Light and AtomsPhysics 113 Goderya

Chapter(s): 7Learning Outcomes:

The Atom

• Smallest part of matter• Nucleus, Protons,

Neutrons and Electrons• The elementary particle

Quark

• Symbol - Z X A

• Z = Atomic NumberNumber of protons

• A = Mass Number

Electron (-)

Nucleus

Protons (+) NeutronsQuarks

Hydrogen Atom

• One proton in the nucleus

• one electron orbiting the nucleus

• Symbol - 1 H1

Electron (-)

Nucleus

Proton (+)

1H1

Helium Atom

• Two proton and two neutrons in the nucleus

• Two electrons orbiting the nucleus

• Symbol - 2 He 4

2He4

The Chemical Elements

• Each chemical element has a specific atomic configuration.

• The periodic table will show this for any known chemical element.

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The Bohr Atom Model

• The Energy of the electron orbits are quantized

• That is each orbit carries a discrete energy value

• The closer the orbit to the nucleus, the more negative is its energy value

-E1

-E2

-E3

-E4

n1

n4

n3

n2

-E1 < -E2 < -E3 < -E4

The Bohr Atom Model

• Electrons can jump from one energy orbit to the other and absorb or emit energy

• The energy of each orbit is calculated by En = -13.6 / n2 (Z2) eV

• n is orbit number and Z is the atomic number

• eV = electron Volts

• 1 eV = 1.602 x 10 -19 J

-E l

-E h

n l

n h

The Bohr Atom Model

• When an electron jumps from a higher orbit to a lower orbit the change in energy is ∆E = E h - E l

• The wavelength of emitted light is 1/= Z 2 R (1/nl

2 - 1/nh2)

• R is Rydberg constant = 1.097 x 10-2 nm-1-E l

-E h

n l

n h

Spectral line

The Bohr Atom Model

• Electrons can only jump to certain defined levels when it gains or loses energy. Because n h and n l are integers

-E l

-E h

n l

n h

∆E = E h - E l

The Bohr Atom Model

• Emission: (colored spectral lines) Transition from nh to nl level. ∆E = E h - E l

• Absorption: (dark spectral line) Transition from nh to nl level. -∆E = E h - E l -E l

-E h

n l

n h

Spectral line

F4-8 The Atomic Spectrum

• The emission of light from an atom can be mapped in a diagram called the spectrum

300nm 700nm500nm

Orbit

6->2

Orbit

5->2

Orbit

4->2

Orbit

3->2

Hydrogen

Helium

Lines of HydrogenMost prominent lines in many astronomical objects: Balmer lines of hydrogen

The Balmer Linesn = 1

n = 2

n = 4

n = 5n = 3

H H H

The only hydrogen lines in the visible wavelength range.

Transitions from 2nd to higher levels of hydrogen

2nd to 3rd level = H (Balmer alpha line)2nd to 4th level = H (Balmer beta line)

…

F4-9 Finger print of an Atom

• Every atom and molecule has its own distinct spectrum

• Spectra differ in number of lines and the spacing between the lines