Electron Arrangement

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Electron Arrangement
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Subatomic Particles

• All atoms consist of the same three subatomic
  particles protons, neutrons, and electrons.
• Only the number of each varies from element to
  element.
• Atoms are neutral, so the number of protons
  (positively charged) must be equal to the number
  of electrons (negatively charged).
• Based on Rutherfords Gold Foil Experiment, it
  was determined that the protons are located in
  the nucleus and the electrons are moving about
  the nucleus.

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Planetary Model

• According to the planetary model, electrons orbit
  the nucleus much like planets orbit the sun.
• The different orbits the electrons occupy are
  referred to as energy levels.
• The lowest energy level is the orbit closest to
  the nucleus.

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Energy Levels

• The lowest energy level (energy level 1) can hold
  a maximum of 2 electrons.
• Energy level 2 can hold a maximum of 8 electrons.
  Energy level 2 is farther from the nucleus than
  energy level 1 and is therefore larger in size.
• Energy level 3 can hold a maximum 18 electrons.
• The equation 2n2 can be used to determine the
  maximum number of electrons in any energy level.
  (n is the energy level number)
• Note There is never more than 8 electrons in the
  outer level. (We will learn why in the next
  unit)
• Electrons found in the outer level are called
  valence electrons.

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Bohr Diagrams

• A Bohr Diagram is a method for representing the
  arrangement of electrons within an atom.

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Atomic Number

• Moseley, in an experiment called the X-ray tube
  experiment, determined that each element has a
  different and unique number of protons.
• The number of protons is referred to as the
  elements atomic number.
• The atomic number can be used to identify the
  element.
• Atomic Number of protons of electrons.

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Mass Number

• The mass number is the total number of particles
  located in the nucleus.
• Mass Number of protons of neutrons
• To determine the number of neutrons, subtract the
  atomic number from the mass number.
• Mass - atomic of neutrons

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Nuclear Symbol

• A nuclear symbol can be used to represent the
  number of subatomic particles found in an atom.
• Example
• 35 Mass Number
• 17 Cl Atomic Number
• How many protons, neutrons, and electrons are
  found in this atom?

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Bohr Diagram

• Draw the Bohr diagram for the atom of chlorine.

17 p 18 n
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More Bohr Diagrams

• Draw the Bohr diagrams for the elements
  represented by the following nuclear symbols

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Isotopes

• In an experiment working with neon atoms, Thomson
  discovered some of the atoms had different
  masses.
• Because they all were neon, the number of protons
  and electrons were all the same.
• Therefore, the difference in mass was due to
  different numbers of neutrons.
• He called these different atoms isotopes.
• This discovery led to a revision of what part of
  Daltons theory?

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Examples of Isotopes
Determine the number of protons, electrons, and
neutrons for each of the isotopes of carbon.
The atomic number identifies the element. The
mass number identifies the isotope. Therefore,
when identifying an isotope by name, the mass
number is shown. Example carbon-12