Chapter 5Periodicity and Atomic Structure

LIGHT AND THE ELECTROMAGNETIC SPECTRUM

Electromagnetic energy (“light”) is characterized by wavelength, frequency, and amplitude.

LIGHT AND THE ELECTROMAGNETIC SPECTRUM

Chapter 5/3

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LIGHT AND THE ELECTROMAGNETIC SPECTRUM

Wavelength x Frequency = Speed

=

m

s

m

s

1

cx

c is defined to be the rate of travel of all electromagnetic energy in a vacuum and is a constant value—speed of light.

c = 3.00 x 108

s

m

EXAMPLESThe light blue glow given off by mercury

streetlamps has a frequency of 6.88 x 1014 s-1 (or, Hz). What is the wavelength in nanometers?

THE ENERGY LEVEL OF HYDROGEN

1.– Particlelike Properties of Electromagnetic Radiation: The Planck Equation

PARTICLELIKE PROPERTIES OF ELECTROMAGNETIC ENERGY

Photoelectric Effect: Irradiation of clean metal surface with light causes electrons to be ejected from the metal. Furthermore, the frequency of the light used for the irradiation must be above some threshold value, which is different for every metal.

EXAMPLES

Solar energy, which is produced by photovoltaic cells.These are made of semi-conducting material which produce electricity when exposed to sunlight

it works on the basic principle of light striking the cathode which causes the emmision of electrons, which in turn produces a current.

PARTICLELIKE PROPERTIES OF ELECTROMAGNETIC ENERGY

PARTICLELIKE PROPERTIES OF ELECTROMAGNETIC ENERGY

Ephoton = hνE

Electromagnetic energy (light) is quantized.

h (Planck’s constant) = 6.626 x 10-34 J s

Einstein explained the effect by assuming that a beam of light behaves as if it were a stream of particles called photons.

* 1mol of anything = 6.02 x 1023

EMISSION OF ENERGY BY ATOM

How does atom emit light? Atoms absorbs energy Atoms become excited Release energy Higher-energy photon –>shorter wavelength Lower-energy photon -> longer wavelength

EXAMPLES

What is the energy (in kJ/mol) of photons of radar waves with ν = 3.35 x 108 Hz?

Calculate the wavelength of light that has energy 1.32 x 10-23 J/photon

Calculate the energy per photon of light with wavelength 650 nm

PARTICLELIKE PROPERTIES OF ELECTROMAGNETIC ENERGY

Niels Bohr proposed in 1914 a model of the hydrogen atom as a nucleus with an electron circling around it.

In this model, the energy levels of the orbits are quantized so that only certain specific orbits corresponding to certain specific energies for the electron are available.

NIELS BOHR MODEL

In each case the wavelength of the emitted or absorbed light is exactly such that the photon carries the energy difference between the two orbits

Excitation by absorption of light and de-excitation by emission of light

WAVELIKE PROPERTIES OF MATTER

The de Broglie equation allows the calculation of a “wavelength” of an electron or of any particle or object of mass m and velocity v.

mvh =

Louis de Broglie in 1924 suggested that, if light can behave in some respects like matter, then perhaps matter can behave in some respects like light.

In other words, perhaps matter is wavelike as well as particlelike.

EXAMPLES

Calculate the de Broglie wavelength of the “particle” in the following case A 25.0 bullet traveling at 612 m/s

What velocity would an electron (mass = 9.11 x 10-31kg) need for its de Broglie wavelength to be that of red light (750 nm)?