucleus.the nucleus.

2.2. As long as electrons stay in one energy As long as electrons stay in one energy level they don’t gain or lose energy.level they don’t gain or lose energy.

Bohr Cont.Bohr Cont.

3. Adding energy can put atoms in an 3. Adding energy can put atoms in an excited excited statestate, the electrons move to a higher energy , the electrons move to a higher energy level, farther from the nucleus.level, farther from the nucleus.

4. Electrons can “fall” back from higher (excited) to 4. Electrons can “fall” back from higher (excited) to lower energy levels. The difference in energy is lower energy levels. The difference in energy is released as electromagnetic radiation (light).released as electromagnetic radiation (light).

Explains the color of the lines in the hydrogen Explains the color of the lines in the hydrogen spectrum.spectrum.