Modern Theory of the Atom: Quantum Mechanical Model

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Modern Theory of the AtomQuantum Mechanical
Model
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Recap of Bohr Model

• Electrons are particles moving in circular orbits
• Specific speed, position, energy
• Quantization of energy levels is imposed
• Ground state electrons closest to nucleus
• Electrons can move between energy levels
• higher energy levels farther from nucleus
• moving up to higher E level electron absorbs
  energy
• moving down to lower E level electron emits
  light energy

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1924 De Broglie

• Proposed
• if light can show both particle and wave
  behavior, maybe matter can too

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2 kinds of waves

• Traveling wave
• Wave not confined to given space
• Travels from one location to another
• Interrupted by boundary or another wave

• Standing wave
• Confined to given space (ends are pinned)
• Interference between incident reflected waves
• At certain frequencies
• certain points seem to be standing still
• Other points - displacement changes in regular way

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Traveling Wave 1

• Traveling Wave 2

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Guitar string

• Standing wave 1

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DeBroglie Electron-Wave
wavelength describing electron depends on energy
of electron At certain energies, electron waves
make standing waves in atom wave does not
represent electron path
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Modern Theory

• Electron treated as wave
• Cannot specify both position speed of electron
• Can determine probability of locating electron in
  given region of space
• Quantized energy levels arise naturally out of
  wave treatment

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Bohr Model vs. Modern Theory

• Electron particle
• Orbit
• Holds 2n2 electrons
• Circular
• Each orbit has specific energy
• Can find exact position/ speed

• Electron Wave
• Orbital
• Holds 2 electrons
• Not necessarily circular
• Each orbit has specific energy
• Probable location

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Orbital Modern Theory

• Orbital term used to describe region where
  electron might be
• Each orbital has specific energy and specific
  shape
• Described by 4 parameters of wave function (like
  an address)
• quantum numbers n, l, m, s

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What can orbitals do for us?

• Physical structure of orbitals explain
• Bonding
• Magnetism
• Size of atoms
• Structure of crystals

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Heisenberg uncertainty principle

• Fundamentally impossible to know velocity and
  position of particle at same time
• Impossible to make observation without
  influencing system

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n principal quantum number

• Specifies atoms principal energy levels
• whole number values 1, 2, 3, 4,
• Maximum electrons in any principal energy level
  2n2

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l Describes sublevels

• Principal energy levels have sublevels
• sublevels depends on principal energy level
• 1st principal energy level has 1 sublevel
• 2nd
  2
• 3rd 3
  
• 4Th 4
  , etc.

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Naming sublevels

• Sublevels are labeled by shapes
• s, p, d, f
• s orbitals spherical
• p orbitals dumbbell shaped
• d f orbitals more complex shapes

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m 3rd quantum number

• Sublevels made up of orbitals
• Each sublevel has specific of orbitals

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s orbitals
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p orbitals
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d orbitals
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4th quantum number s

• Electron spin 2 possible values
• 4 quantum numbers address for each electron
• No 2 electrons in atom can have same 4 quantum
  numbers
• only 2 electrons per orbital
• Pauli exclusion principle

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3rd principal energy level, 3 sublevels
2nd principal energy level, 2 sublevels s p
1st principal energy level, 1 sublevel s
Each box represents an orbital and holds 2
electrons
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Order of fill Aufbau principle

• Each electron occupies lowest energy orbital
  available
• Learn sequence of orbitals from lowest to highest
  energy
• Some overlap between sublevels of different
  principal energy levels

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Sequence of orbitals 1s, 2s, 2p, 3s, 3p, 4s, 3d,
4p, 5s, 4d, Follow arrows Exceptions do
occur - half-filled orbitals
have extra stability

• 1s
• 2s 2p
• 3s 3p 3d
• 4s 4p 4d 4f
• 5s 5p 5d 5f
• 6s 6p 6d 6f
• 7s 7p

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Electron Configurations
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Compare Bohr Schrodinger
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Frequencies in Chemistry
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Electron Configuration PT
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