Atomic Structure

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Atomic Structure

• Chapter 3

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Father of Atomic TheoryJohn Dalton
1766-1844
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Daltons Model of the Atom
An atom is a hard sphere that is uniform
throughout.
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Daltons Theory

• Atoms are indestructible particles ( False)
• Atoms are the smallest particle of an element and
  still have all the properties of that element
• All atoms of the same element are alike (False)
• Elements differ because atoms differ. They
  always differ in the number of protons.
• Atoms will combine to form compounds.
• Only whole atoms combine
• Atoms combine in a ratio of small whole numbers.

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J.J. Thomson (1856-1940)

• Discovered the electron by passing electric
  current through gases at low pressure using a
  cathode -ray tube

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J.J. Thompson

• He proposed that an atom consisted of a solid,
  positively charged sphere and the negatively
  charged electrons were imbedded in the surface of
  the atom
• cookie dough or plum pudding model

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Robert Millikan (1868-1953)

• He measured the charge of an electron and
  calculated the mass by spraying oil droplets,
  which fell on electrically charged plates
• He calculated the charge of an electron to be 1.6
  x 10-19 coulomb and the mass of an electron to be
  9.11 x 10-28 gram, which is about 2000 times
  lighter than an atom of hydrogen

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Ernest Rutherford (1831-1937)

• In 1911, he performed the gold foil experiment by
  directing a narrow beam of alpha particles at a
  very thin sheet of gold foil
• To his surprise, the majority of the alpha
  particles passed straight through the gold atoms,
  without deflection and a small fraction bounced
  off the gold foil at large angles

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Gold Foil Experiment
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Gold Foil Experiment
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Rutherford

• He proposed that the atom is mostly empty space
  and that all the positive charge and almost all
  the mass are concentrated in a small region which
  he called the nucleus.
• The nucleus is composed of protons and neutrons
• If an atom were the size of a football stadium,
  then its nucleus would be the size of a marble

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Rutherfords model of the Atom
protons
neutrons
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Neils Bohr (1885-1962)

• Proposed that electrons orbit the nucleus in
  fixed orbits

The protons and neutrons are in the nucleus. The
electrons are in specific orbits around the
nucleus. (Sorry, the colors are not
cooperating!!!)
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Rutherford video

• http//cwx.prenhall.com/petrucci/medialib/media_po
  rtfolio/text_images/006_RUTHERFORD.MOV

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Excited state Vs. ground state

• When an electron absorbs energy, it jumps to a
  higher energy, called the excited state. When an
  electron emits energy, it falls from the excited
  state, back to the ground state.
• The emission of energy creates spectral lines
  that are specific to each element
• An atom in the ground state has the same number
  of electrons that it has in the excited state.
  The number of electrons does not change, only the
  energy that the electrons have changes.

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Modern Atomic theory

• All elements are composed of atoms. An atom is
  the smallest particle of an element that can
  enter into combination with other elements.
• All atoms of the same element are chemically
  alike. They may differ in mass.
• Atoms of different elements are chemically
  different.
• Atoms are not divided, created, or destroyed by
  ordinary chemical or physical means.

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Subatomic particles
symbol
charge
relative mass
location
Particle
nucleus
1
Proton
H1

nucleus
1
Neutron
0
n1
-
shells energy levels
e-
0
Electron
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A
? Atomic mass
X
Symbol ?
Z
? Atomic number
Atomic number protons
Mass number protons neutrons
In an atom the number of electrons the number
of protons or total total -
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23
Na
11
Atomic number
11
Mass number
23
protons
11
neutrons
23 11 12
electrons
11
(only for atoms in the ground state)
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Isotopes

• Atoms of the same element that have different
  numbers of neutrons
• Example 12C and 14C are isotopes
• Carbon-12 has 6 protons and 6 neutrons
• Carbon-14 has 6 protons and 8 neutrons

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Isotopes

• Isotopes are uniformly mixed in nature
• The atomic mass on the periodic table is the
  weighted average mass of the naturally occurring
  isotopes of that element
• The closer the number of neutrons is to the
  number of protons, the more stable the isotope is

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Isotope sample problem

• Imaginary element X has two isotopes, 17X and
  21X. 65 of all isotopes of X are 17X. What is
  the atomic mass of X?
• Change your percent to a decimal and multiply by
  the mass. Then, add answers together.
• 17X .65 x 17 11.05
• 21X .35 x 21 7.35
• 18.40 amu
• Your answer should be somewhere between the
  values of your isotopes. Always remember to
  multiply by the percentages.

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One for you to try

• Imaginary element Z has three isotopes, 30Z,
  34Z and 35Z. 20 are 30Z, 50 are 34Z, and 30
  are 35Z. What is the atomic mass of Z?
• 30Z .20 x 30 6
• 34Z .50 x 34 17
• 35Z .30 x 35 10.5
• 33.5 amu

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Bohr diagrams

• When drawing a Bohr diagram, you need to know the
  number of protons, neutrons, and the electron
  configuration of the species youre drawing.
• Remember, the protons and neutrons are located in
  the nucleus.
• The electrons are drawn on the correct orbitals
  around the nucleus according to their electron
  configurations.

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Atoms vs. ions

• All atoms are neutral.
• All atoms have the same number of protons as
  electrons.
• Ions are charged particles.
• Atoms form ions when they gain or lose electrons.

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Metal ions vs. Nonmetal ions

• Metals form positive ions because they lose
  electrons.
• Nonmetals form negative ions because they gain
  electrons.

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Ions

• Atoms want to form ions in order to have a
  completed outer shell consisting of 8 electrons.
• (Elements 1-5 only need 2 electrons to have a
  completed outer shell since the first energy
  level can only hold 2)

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How many electrons to gain or lose?

• The last number in the electron configuration is
  the number of valence electrons.
• Use this to figure out how many electrons to gain
  or lose.
• Atoms with 1-3 valence electrons will lose all
  their valence electrons.
• Atoms with 5-7 will gain as many electrons as
  needed to make 8.
• Atoms with 4 can either gain or lose 4.

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Lewis electron dot diagrams

• Use the number of valence electrons of the atom
  to place the corresponding number of dots around
  the symbol for the atom.
• Al
• F

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