Atomic Models

1
Atomic Models

• Recall that atoms are really tiny
• With a golf ball size nucleus, the atom is 2.5
  miles in diameter, so it is mostly empty space
• There are some good pictures and discussion at
  the beginning of Chapter 15 about sizes and how
  to see atoms

2
Spectroscopy

• We want to examine evidence of atoms behavior in
  order to build a conceptual model of the
  structure of the atom
• If we put atoms in a large electric field (like
  an electric arc) or feed a flame, we can cause
  rearrangements of the electrons in the atoms
• As the atoms return to their resting states they
  emit light

3
Spectroscopy

• We can look at the light from atoms placed in an
  electric arc with a spectroscope
• Recall how a prism or diffraction grating can
  separate out the different colors of light

4
Spectroscopy
Images of the slit
Arc
Red image
Blue image
Slit
5
Spectroscopy
6
Spectroscopy

• So, we can ask the question, Why do we get the
  particular colors from any element?
• Why are the colors different for each element?
• For a given element, what does the pattern say
  about the underlying structure of that atom?

7
Spectroscopy

• Start with what we think is the simplest atom,
  namely Hydrogen
• The spectrum is a series of lines getting closer
  together and converging on a value

8
Spectroscopy

• It is possible to relate the frequencies of the
  light represented by these colored lines in a
  regular mathematical sequence

9
Spectroscopy

• Another series was discovered in the ultraviolet
  where the two became a 1
• Still others were discovered with a 3, then a 4,
  etc. (in the infrared)

10
Atomic Models

• Clearly, something interesting is going on
• We should be able to figure out a mechanism to
  explain this regularity
• Looks similar to the overtones we get when we
  pluck a guitar string, but with a different
  mathematical formula
• For the guitar f f0n where n 1, 2, 3,

11
Atomic Models

• What else might help us figure this out?
• Einsten explained the photoelectric effect by
  saying that light must also be considered as a
  particle (a photon)
• Max Planck explained the spectrum obtained upon
  heating an object and watching the color change
  as the object got hotter
• Deep red to red to orange to yellow to white
• His explanation involved light as particles

12
Atomic Models

• People also noted that some things in nature
  seemed fixed in size to particular values
• The charge on an electron was of fixed size
• All charged objects have a charge that is an
  integer multiple of the charge on the electron
• Things seem to be quantized

13
Quantum Hypothesis

• So, electric charge is quantized
• Light is quantized
• Energy h x frequency
• Since the light coming from atoms observed in a
  spectroscope has particular frequencies then the
  light has particular energies!!

14
Quantum Hypothesis

• If light is absorbed by an atom, it occurs at the
  same frequencies as the light emitted by the atom
  when it is placed in a flame or an electric
  arc!!!
• If an atom absorbs the energy of a photon, where
  does it go and what happens?

15
Quantum Hypothesis

• An electron some distance from a nucleus has a
  particular value of potential energy
• Based on distance from the nucleus and the size
  of the electric field
• If you give the electron more energy, it gain
  potential energy and will move farther away from
  the nucleus!!!
• The opposite is also true. When an electron
  gives up energy to move closer, it will be
  emitted as a photon of light

16
Quantum Hypothesis
17
Quantum Hypothesis

• Bohr said that if the light energy coming from
  the atom is quantized to particular values, the
  energy of the electrons in the atom must be
  quantized to particular values as well!!!
• His hypothesis is called the planetary model

18
Planetary Model
19
Planetary Model

• The electron is not allowed a leisurely journey
  from one orbit to another
• It jumps instantaneously
• Also explains why some frequencies of light
  emitted by hydrogen are the sum of two other
  frequencies that are observed

20
Quantum Hypothesis
21
Quantum Hypothesis

• Bohrs model of hydrogen explained the hydrogen
  spectrum
• However, if you tried to apply it to helium, the
  next simplest atom, it didnt work very well
• It also didnt provide a real basis for
  understanding WHY the electron energies were
  quantized
• Next time we will improve on the model by
  supposing the electron has WAVE properties