Atoms, Molecules and Ions

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Atoms, Molecules and Ions

• Chapter 2

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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.

HgO ? Hg O 433.2 g 401.2g 32g
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Daltons Atomic Theory (1808)

1. Elements are composed of extremely small
   particles called atoms. 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.

1. Compounds are composed of atoms of more than one
   element. The relative number of atoms of each
   element in a given compound is always the same.

1. Chemical reactions only involve the
   rearrangement of atoms. Atoms are not created or
   destroyed in chemical reactions.

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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.
• The electrode is named by what type of particle
  it attracts
• Cathode Negative (-)
• Anode Positive ()
• 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.

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• 1897 JJ Thomson used magnets to deflect the beam
  proving that particles had a NEGATIVE CHARGE.

JJ THOMSON DISCOVERED A NEGATIVE PARTICLE CALLED
THE ELECTRON!
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• Now with the knowledge of electrons, and knowing
  the atom is neutral, there must be a particle
  that is positive to balance the negative charge.
• A small paddle wheel was placed inside and it
  rolled toward the anode, providing evidence that
  some PARTICLE MUST BE STRIKING THE WHEEL to make
  it move.

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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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Radioactivity

• Becquerel
• discovered RADIATION
• uranium would expose photographic plates in the
  dark
• The properties of an element changed as it gave
  off radiation
• Curie
• Discovered radium and polonium
• The radioactive emissions of alpha, beta and
  gamma rays were identified.

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(Uranium compound)
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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.

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Rutherfords Gold Foil Experiment
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Rutherford Its like shooting a cannon at a
piece of tissue paper and having it bounce back
at you!
MOST particles went through the gold foil SOME
particles BOUNCED back
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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
If the atom is the size of Giants Stadium Then
the nucleus is a marble on the 50 yard line
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Goldstein and Wien - 1886

• Cathode Ray Tube with perforated cathode
• Discovered collection of positively charged
  particles
• DISCOVERED THE PROTON!!

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Chadwicks Experiment - 1932

• Found that alpha particles shot at beryllium made
  a beam form
• The beam had the same mass of a proton but was
  electrically neutral
• DISCOVERED THE NEUTRON!!

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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
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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 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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Molecule an aggregate of two or more atoms in a
definite arrangement held together by chemical
bonds
diatomic molecule contains only two atoms
H2, N2, O2, Br2, HCl, CO
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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monatomic ion contains only one atom
Na, Cl-, Ca2, O2-, Al3, N3-
polyatomic ion contains more than one atom
OH-, CN-, NH4, NO3-
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Do You Understand Ions?
13 protons, 14 neutrons, 10 (13 3) electrons
34 protons, 44 neutrons, 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 ?? 134Cs