Atoms and Atomic Theory

1
Atoms and Atomic Theory

• Democritus around 400 BC, said that matter is
  made of small indivisible particles.
• Lavoisier late 18th century, proposes law of
  conservation of mass
• Proust late 18th century, proposes law of
  constant composition
• Dalton early 20th century, proposes first
  atomic theory.

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Daltons Atomic Theory

• Elements are made of small particles called
  atoms.
• All atoms of a given element are identical.
• The atoms of different elements are different
  from those of any other element.
• Atoms of different elements combine to form
  compounds.
• Atoms cannot be created or destroyed in a
  chemical reaction.

3
Scientist Experiment Knowledge
Crookes Cathode Ray Tube Negative particles of some kind exist
J.J. Thomson Cathode Ray Deflection Mass/charge ratio of the electron determined
Millikan Oil Drop Experiment Charge on electron
Rutherford, Marsden, Geiger Gold Foil Experiment Nucleus present in atom contains positive protons
4
After Chadwick discovered the presence of the
neutron, Bohr proposed the idea that the atom was
being orbited by electrons in specific orbits.
Eventually, this model was modified to include
modern understanding of both the wave and
particle behavior of electrons.
Particle Charge Mass Position
Proton 1 1u In nucleus
Neutron 0 1u In nucleus
Electron -1 1/1836u Outside nucleus
u denotes atomic mass units (1/12 the mass of
one C-12 atom)
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Bohrs Model

• Bohr answered this question
• What prevents negatively charged electrons from
  crashing in on the positively charged nucleus?
• ENERGY LEVELS! He proposed that electrons reside
  in energy levels
• 1st level 2 electrons 2nd level 8 electrons
• 3rd level 18 electrons 4th level on 32
  electrons
• Demo Geissler Tube Apparatus

6
Modern Atomic Theory (aka quantum theory)

• While Bohrs model was good, it was not really
  complete. We now know that electrons reside in
  orbitals of which there are four types, s, p, d,
  and f, on different energy levels in the atom.
  The shapes of these orbitals will be shown in
  lecture.

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Periodic Table Information

• The atomic number (printed above symbol on
  periodic table, designated Z) indicates the
  number of protons in the nucleus of one atom of
  that element. Since atoms are neutral, this
  number also indicated the number of electrons in
  the atom.

• 6
• C
• 12.01

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More Info in the Periodic Table

• The mass number (below the symbol in the periodic
  table, usually designated with an A) tells the
  number of protons and neutrons in one atom of the
  element.

• 6
• C
• 12.01

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Isotopes

• In nature, not all atoms of an element have the
  same number of neutrons. Atoms with the same
  number of protons but different numbers of
  neutrons are called isotopes.
• Determine the number of protons, neutrons, and
  electrons in
• 40K19 210Pb80

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Average Atomic Mass

• So, why are the atomic mass numbers on the
  periodic table not whole numbers?
• Because they represent the average of all the
  isotopes of an element based on their prevalence
  in nature.
• Average atomic mass is calculated like this
• ?(of each element in decimal form)(atomic mass
  of isotope)

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Electron Configurations

• Every element has a specific order in which its
  orbitals are filled with electrons. This is
  called the electron configuration.
• There are four orbital types
• 1 s orbital holds 2 electrons (e-)
• 3 p orbitals hold 2 e- for a total of 6 e-
• 5 d orbitals hold 2 e- for a total of 10 e-
• 7 f orbitals hold 2 e- for a total of 14 e-

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Electron Configuration

• To determine the electron configuration, look at
  the number of electrons in the atom and simply
  fill each orbital in the following order
• 1s22s22p63s23p64s23d104p65s24d105p66s2
• 4f145d106p67s25f14