Ch. 5 How are an atom

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Ch. 5 How are an atoms electrons configured?
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Electrons and Light

• To chemists, electrons are the most important
  part of the atom
• Why ?
• Electrons emit light

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Light is an electromagnetic wave

• Sunlight, when passed through a glass prism,
  produces the visible spectrum all the colors of
  light that the human eye can perceive

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Electromagnetic Spectrum

• Visible portion
• Gamma
• X-rays
• Ultraviolet
• Infrared
• Microwave
• Radio

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Light

• Light and all other electromagnetic radiation,
  can be thought of as moving waves
• Waves can be described by three characteristics
• Speed, wavelength, and frequency

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Line Emission Spectrum

• Light can be separated into distinct lines of
  different colors
• Elements emit characteristics colors of light
  that produce a distinctive line-emission spectrum
  for that element

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Speed of Light (c)

• All electromagnetic waves travel at the same high
  speed in empty space
• 2.99 x 108 m/s
• Light travels 150 million kilometers in 500
  seconds (the distance between the sun and the
  Earth)

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Wavelength (?)

• Distance between two consecutive peaks or troughs
  of a wave
• Measured in meters
• Range from less than 10-13 m for gamma rays to
  more than 105 m for radio waves.

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Frequency (f)

• The frequency of a wave is the number of waves
  that pass a stationary point in one second.
• One wave per second is called a hertz, Hz (the
  unit for frequency)
• Range from less than 1000 Hz to more than 1022 Hz

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Relationship between speed, wavelength, and
frequency

• Frequency x wavelength speed of light
• Frequency (f)
• Speed of light (c)
• Wavelength (? )
• f c/?

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Excited Electrons Emit Light

• Niels Bohr, Danish physicist
• An electron in an atom is able to exist in any
  one of a number of energy states
• An electron can move from a low energy state to a
  high energy state by absorbing energy
• An electron can move from a higher energy state
  to a lower energy state by releasing energy

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Excited State

• If an electron acquires additional energy, then
  it is an excited state
• Unstable state
• Electron quickly falls back to its ground state
• Excess energy is released as light

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Ground State

• Each electron in an atom is in a state of lowest
  possible energy

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Quantum Theory

• Electrons are located in orbitals.
• Orbitals are region of space in which you can
  expect to find electrons of specific energy.
• Region of high probability for finding a
  particular electron

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Orbitals replace orbits

• Electrons have the properties of both particles
  and waves
• According to quantum theory, each electron in an
  atom is assigned three quantum numbers,
  symbolized n, l and m

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The principal quantum number (n)

• The principal quantum number (n) can take values
  of 1,2,3,4,5,6,7
• The larger the (n) the further the orbital from
  the nucleus

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The l quantum number

• l quantum number can take any whole-number value
  from 0 to n-1
• Ex if n 3 then l can be 0, 1, or 2
• When discussing the l quantum number, chemists
  use a letter code.

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• l 0 corresponds to the s-orbital
• l 1 corresponds to the p-orbital
• l 2 corresponds to the d-orbital
• l 3 corresponds to the f-orbital

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Shapes of Orbitals s, p, and d

• The s orbital is spherically shaped. There is
  only one s orbital for each value of n1,2,3. of
  the principle quantum number.

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• For each of the values n 2, n 3,n 4, there
  are three p orbitals. Each is dumbbell shaped,
  but they differ in orientation.

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• For each of the values n 3, n 4, n 5., there
  are 5 d orbitals.
• Four of the five are similar in shape, but have
  different orientation.

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No more than two electrons occupy an orbital

• Pauli Exclusion Principle states that no more
  than two electrons can occupy a single orbital
• This means that an orbital can occupy 0, 1, or 2
  electrons.

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Spin quantum number (m)

• Chemists imagine that electrons spin, and they
  imagine that if 2 electrons are in the same
  orbital, they must spin in opposite directions.
• ms describes the direction of spin

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• There are only two possible values for the spin
  quantum number ms
• ms -1/2 and
• ms- 1/2

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Electron Configuration

• Aufbau principle the electrons in an atom will
  occupy the lowest available orbitals first.

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How are the orbitals filled?

• The order in which the orbitals are filled
  matches the order of energies, which starts out
  as follows.
• 1s2 lt 2s2 lt 2p6 lt 3s lt 3p
• See handout

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Orbital Diagrams

• Orbital diagrams are also used to show how
  electrons are distributed within sublevels.
• Hunds Rule A single electron with the same spin
  must occupy each equal energy orbital before
  additional electrons with opposite spins can
  occupy the same orbital
• In an orbital diagram, each orbital is
  represented by a box, and each electron is
  represented by an arrow.

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• You must consider the aufbau principle when
  constructing the boxed orbital diagram.
• Lets construct a boxed orbital diagram for Carbon

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