Arrangement of Electrons in Atoms

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Arrangement of Electrons in Atoms

• Principles of electromagnetic radiation led to
  Bohrs model of the atom.
• Electron location is described using
  identification numbers called quantum numbers.
• Rules for expressing electron location results in
  a unique electron configuration for each element.

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Building electron configurations for the ground
state of an atom Aufbau Lowest energy level
1st Pauli Exclusion Only 2 e- per orbital,
opposite spin Hund One electron per orbital
until that level is full (same spin)
Theater analogy 1. Enter at front. Climb
stairs and sit in first empty section you
reach. 2. Each bench seats 2 people. One male
and one female per bench. 3. Within a level,
ladies first. One lady per bench until each
bench is occupied, then one man per bench to
fill that level.
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Example Building electron configurations for
the ground state of an atom Phosphorous 15
electrons Start with 1s. Each s can hold 2
electrons. Each p can hold 6 electrons. Place
one electron per orbital, then pair until that
level is full. Only then go to the next level.
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Wave Description of Light

• Electromagnetic radiation is a form of energy
  that exhibits wavelike behavior as it travels
  through space
• Wavelength (?)
• Distance between corresponding points on adjacent
  waves.
• Unit nm,cm,m
• Frequency (?)
• Number of waves that pass a specific point in a
  given time
• Unit Hz or waves/sec
• Recall that Speed Distance/time (m/sec)
• Speed of light (c)
• C ? ?

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Behavior of Light

• Photoelectric effect
• The emission of electrons when light shines on
  the metal
• Scientists found that below a certain frequency,
  no electrons were emitted.
• Light also behaves as a particle Since hot
  objects do not emit em energy continuously, they
  must emit energy in small chunks called quanta.
• Quantum
• Minimum quantity of energy that can be gained or
  lost by an atom

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Light as a particle and a wavePlanck and Einstein

• Max Planck Relationship between quantum of
  energy and wave frequency
• Plancks constant h 6.626 x 10-34 J-s
• E h? E is energy, ? is frequency
• Albert Einstein Established dual wave-particle
  nature of light 1st
• Einstein explained PE effect by proposing that EM
  radiation is absorbed by matter only in whole
  numbers of photons.
• Electron is knocked off metal surface only if
  struck by one photon with certain minimum energy.

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Quantum Theory

• Ground state An atoms lowest energy state
• Excited state Higher potential energy than
  ground state.
• Photon A particle of electromagnetic radiation
  having zero mass and carrying a quantum of energy
  (i.e., packet of light)
• Only certain wavelengths of light are emitted by
  hydrogen atoms when electric current is passed
  throughWhy?

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Niels Bohr links hydrogens electron with photon
emission

• Bohr proposed that an electron circles the
  nucleus in allowed orbits at specific energy
  levels.
• Lowest energy is close to nucleus
• Bohrs theory explained the spectral lines seen
  in hydrogens line emission spectrum, but it did
  not hold true for other elements.

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Quantum Numbers

• Principal quantum number
• Angular momentum quantum number
• Magnetic quantum number
• Spin quantum number

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Quantum numbers
1s ____ 2s ____ 2p ____ ____ ____ 3s _____
Magnetic quantum number
Principal quantum number
Angular momentum quantum number
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Magnetic quantum number

• Magnetic quantum number is the orientation of an
  orbital around the nucleus.
• It is the number of orbitals in a sublevel.
• The s sublevel has 1 orbital.
• The p sublevel has 3 orbitals.
• The d sublevel has 5 orbitals.
• The f sublevel has 7 orbitals.

Orbitals per sublevel s 1 p 3 d 5 f 7
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Atomic StructureSummary of Contributions

• Max Planck
• A hot object emits energy in small, specific
  amounts called quanta.
• Albert Einstein
• E mc2
• Light behaves as both a wave and a particle.
• Each particle of light carries a photon ( a
  quantum of energy).
• Niels Bohr
• Created a model of the atom that showed a single
  electron of hydrogen orbits the nucleus only in
  allowed orbits with a fixed energy.

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Atomic StructureSummary of Contributions,
continued

• Werner Heisenberg
• Uncertainty principle It is impossible to
  determine simultaneously both the position and
  velocity of an electron.
• Erwin Schrödinger
• Helped lay the foundation for quantum theory with
  an equation that treats electrons like waves.

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Electron Configuration The Rules

• Aufbau principle
• An electron occupies the lowest energy orbital
  that will receive it.
• Pauli exclusion principle
• No two electrons in the same atom can have the
  same set of 4 quantum numbers.
• Therefore, electrons can pair in an orbital as
  long as their spins are opposite.
• Hunds rule
• Each of the orbitals at a particular level have
  one electron before any of them can have two
  electrons.
• All single electrons in the orbitals at a
  particular level have the same spin.