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Chapter 2 Atoms _ Elements

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					    Chapter 2
 Atoms & Elements
CHE 123: General Chemistry I
 Dr. Jerome Williams, Ph.D.
     Saint Leo University
              Overview
• Historical Overview of Atom
• Fundamental Chemical Laws
• The Structure of Atoms
            Historical Overview of Atom
   • Some early philosophers believed matter had
     an ultimate, tiny, indivisible particle (atomos)
         – Leucippus and Democritus
   • Other philosophers believed that matter was
     infinitely divisible
         – Plato and Aristotle
   • Because there was no experimental way of
     proving who was correct, the best debater was
     the person assumed correct, i.e., Aristotle
Tro: Chemistry: A Molecular Approach, 2/e   3
                        Scientific Revolution

   • In the late 17th century, the scientific
     approach to understanding nature became
     established
         – Robert Boyle (The Sceptical Chemist)
   • For the next 150+ years, observations about
     nature were made that could not easily be
     explained by the infinitely divisible matter
     concept

Tro: Chemistry: A Molecular Approach, 2/e   4
          Law of Conservation of Mass
   • In a chemical reaction,
     matter is neither created nor
     destroyed
   • Total mass of materials you
     have before the reaction
     must equal the total mass of
     materials you have at the
     end
                                                Antoine Lavoisier
         – total mass of reactants =
           total mass of products                  1743-1794


Tro: Chemistry: A Molecular Approach, 2/e   5
  Fundamental Chemical Laws
• Law of Definite Proportions

  – Different samples of a pure chemical
    substance always contain the same
    proportion of elements by mass.
  – Example is water (11.1% H by mass; 88.9% O
    by mass)
  – Proposed by Joseph Proust (1754 – 1826)
        Proportions in Sodium Chloride
  A 100.0 g sample of sodium
  chloride contains 39.3 g of
  sodium and 60.7 g of
  chlorine
  A 200.0 g sample of sodium
  chloride contains 78.6 g of
  sodium and 121.4 g of
  chlorine
  A 58.44 g sample of sodium
  chloride contains 22.99 g of
  sodium and 35.44 g of
  chlorine

Tro: Chemistry: A Molecular Approach, 2/e   8
 Example 2.1: Show that two samples of carbon
  dioxide obey the Law of Definite Proportions
               Given:           Sample 1: 25.6 g O and 9.60 g C
                                Sample 2: 21.6 g O and 8.10 g C
           Find:                proportion O:C
      Conceptual                      g O1, g C1
                                                            O:C in each sample
           Plan:                      g O 2, g C2
                               all samples of a compound have the same
   Relationships:                    proportion of elements by mass
          Solution:




              Check: both samples have the same O:C ratio, so the result is
                                   consistent with the Law of Definite Proportions
Tro: Chemistry: A Molecular Approach, 2/e     9
        Practice – If a 10.0 g sample of calcite
         contains 4.0 g of calcium, how much
          calcite contains 0.24 g of calcium?




Tro: Chemistry: A Molecular Approach, 2/e   10
       Practice – How much calcite contains 0.24 g
                      of calcium?
               Given:           Sample 1: 4.0 g Ca and 10.0 g calcite
                                Sample 2: 0.24 g Ca
           Find:                mass calcite in Sample 2, g
      Conceptual                  g Ca1, g calcite1
                                                                 g calcite2
           Plan:                       g Ca2
                             because
  Relationships:


          Solution:




        Sig Figs &
           Round:
                                            0.6 g calcite = 0.60 g calcite
Tro: Chemistry: A Molecular Approach, 2/e       11
  Fundamental Chemical Laws
• Law of Multiple Proportions

  – Elements can combine in different ways to
    form different substances whose mass ratios
    are small whole-number multiples of each
    other.
  – Proposed by John Dalton (1766-1844)
              Example 2.2: Show that two oxides of
              nitrogen are consistent with the Law of
                         Multiple Proportions
               Given: nitrogen dioxide: 2.28 g O per 1 g N
                  dinitrogen monoxide: 0.570 g O per 1 g N
            Find: O in nitrogen dioxide:O in dinitrogen monoxide
       Conceptual        g O in nit. dioxide
                                                     O1:O2
            Plan:       g O in dinit. monox.
                               samples of different compounds that have
  Relationships:                the same elements show proportions by
                                mass that are small whole number ratios
           Solution:



