Atomic Structure and Periodicity

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A sample of CuCl emitting light at 450 nm can only lose energy in increments of 4.41 x 10-19J, the size of the quantum in this case.

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Energy and Mass

According to Einstein theory of relativity-

Energy has mass; Einstein equation,

E = mc2 where, E = energy, m = mass

c = speed of light

After rearrangement of the equation,

Now we can calculate the mass associated

with a given quantity of energy

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Einstein suggested that electromagnetic radiation can be viewed as a stream of “particles” called photons. The energy of each photon is given by,

It was Einstein who realized that light could not be explained completely as waves but had to have particle properties. This is called the dual nature of light.

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Electromagnetic Radiation

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Wavelength and Mass

de Broglie thought if waves like light could have particle properties that particles like electrons could have wave properties. We have,

de Broglie’s equation,

 = wavelength (m); m = mass (kg);  = velocity (m/s)

h = Planck’s constant, 6.626  1034 J s = kg m2 s1

This equation allows us to calculate the wavelength of a particle. Matter exhibits both particulate and wave properties.

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Example: Compare the wavelength for an electron (mass = 9.11 x 10-31 kg) traveling at a speed of 1.0 x 107 m/s with that for a ball (mass = 0.10 kg) traveling at 35 m/s.

We use the equation  = h/m, where

h = 6.626  1034 J.s or 6.626  1034 kg m2 /s

since, 1 J = 1 kg. m2 /s2

For the electron,

For the ball,

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Atomic Spectrum of Hydrogen

When H2 molecules absorb energy, some of the H-H bonds are broken and resulting hydrogen atoms are excited. The excess energy is released by emitting light of various wavelengths to produce the emission spectrum of hydrogen atom.

Continuous spectrum: Contains all the wavelengths of light.

Line (discrete) spectrum: Contains only some of the wavelengths of light. Only certain energies are allowed, i.e., the energy of the electron in the hydrogen atom is quantized.

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A Continuous Spectrum (a) and A Hydrogen Line Spectrum (b)

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