Atoms, Molecules and Ions

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Atoms, Molecules and Ions

• Chapter 3

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Foundations of Atomic Theory

• Law of conservation of mass Antoine Lavoisier
• Mass is neither created nor destroyed. The total
  mass of a compound must be the same as the total
  mass of individual elements.
• Law of definite composition Joseph Proust
• a chemical compound contains the same elements in
  exactly the same proportions by mass regardless
  of the size of the sample or source of the
  compound

HgO ? Hg O 433.2 g 401.2g 32g
Sugar 42.1 Carbon 51.4 Oxygen 6.5
Hydrogen Whether you have a teaspoon or a
truckload!
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Law of Conservation of Mass
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• Law of multiple proportions John Dalton
• Applies to different compounds made from the same
  elements
• The mass ratio for one of the elements that
  combines with a fixed mass of the other element
  can be expressed as a whole number ratio.

H2O H2O2 Water Peroxide 2g H
2g H 16g O 12 Ratio 32g O
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Law of Multiple Proportions
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Daltons Atomic Theory (1808)

1. Elements are composed of extremely small
   particles called atoms.
2. All atoms of a given element are identical,
   having the same size, mass and chemical
   properties. The atoms of one element are
   different from the atoms of all other elements
3. Atoms cannot be subdivided, created or destroyed

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Daltons Atomic Theory (1808) contd

• Atoms of different elements combine in simple
  whole-number ratios to form chemical compounds.
• 5. Chemical reactions only involve the
  rearrangement of atoms.

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JJ Thomsons Cathode Ray Tube
Negatively Charged Electrode
Positively Charged Electrode
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Cathode Ray Tube

• Scientists studied the flow of electric current
  in a glass vacuum tube with electrodes at each
  end.
• When connected to electric current the remaining
  gas glowed forming a BEAM OF LIGHT.
• The beam always originated at the NEGATIVE
  electrode and toward the POSITIVE electrode.
• 1897 JJ Thomson used magnets to deflect the beam
  proving that particles had a NEGATIVE CHARGE.

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JJ THOMSON DISCOVERED A NEGATIVE PARTICLE CALLED
THE ELECTRON!
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CRT Video
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Plum Pudding Model
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Plum Pudding OR Chocolate Chip Cookie
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Robert A. Millikan

• Performed the Oil Drop Experiment
• Determined the exact charge of an electron

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Measured Charge of e- (1923 Nobel Prize in
Physics)
e-
charge -1.60 x 10-19 C Thomsons charge/mass of
e- -1.76 x 108 C/g
e- mass 9.10 x 10-28 g
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Oil Drop Experiment Video
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Ernest Rutherfords Gold Foil Experiment

• Set up Gold Foil with a detection sheet around
  it.
• Set up radioactive source emitting alpha
  particles.
• ALPHA PARTICLES shot at gold foil.
• MOST particles went through the gold foil
• SOME particles BOUNCED back

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Rutherfords Gold Foil Experiment
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Gold Foil Conclusions

• The atom is made up of mostly EMPTY SPACE
• The center of the atom contains a POSITIVE CHARGE
• Rutherford called this positive bundle of matter
  the NUCLEUS

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Gold Foil Experiment Video
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Rutherfords Model of the Atom
atomic radius 100 pm 1 x 10-10
m nuclear radius 5 x 10-3 pm 5 x 10-15 m
If the atom is the size of Giants Stadium Then
the nucleus is a marble on the 50 yard line
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Ob-scertainer

• You cant see something, but you know its there
  you feel it moving How is it moving?
• Do your best to determine the movement of the
  particle in your containers

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Ob-scertainer Solutions
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Niels Bohr 1913

• Developed a new diagram of the atom
• Electrons can only be at certain energies
• Electrons must gain a specific amount of energy
  to move to a higher level, called a quantum
• DISCOVERED ENERGY LEVELS!!

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Bohrs Model of the Atom
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Subatomic Particles (Table 2.1)
mass p mass n 1840 x mass e-
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Atomic number (Z) number of protons in nucleus
Mass number (A) number of protons number of
neutrons
atomic number (Z) number of neutrons Isotopes
are atoms of the same element (X) with different
numbers of neutrons in their nuclei
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Do You Understand Isotopes?
6 protons, 8 (14 - 6) neutrons, 6 electrons
6 protons, 5 (11 - 6) neutrons, 6 electrons
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A molecule is an aggregate of two or more atoms
in a definite arrangement held together by
chemical bonds
A diatomic molecule contains only two atoms
H2, N2, O2, Br2, HCl, CO
A polyatomic molecule contains more than two atoms
O3, H2O, NH3, CH4
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An ion is an atom, or group of atoms, that has a
net positive or negative charge.
cation ion with a positive charge If a neutral
atom loses one or more electrons it becomes a
cation.
anion ion with a negative charge If a neutral
atom gains one or more electrons it becomes an
anion.
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A monatomic ion contains only one atom
Na, Cl-, Ca2, O2-, Al3, N3-
A polyatomic ion contains more than one atom
OH-, CN-, NH4, NO3-
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Do You Understand Ions?
13 protons, 10 (13 3) electrons
34 protons, 36 (34 2) electrons
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Atomic Mass Unit
Relative Atomic Mass

• One atom is the standard Carbon
• Mass of other elements are based off of the
  standard
• Carbon 6 p and 6 n 12 amu

• 1/12 mass of Carbon atom
• Periodic table lists weighted average atomic
  masses of elements (like a GPA calculation)

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Calculation AVERAGE Atomic Mass

• Steps
• 1. Percent to decimal
• 2. Multiply by mass
• 3. Add it up!

75 133Cs 20 132Cs 5 134Cs
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