Chapter 13 Notes

1
Chapter 13 Notes

• Electron Models

2
Evolution of Electron Models

• The first model of the electron was given by J.J.
  Thompsonthe electrons discoverer. His was the
  plum pudding model.

3
The Rutherford Model

• With Rutherfords discovery of the nucleus of an
  atom, the atomic model changed.

4
The Bohr Model

• Niels Bohr introduced his model, which answered
  why electrons do not fall into the nucleus.
• He introduced the concept of energy levels, where
  the electrons orbited similar to the way the
  planets orbit the sun.

5
Bohr Model and Energy Levels

• In the Bohr model, electrons are in energy
  levels, or regions where they most probably are
  orbiting around the nucleus.
• The analogy is that energy levels are like
  the rungs of a ladderyou
  cannot be between
  rungs, just like an electron
  cannot be between energy levels.
• A quantum of energy is the amount of
  energy it takes to move from
  one energy level to the
  next.

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Bohr Model and Energy Levels

• The Bohr model worked well for explaining the
  behavior of electrons in hydrogen, but for all
  other elements, the equations he used to predict
  the electron location did not work.

7
Quantum Mechanical Model

• In 1926, Erwin Schrodinger used the new quantum
  theory to write and solve mathematical equations
  to describe electron location.

8
The Quantum Mechanical Model, cont.

• Todays model comes from the solutions to
  Schrodingers equations.
• Previous models were based on physical models of
  the motion of large objects.
• This model does not predict the path of
  electrons, but estimates the probability of
  finding an electron in a certain position.
• There is no physical analogy for this model!

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Where are the electrons?

• In an atom, principal energy levels (n) can hold
  electrons. These principal energy levels are
  assigned values in order of increasing energy
  (n1,2,3,4...).
• Within each principal energy level, electrons
  occupy energy sublevels. There are as many
  sublevels as the number of the energy level
  (i.e., level 1 has 1 sublevel, level 2 has 2
  sublevels, etc.)

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Where are the electrons?

• There are four types of sublevels we will talk
  abouts,p,d and f. Inside the sublevel are
  atomic orbitals that hold the electrons. Every
  atomic orbital can hold two electrons.
• S has one orbital, P has three, D has five and F
  has seven. How many electrons can each one hold?

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Orbital Shapes
s orbital s sublevel
p sublevel


pz orbital
px orbital
py orbital
12

• http//winter.group.shef.ac.uk/orbitron/AOs/1s/ind
  ex.html

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Where are the electrons?

• So how many electrons can each energy level hold?
• Level 1 has an s sublevel2 e-
• Level 2 has an s and a p sublevel8e-
• Level 3 has an s, p and d sublevel18e-
• Level 4 has an s, p, d and f sublevel32e-

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Electron Configuration
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Electron Configuration

• In the atom, electrons and the nucleus interact
  to make the most stable arrangement possible.
• The ways that electrons are arranged around the
  nucleus of an atom is called the electron
  configuration.

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Aufbau Principal

• Electrons enter orbitals of the lowest energy
  first.

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He
1s
2s
2p
3s
3p
4s
4p
3d
5s
5p
4d
6s
6p
5d
7s
7p
6d
4f
5f
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EMR and Quantum Theory
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What does a wave look like?

• With your partner, label all the parts of a wave
  you can remember.

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A Quick Look at WavesParts of Waves
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A Quick Look at Waves

• The number of waves to pass a point in a given
  time is called frequency (n) and is measured in
  1/s or Hertz (Hz).

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Electromagnetic Radiation (EMR)

• According to the wave model, visible light
  consists of electromagnetic waves and is just a
  small fraction of waves classified as
  electromagnetic radiation.
• Other EMR includes radio waves, microwaves,
  infrared, ultraviolet, X-rays, gamma rays
  and cosmic rays.

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Electromagnetic Radiation (EMR)

• All of these waves travel at the same speed,
  3.0x108m/s!
• The waves differ in their frequencies and
  wavelengths, and obey the equation c l n
• This is an inverse relationshipas the frequency
  increases, the wavelength decreases.

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Practice Problems

• What is the wavelength of an electromagnetic wave
  with a frequency of 4.45x1015Hz?
• What is the frequency of a light wave with a
  wavelength of 497nm?
• What is the wavelength in nanometers of an
  electromagnetic wave with a frequency of
  2.97x1014Hz?

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Atomic Emission Spectra

• When sunlight is broken down into the waves it is
  made of, it creates a continuous spectrum
• Scientists used a hydrogen lamp to produce light,
  they expected a continuous spectrum but it
  wasnt! They had an atomic emission spectrum.

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• When we previously found the electron
  configuration for elements, it was for electrons
  at ground state (the lowest energy possible).
• As energy is added to atoms, they absorb the
  energy by electrons going from ground state to an
  excited state, where electrons are no longer in
  the lowest energy orbitals.

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• Electrons can then only go back to ground state
  by releasing the energy, usually in the form of
  light in discreet packets called photons.
• These packets defied classical physics, that said
  electrons would go back to ground state
  continuously.

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Max Planck

• To understand why this points towards the concept
  of energy levels, we need to know about Max
  Plancks discovery
• E h n
• Plancks constant (h6.6262x10-34Js)

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Practice Problems

• How much energy is associated with a wave with a
  frequency of 4.4x1014Hz?
• An electromagnetic wave is found to have
  1.18x10-19J of energy. What is its frequency?
• How much energy is associated with a wave of red
  light with a wavelength of 697nm?

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Putting It Together

• So, if only specific frequencies of light are
  emitted when electrons fall back to ground state
  from being excited, then there are only certain
  energies that electrons can have. This explains
  atomic emission spectra!

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Even Stranger

• Louis de Broglie predicts yet another property of
  electronsthat they have both a wave nature and a
  particle nature.
• Any moving particle can be described to have a
  wave nature described by de Broglies equation
• l h / mv

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• Even stranger still is the Heisenberg Uncertainty
  Principle.
• It states that you cannot know both a particles
  exact position and exact velocity (the more you
  know about one the less you know about the other).