chapt 4 early atomic theory by pWh1aG

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									       Chapter 4




Atoms and their structure


                            1
                History of the atom
   Not the history of atom, but really the idea of the atom
   The original idea - Ancient Greece (400 B.C..)
   Democritus and Leucippus Greek philosophers who
    were debating whether matter was continuous or
    discontinuous.
   Continuous – divide matter forever and always have a
    smaller piece of matter.
   Discontinuous – divide matter only so far and can go
    no farther.


                                                         2
             History of Atom
Start with a box of marble divide in half
eventually you get down to one marble
which if you divide again you no longer
have a marble.




The Greek word for “can not cut” is atomos – thus atom.

                                                          3
                  Another Greek
Aristotle – another famous Greek philosopher
All substances are made of 4 elements

Fire - Hot
Air - light
Earth - cool, heavy
Water - wet



   Aristotle and others believed in 4 elements of
    matter, combined in different proportions rather
    than indivisible particles
                                                  4
              Who Was Right?
 Greek society was slave based and so it was
  beneath the famous to work with their hands.
The Greeks did not experiment, they settled
  disagreements by argument (debates).
 Aristotle was more famous so his ideas carried
  through to the middle ages.
 During the middle ages Alchemists tried to
  change lead to gold


                                              5
    Re-emergence of the Atomic Theory
Late 1700’s - John Dalton- England (a major contributor
  to today’s Atomic Theory)
A teacher who summarized results of his
  experiments and those of other’s
In Dalton’s Atomic Theory he combined ideas of
  elements with that of atoms




                                                    6
          Dalton’s Atomic Theory
 All matter is made of tiny indivisible particles
  called atoms.
 Atoms of the same element are identical, those
  of different atoms are different.
 Atoms of different elements combine in whole
  number ratios to form compounds
 Chemical reactions involve the rearrangement
  of atoms. No new atoms are created or
  destroyed.
                                                     7
     Law of Definite Proportions
• Each compound has a specific whole-number
  ratio of elements; ratio is by mass [Definite
  Proportions]
          Water                  H2O

      Carbon dioxide             CO2

        Methane                  CH4

8.0 g oxygen reacts with 1.0 g hydrogen (H2O)
Ratio = 8:1 by mass




                                            8
         Law of Multiple Proportions
   if two elements form more that one compound,
    the ratio of the second element that combines
    with 1 gram of the first element in each is a
    simple whole number.
    •In hydrogen peroxide 16.0 g oxygen reacts with 1.0 g
    hydrogen (H2O2)
    •Ratio of the masses of oxygen in hydrogen peroxide and
    water is 16:8 = 2:1 Therefore H2O2 contains twice as many
    oxygen atoms per hydrogen atom than H2O [Multiple
    Proportions]

                                                                9
                             What?
Water is 8 grams of oxygen per gram of hydrogen. (H2O)
Hydrogen Peroxide is 16 grams of oxygen per gram of hydrogen.
  (H2O2)


16 to 8 is a 2 to 1 ratio

Therefore H2O2 contains twice as many oxygen atoms per hydrogen atom than
  H2O [Multiple Proportions]


True because you have to add a whole atom, you can’t add a
  piece of an atom.



                                                                     10
                   Parts of Atoms
   As scientists began to develop methods for more
    detailed probing of the nature of matter, the atom
    (supposedly indivisible) began to show signs of a
    more complex structure
 J. J. Thomson - English physicist. 1897
 Made a piece of equipment called a cathode
  ray tube which was used to study the electrical
  conductivity of gasses.
 The cathode ray tube is a vacuum tube - the air
  has been pumped out.

                                                         11
    Thomson’s Experiment
         Voltage source
-                         +


         Vacuum tube

         Metal Disks
                              12
    Thomson’s Experiment
         Voltage source
-                         +




                              13
    Thomson’s Experiment
         Voltage source
-                         +




                              14
    Thomson’s Experiment
         Voltage source
-                         +




                              15
         Thomson’s Experiment
                 Voltage source
     -                                  +

   Passing an electric current makes a beam
    appear to move from the negative to the
    positive end

                                               16
         Thomson’s Experiment
                 Voltage source
     -                                  +

   Passing an electric current makes a beam
    appear to move from the negative to the
    positive end

                                               17
         Thomson’s Experiment
                 Voltage source
     -                                  +

   Passing an electric current makes a beam
    appear to move from the negative to the
    positive end

