ATOMS (PowerPoint download)

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 Democritus – First person known to suggest that matter
  can be broken down into small particles that he named:
 In the 1790s, chemists are using better instruments and are
  able to make better measurements.
   Using these instruments, theories about matter are
     developed. Some of which are still valid today.
 In the late 19th century and early 20th century, some
  fundamental discoveries were made about the atom due to
  the further development of instruments.
 What did Einstein propose?

 Mass is neither destroyed nor created
  during ordinary chemical reactions or
  physical change.
 Example:
   If you have 50 grams of matter before a chemical
   reaction or physical change, the total mass will
   be 50 grams after the chemical reaction or
   physical change.
 What is the formula for water?

 A chemical compound contains the same elements in
  exactly the same proportions by mass even if the size of
  the samples change. It is the law of Definite Proportions

 Other example:
     Carbon dioxide is always 1 Carbon and 2 Oxygen atoms
     Carbon Monoxide is always 1 Carbon and 1 Oxygen atoms
     Hydrogen Peroxide is always 2 Hydrogen and 2 Oxygen atoms
     What is the ratio of elements in Hydrogen Peroxide and how
      is that different from Water?
What is different between the two examples?
What is the same?
What is the ratio between the mass of Oxygen in the first
example and that in the second example?
 If we have two different substances that have the
 same two elements, if we combine the second
 element with the same amount of the first, the
 ratio of the elements are always small whole
   2:1, 3:1, 3:2, etc….
 In the examples below, find the whole number
 ratio between:
   C2H2 and C2H6
   H2O2 and H2O
Dalton’s Atomic Theory
        year 1808                            Today - 2010
1.   Matter made of extremely        Statement 1, 4 and 5 are still
     small particles called atoms     accepted today
2.   Atoms of an element are         Statement 2 has been modified
     identical in: mass, size and     because we now know that
     other properties                 atoms of an element can differ. A
3.   Atoms cannot be subdivided,      given element can have atoms of
     created or destroyed             different masses
4.   Atoms combine in simple         Statement 3 is incorrect. Atoms
     whole number ratios to for       can be subdivided and have
     chemical compounds               been
5.   In chemical reactions, atoms
     combine, separate or are
Discovery of the              Discovery of the Nucleus &                   Discovery of the
Electron                      Proton                                       Neutron
Discover/Experimenter:        Discover/Experimenters: Ernest               Discover/Experimenter:
Joseph John Thomson           Rutherford, Hans Geiger and                  Chadwick
                              Ernest Marsden

Experiment/year:              Experiment/year: commonly                    Experiment/year:
Cathode Tube; 1897            known as “Rutherford’s                       Complete for
                              experiment”; 1911                            homework
                              What was his greatest discovery and
                              what was the controversy this created
                              among scientists?
Showed: existence of a        Showed: existence of a nucleus with          Complete for homework
negatively cathode ray; the   positive particles. A thin piece of gold
ray had mass and force; ray   foil was bombarded with a narrow
deflected by magnetic field   beam of alpha (+) particles. Some of the
and negatively charged        particles were redirected by the gold foil
objects; all cathode rays     back toward the source. Rutherford’s
composed of negatively        hypothesis: negatively charged
charged particles that were   electrons surround positively charged
later named electrons         nucleus
What is the difference between the charge of a proton and an
Which particle has the least mass? By how much less?
Which of them is neutral and where is it located?
Proton-Proton                          Neutron-Neutron
Attraction                             Attraction
 Protons are closely                   Similar attraction
  packed at the center of the             between neutrons in the
  atom                                    nucleus
 When protons are
  extremely close, there is a
  between them. More than
  100 protons can exist close
  together in the nucleus
  The forces that hold the protons and neutrons together are referred to
Why is matter mostly empty space? Hint: look at the radius
difference between the atomic radius and nucleus radius)
Homework: Are there other sub particles in the atom? If there
are, give some examples.
Protons                  Neutrons                     Electrons

