Electrons in Atoms

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Electrons in Atoms

• Chapter 5

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What were early steps in development of atomic
theory?

• John Dalton Billiard Ball Theory
• Atom was indivisible
• J.J. Thomson Plum Pudding Model
• Atom was composed of smaller particles
• Rutherford Nuclear Model

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Rutherford Model

• Nucleus contains
• all the positive charge most of mass of atom
• Nucleus very small
• only 1/10,000th of atomic diameter
• Electrons occupy most of volume

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Later Models

• Bohr Planetary Model
• Schrodinger Wave Mechanical Model

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Problems with the Rutherford Model

• Why dont electrons crash into nucleus?
• How are electrons arranged?
• Why do different elements exhibit different
  chemical behavior?
• How is atomic emission spectra produced?

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Atomic Emission Spectra

• gas in glass tube and apply voltage across ends
• produces light
• color of light depends on gas in tube
• Every element produces its own unique color

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emission spectrum of element is set of
frequencies (or wavelengths) emitted
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Why is emission spectra useful?

• use it to determine if given element is present
  in sample
• Neon lights

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Emission Absorption Spectra of Elements
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Bohr Model

• Bohr - electrons in atom can have only specific
  amounts of energy NEW idea!
• Each specific energy is associated with specific
  orbit
• electrons are restricted to these orbits
• Bohr assigned quantum number (n) to each orbit
• the smallest orbit (n 1)
• is closest to nucleus
• has lowest energy
• larger the orbit, more energy it has

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Bohr Diagram

• Shows all the electrons in orbits or shells about
  the nucleus.

n2
n3
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Bohr Model

• Energy absorbed when electron
• moves to higher orbit (farther from nucleus)
• Endothermic process
• Energy released when electron
• drops to lower orbit (closer to nucleus)
• Exothermic process

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energy levels get closer together the farther
away they are from nucleus
Larger orbits can hold more electrons
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Max Capacity of Bohr Orbits
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Electron Transitions

• If electron gains (absorbs) specific amount of
  energy
• it can be excited to higher energy level
• If electron loses specific amount of energy
• it moves down to lower energy level

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Hydrogen has 1 electron, but it can make many
possible transitions
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Absorption Emission

• cannot easily detect absorption of energy by
  electron
• BUT
• can easily detect emission of energy by electron
• see photon that is kicked out

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Emitted Light

• energy of emitted light (E h??
• matches difference in energy between 2 levels
• dont know absolute energy of energy levels, but
• can observe how far apart are from each other

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ladder often used as analogy for energy levels of
atom How is little bit different?
Potential Energy
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Ground State vs. Excited State

• Ground state
• Lowest energy state of atom
• electrons in lowest possible energy levels
• Configurations in Reference Tables are ground
  state
• Excited state
• Many possible excited states for each atom
• One or more electrons excited to higher energy
  level

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Success of Bohrs Model

• Bohrs model could predict frequencies in
  emission spectrum of hydrogen
• Predicted correct size of H atom
• Unfortunately, didnt work for anything with more
  than 1 electron

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Which principal energy level of an atom contains
electron with the lowest energy?

• n1
• n2
• n3
• n4

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What is total of occupied principal energy
levels in atom of neon in ground state?

1. 1
2. 2
3. 3
4. 4

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What is total of fully occupied principal
energy levels in atom of nitrogen in ground state?

1. 1
2. 2
3. 3
4. 4

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What is total of electrons in completely
filled fourth principal energy level?

1. 8
2. 10
3. 18
4. 32

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Which atom in ground state has five electrons in
its outer level and 10 electrons in its kernel?

1. C
2. Cl
3. Si
4. P

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Which electron configuration represents atom in
excited state?

• a) 2-8-2
• 2-8-1
• 2-8
• 2-7-1

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Which electron configuration represents atom of
Li in an excited state?

1. 1-1
2. 1-2
3. 2-1
4. 2-2

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The characteristic bright-line spectrum of atom
is produced by its

1. Electrons absorbing energy
2. Electrons emitting energy
3. Protons absorbing energy
4. Protons emitting energy