                                               18
         Thomson’s Experiment
                 Voltage source
     -                                  +

   Passing an electric current makes a beam
    appear to move from the negative to the
    positive end

                                               19
        Thomson’s Experiment
                 Voltage source




   By adding an electric field




                                  20
       Thomson’s Experiment
                Voltage source

                        +



                         -
 By adding an electric field




                                 21
       Thomson’s Experiment
                Voltage source

                        +



                         -
 By adding an electric field




                                 22
       Thomson’s Experiment
                Voltage source

                        +



                         -
 By adding an electric field




                                 23
       Thomson’s Experiment
                Voltage source

                        +



                         -
 By adding an electric field




                                 24
       Thomson’s Experiment
                Voltage source

                        +



                         -
 By adding an electric field




                                 25
        Thomson’s Experiment
                 Voltage source

                         +



                           -
 By adding an electric field he found that the
  moving pieces were negative because the
  rays were attracted to the positive electrode
  in the external field.                          26
Cathode Ray Tube




                   27
    • Thompson concluded that:
       » Cathode rays consist of beams of particles

       » The particles have a negative charge
   Based on his findings, a new fundamental
    particle of matter was discovered – The
    Electron!




                                                      28
             Thomsom’s Model
 Couldn’t find positive (for a
  while)
 Said the atom was like plum
  pudding or (blueberry muffin)
 A bunch of positive stuff, with
  the electrons embedded
  (able to remove the
  embedded electrons)


                                    29
             Other pieces
 Proton - positively charged pieces 1840
  times heavier than the electron
 Neutron - no charge but the same mass
  as a proton.
 Where are the pieces?




                                            30
         Rutherford’s experiment
 Ernest Rutherford English physicist. (1910)
 Believed in the plum pudding model of the
  atom.
 Used radioactivity
 Alpha particles - positively charged pieces
  given off by uranium
 Shot them at gold foil which can be made a few
  atoms thick

                                             31
     Rutherford’s experiment
 When the alpha particles hit a florescent
  screen, it glows.
 Here’s what it looked like




                                              32
                       Florescent
Lead    Uranium          Screen
block

                  Gold Foil




                                    33
             He Expected
 Rutherford believed that the a particles
  would pass through unhindered. (The
  alpha particles to pass through without
  changing direction very much.)
 Because the positive charges were
  spread out evenly within the atom. The
  positive charge in the atom was not
  enough to stop the alpha particles


                                             34
What he expected




                   35
Because




          36
  Because, he thought the mass was
  evenly distributed in the atom


a particles




                                     37
Because, he thought
the mass was evenly
distributed in the atom




                          38
                What he got




A small percentage of the particles were being reflected at unexpected
angles, inconsistent with the “muffin model”
                                                                         39
   How he explained it
 Atom is mostly empty
 Small dense, positive piece at center
 Alpha particles are deflected by it if they
  get close enough



                       +


                                                40
Rutherford explained his observations as follows:
•Atom is mostly empty space
• Small, dense, and positive at the center
• Alpha particles were deflected if they got close enough




   a particles
                                               +




                                                            41
                 Modern View
The atom is mostly empty space




Two regions
Nucleus- protons and neutrons
Electron cloud- region where you might find an electron
                                                     42
           Density and the Atom
 Since most of the alpha particles went through,
  the atom is mostly empty.
 Because the alpha particles turned so much,
  the positive region of the atom is heavy.
 Small volume, big mass, big density
 This small dense positive area is the nucleus




                                               43
           Subatomic particles
                  Relative Actual
Name Symbol Charge mass    mass (g)
Electron    e-    -1   1/1840 9.11 x 10-28

Proton     p+    +1      1    1.67 x 10-24

Neutron    nº     0      1    1.67 x 10-24



                                             44
           Structure of the Atom
There are two regions
1. The nucleus – with protons and neutrons so
   that it has a Positive charge and almost all the
   mass
2. Electron cloud- Most of the volume of an atom
   and is the region where the electron can be
   found (extra nuclear)



                                                45
               Size of an atom
 Atoms are small and are measured in
  picometers, 10-12 meters
 Hydrogen atom, 32 pm radius
 Nucleus tiny compared to atom. If the atom was
  the size of a stadium, the nucleus would be the
  size of a marble.
 Radius of the nucleus near 10-15m.
 Density near 1014 g/cm3