The atomic number        The number of neutrons       The number of
of an element            can differ in atoms of the   electrons is equal to
represents the number    same element. For            the atomic number
of protons . The         example: Hydrogen-2          when the atom is
atomic number (Z) is     (Deuterium) has 1 neutrons   neutral
the same for all atoms   while Hydrogen-3 has 2
of a specific element.   neutrons.
For example, all         Helium-3 has 1 neutron and
Hydrogen atoms have 1    Helium-4 has 2 neutrons.
proton, all helium       These examples above are
atoms have 2 protons.    called ISOTOPES or atoms
Elements are listed in   of the same element with
the periodic table       different masses.
according the atomic     # of neutrons = mass
number                   number – atomic number
Mass number is equal to the total number of
protons and neutrons or the atomic number + the
number of neutrons
1.        Two ways of writing isotopes:                       Formula
     1.         Hyphen notation: example:
           1.      Chlorine-37; uranium-235; carbon-14
                                                              Top number
                                                              or mass
                                                              number –
     2.         Nuclear Symbol: example:                      the bottom
           1.     235 U   the top number is the atomic mass
                  92        and the bottom number is the
                                                              number or
                          atomic number                       atomic
                                                              number =
      Example Find the number of protons, neutrons
                                                              the number
      and electrons of: Chlorine-37:
                                                              of neutrons
Find the atomic number from the Periodic Table. The
atomic number = Number of protons = number of
Mass number = number of neutrons –atomic number

Atomic number from the Periodic Table = 17
The number of electrons is = 17
The number of neutrons is:
37 – 17 protons = 20 neutrons.

How many protons, neutrons and electrons are there is
atom of bromine-80?
 The standard used by scientists to apply to atomic
  mass is based on an atom: carbon-12 nuclide.
 The mass assigned to this nuclide is 12 atomic mass
 One atomic mass unit or 1 amu is 1/12 the mass of a
  carbon-12 amu
Average Atomic Mass        Example
 Weighted           Copper has two naturally
 average of the       occurring isotopes:
 atomic masses       69.17% of Copper-63 with a mass
 of the naturally     of 62.929599 amu and 30.83% of
 occurring            Copper-65 with atomic mass
 isotopes of an       64.927793 amu.
 element             Calculating>
                     .6917 X 62929599 amu + .3083 X
                      64.927793 amu = 63.55 amu
The Mole                  Avogadro’s Number           Molar Mass
Like a dozen, the mole    The number of particles     The mass of one mole of a
represents a certain      in a mole or                pure substance in grams.
amount of a substance.    6.022 X 1023 particles in   It is equal to the atomic
                          one mole of a pure          mass that we find in the
                          substance                   periodic table for each
Example: for example,     Example: one mole of        Examples: one molar
number of carbon has 12   copper has 6.022 X 1023     mass of Lithium is 6.94
grams of carbon           atoms                       grams/mole
                                                      One molar mass of
                                                      mercury (Hg) is 200.59
What is the mass in grams of 3.50 mol of
the element copper, Cu?

 Given: 3.50 mol Cu
 Unknown: mass of Cu in grams
 Solution: the mass of an element in grams
 can be calculated by multiplying the
 amount of the element in moles by the
 element’s molar mass.

 The molar mass of copper from the
 periodic table is rounded to 63.55 g/mol.
   A chemist produced 11.9 g of aluminum, Al.
   How many moles of aluminum were

   Given: 11.9 g Al
   Unknown: amount of Al in moles

The molar mass of aluminum from the
   periodic table is rounded to 26.98
How many moles of silver, Ag, are in 3.01 
1023 atoms of silver?

Given: 3.01 × 1023 atoms of Ag
Unknown: amount of Ag in moles
What is the mass in grams of 1.20  108
atoms of copper, Cu?

Given: 1.20 × 108 atoms of Cu
Unknown: mass of Cu in grams

        The molar mass of copper from the
          periodic table is rounded to 63.55

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