               Check: because the compounds have O:O ratio that is
                         a small whole number, so the results are
                      consistent with the Law of Multiple Proportions.
Tro: Chemistry: A Molecular Approach, 2/e   13
        Dalton’s Atomic Theory
• Proposed theory of matter (four postulates)

1. Elements are made of tiny indestructible particles called
   atoms.
2. Atoms of a given element have the same mass and
   other properties that distinguish them from atoms of
   other elements.
3. Chemical combinations of elements to make different
   substances occur when atoms join together in small
   whole-number ratios.
4. Chemical reactions only rearrange the way that atoms
   are combined; the atoms themselves are unchanged.
      Practice – Decide if each statement
        is correct according to Dalton’s
               model of the atom
     • Copper atoms can combine with zinc
       atoms to make gold atoms

     • Water is composed of many identical
       molecules that have one oxygen atom and
       two hydrogen atoms




Tro: Chemistry: A Molecular Approach, 2/e   15
    Practice – Decide if each statement
       is correct according to Dalton’s
                model of the zinc atoms to make
   • Copper atoms can combine with atom
     gold atoms – incorrect; according to Dalton, atoms of
     one element cannot turn into atoms of another
     element by a chemical reaction. He knew this
     because if atoms could change it would change the
     total mass and violate the Law of Conservation of
     Mass.
   • Water is composed of many identical molecules that
     have one oxygen atom and two hydrogen atoms –
     correct; according to Dalton, atoms combine together
     in compounds in small whole-number ratios, so that
     you could describe a compound by describing the
     number of atoms of each element in a molecule. He
Tro: Chemistry: A Molecular Approach, 2/e   16
      Practice – Decide if each statement
        is correct according to Dalton’s
               Model of the Atom
     • Some carbon atoms weigh more than
       other carbon atoms

     • Because the mass ratio of Fe:O in wüsite
       is 1.5 times larger than the Fe:O ratio in
       hematite, there must be 1.5 Fe atoms in a
       unit of wüsite and 1 Fe atom in a unit of
       hematite


Tro: Chemistry: A Molecular Approach, 2/e   17
   Practice – Decide if each statement
     is correct according to Dalton’s
            model of the atom
      • Some carbon atoms weigh more than other carbon
         atoms – incorrect; according to Dalton, all atoms of
         an element are identical.
      • Because the mass ratio of Fe:O in wüsite is 1.5
         times larger than the Fe:O ratio in hematite, there
         must be 1.5 Fe atoms in a unit of wüsite and 1 Fe
         atom in a unit of hematite – incorrect; according to
         Dalton, atoms must combine in small whole-number
         ratios. If you could combine fractions of atoms, that
         would mean the atom is breakable and Dalton’s first
         premise would be incorrect. You can get the Fe:Fe
         mass ratio to be 1.5 if the formula for wüsite is FeO
         and the formula for hematite is Fe O .
Tro: Chemistry: A Molecular Approach, 2/e 18
             Structure of Atoms
• The Players found in atom
  – Electrons are negatively charged particles. Discovered by J.J.
    Thompson, who also proposed model of atom (Plum Pudding
    Model of Atom)
  – Protons are positively charged particles. Much larger than
    electron in mass (about 1800 times larger)
  – Neutrons are neutral (no charge).


• Atoms have an overall neutral charge.
             Structure of Atoms
• Two Main Structures found in atom

  – Nucleus is small, positively charged central core where protons
    and neutrons reside. Most of mass in atom found here.
    Discovered by E. Rutherford who disproved Thompson Model of
    Atom (Gold Foil Experiment)


  – Electron Cloud is external to nucleus and is where electrons are
    found. Electrons are negatively charged.

				
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