                                              46
           Counting the Pieces
Atomic Number = number of protons (p+)
The number of protons determines kind of atom –
  2 protons in the nucleus means that this is a
  Helium atom.
Chemists use Z as a symbol for atomic number.
In a neutral atom there is the same number of
  electrons (e-) and protons (atomic number)
Mass Number = number of protons + neutrons
  [Sum of p+ and nº (p+ + nº)] The symbol used
  for mass number is A.
The neucleons (p+ and nº) make up the mass of
  the atom.
                                            47
                   Isotopes
 Dalton was wrong when he said that all atoms
  of one element are the same.
 Atoms of the same element can have different
  numbers of neutrons and therefore have
  different mass numbers and different masses.
 The atoms of the same element that differ in the
  number of neutrons are called isotopes of that
  element.


                                               48
            Nuclear Symbols
   Contain the symbol of the element, the
    mass number and the atomic number




                                             49
                    Nuclear Symbols
 Contain:
                             A

                                 E
the mass number
                                      the symbol of
the atomic number                     the element


                             Z



                                                      50
                 Symbols
   Find the
    – number of protons
    – number of neutrons
    – number of electrons   19
    – Atomic number          9   F
    – Mass Number




                                     51
            Symbols
 Findthe
  –number of protons
  –number of neutrons
                         80
  –number of electrons
                         35   Br
  –Atomic number
  –Mass Number


                                   52
            Symbols
 ifan element has an atomic
  number of 34 and a mass number
  of 78 what is the
  –number of protons
  –number of neutrons
  –number of electrons
  –Complete nuclear symbol

                                   53
            Symbols
   an element has 91 protons and
 if
  140 neutrons what is the
  –Atomic number
  –Mass number
  –number of electrons
  –Complete nuclear symbol


                                   54
             Symbols
 ifan element has 78 electrons and
  117 neutrons what is the
   –Atomic number
   –Mass number
   –number of protons
   –Complete nuclear symbol


                                      55
         Naming Isotopes
 Put the mass number after the name of
  the element
 carbon- 12
 carbon -14
 uranium-235




                                          56
               Atomic Mass
 How heavy is an atom of oxygen?
 There are different kinds of oxygen atoms.
 More concerned with average atomic mass.
 Based on abundance of each isotope in nature.
 Don’t use grams because the numbers would
  be too small




                                            57
         Measuring Atomic Mass
 Unit is the Atomic Mass Unit (amu)
 One twelfth the mass of a carbon-12 atom.
 Each isotope has its own atomic mass we
  need the average from percent abundance.




                                              58
         Calculating averages
 You have five rocks, four with a mass of 50
  g, and one with a mass of 60 g. What is the
  average mass of the rocks?
 Total mass =    4 x 50 + 1 x 60 = 260 g
 Average mass = 4 x 50 + 1 x 60 = 260 g
                           5          5
 Average mass = 4 x 50 + 1 x 60 = 260 g
                     5         5      5


                                            59
                  Calculating averages
   Average mass = 4 x 50g + 1 x 60g = 260 g = 52g
                   5         5           5
 Average mass = .8 x 50g + .2 x 60g = 52g
 80% of the rocks were 50 grams
 20% of the rocks were 60 grams


Average = % as decimal x mass1+ % as decimal x mass2 + % as decimal x mass3 +…




                                                                          60
                Atomic Mass
   Calculate the atomic mass of copper if
    copper has two isotopes. 69.1% has a mass
    of 62.93 amu and the rest (30.9%) has a
    mass of 64.93 amu.

    0.691 x 62.93amu + 0.309 x 64.93amu =
       43.48463amu + 20.06337amu = 63.548amu




                                            61
            Atomic Mass
 Magnesium has three isotopes. 78.99%
  magnesium 24 with a mass of 23.9850
  amu, 10.00% magnesium 25 with a mass of
  24.9858 amu, and the rest magnesium 26
  with a mass of 25.9826 amu. What is the
  atomic mass of magnesium?
 If not told otherwise, the mass of the
  isotope is the mass number in amu


                                       62
                   Atomic Mass
   is not a whole number because it is an average
    (are the decimal numbers on the periodic table).

Isotopes - atoms of the same element can have
different numbers of neutrons and therefore have
different mass numbers

When naming, write the mass number after the name of
the element

        1              2              3
        1   H          1   H          1   H
         Protium       Deuterium        Tritium
                                                  63

								
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