Chapter 2 Atoms_ Molecules_ and Ions

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					Chem 121 Chapters 2 Atoms, Molecules and Ions (and Compounds)

Note-information on writing formulas and naming compounds is contained in the week 1
laboratory assignment and will not be covered in lecture.

1. Atomic Theory of Matter
Daltons Atomic Theory is based on the ideas of the Greek philosopher Democritus and three
―Natural Laws‖ of science.
       –   Democritus- believed that matter could not be subdivided infinitely, but only
           subdivided to small indivisible particles that he called ―atomos‖ meaning not
       –   Law of Conservation of Mass (Lavosier)- Matter is neither created nor destroyed in a
           chemical reaction.
       –   Law of Definite Proportions (Proust) a.k.a. ―the law of constant compostion‖- The
           relative number and kind of atoms in a compound is always the same.
       –   Law of Multiple Proportions (Dalton)- When elements form more than one
           compound, the atom ratio differs. (Different compounds have different atom ratios)

Daltons Atomic Theory
       –   1. An element is composed of tiny particles called atoms
       –   2. All atoms of a given element have the same chemical properties.
       –   3. Atoms are neither created nor destroyed nor changed into other atoms in normal
           chemical reactions.
       –   4. Compounds contain atoms of two or more elements in a fixed ratio.
       –   5. When elements form more than one kind of compound, the atom ratio differs.
           Different compounds have different atom ratios.

2. The Discovery of Atomic Structure
Rutherford, Thomson, Curie’s, Chadwick, Dalton and others contributed to the modern model.

Rutherford Model of the Atom-a dense nucleus surrounded by a very diffuse cloud of electrons.
       –   Nucleus- dense central core of an atom that consists of protons and neutrons.
       –   electron cloud- large volume around the nucleus in which the electrons move about.

3. The Modern View of Atomic Structure Characteristics of the Fundamental Subatomic

Particle          Symbol            Charge             Mass              Location
proton            p+ or H+          +1                 1 amu             Inside nucleus
neutron           n                 0                  1 amu             Inside nucleus
electron          e-                -1                 1/1836 amu        Outside nucleus

Ch 121 Ch 2,--F08—rt                                                              pg. 1/4
Atomic Weight- the relative mass of an average atom of the element expressed in atomic mass
Atomic Mass Unit (amu)- A unit used to express the relative mass or atomic weight of an
element. The mass of 1 proton or 1 neutron is equal to 1 amu. The mass of an electron is
negligible by comparison (1/1836). It would take 1836 electrons to equal the mass of one amu.
Isotopes: Notation for elements zAE
           –   atomic number, Z.
                         • Represents the number of protons

           –   Mass number, A.
                        • Represents the number of protons plus the number of neutrons.
Number of neutrons= A-Z.

Examples: Fill in the blanks in the following table:
Atom     name             Z       A          protons         neutrons          electrons
30 Zn                     30      64

63    Eu                            153        63

                                                             6                 6

                             92                              143

isotopes and atomic weights
           –   isotope percent abundance- the percent abundance represents the number of atoms of
               that particular isotope out of a sample of 100 atoms.
           –   average mass- the weighted average of all isotopes of an element.

To calculate average mass for an element:
   1. Take the mass of each isotope x % abundance/100
   2. add all isotope values together.

Example: Calculate the atomic weight of boron, B, from the following data:
ISOTOPE                       ISOTOPIC MASS (amu)            FRACTIONAL
                                                             ABUNDANCE (%/100)
B-10                          10.013                         0.1978
B-11                          11.009                         0.8022
                              Average =
4. The Periodic Table
Overhead of periodic table and define groups, periods, metals, nonmetals etc. Handout table.

Ch 121 Ch 2,--F08—rt                                                                 pg. 2/4
5. Molecules and Ions
Molecules and Chemical Formulas
     Compound Formulas-notation for a compound, consisting of the symbols of the atoms
       found in the molecule/formula unit. Atoms present in numbers greater than 1 have the
       numbers indicated by a subscript.
          o Example- phosphoric acid consists of 3 atoms of hydrogen chemically bonded to
              1 phosphorus atom and 4 oxygen atoms. Write the chemical formula for one
              molecule of phosphoric acid.

      A molecule is a definite group of atoms that are chemically bonded together – that is,
       tightly connected by attractive forces.
      A molecular substance is a substance that is composed of molecules, all of which are
      A molecular formula gives the exact number of atoms of elements in a molecule.
      Structural formulas show how the atoms are bonded to one another in a molecule.
      An important class of molecular substances that contain carbon is the organic
           o Organic compounds make up the majority of all known compounds.
           o The simplest organic compounds are hydrocarbons, or compounds containing
               only hydrogen and carbon.
           o Common examples include methane, CH4, ethane, C2H6, and propane, C3H8.

Examples: Identify the name and number of each kind of atom in the following compounds
a. water (dihydrogen oxide), H2O

b. potassium selenide, K2S

c. sulfur trioxide, SO3

d. nitrophenol, C6H5NO3

Ion - A positive or negative charged particle. My be composed of one or more atoms, and has
lost or gained electrons to have a +x or –x charge
       –   negative ions are called anions. Most non-metal atoms tend to gain electrons to
           become negatively charged.
               »   example: O + 2e- ==> O-2 , the oxide ion ,
               »             F + e- ==> F- , the fluoride ion.
       –   positive ions are called cations. Most metal atoms will lose electrons to become
           positively charged.
               »   example: Al ==> Al+3 + 3e- , the aluminum ion

Ch 121 Ch 2,--F08—rt                                                               pg. 3/4
               »              Na ==> Na+ + e- , the sodium ion
       –   Polyatomic ions - ions consisting of more than one atom bonded together as in a
           molecule, but with a net positive or negative charge.

example: PO4-3 , phosphate ion

Predicting Ionic Charge
       –   Most atoms will try to form ions so that they have the same number of electrons as
           the ―Noble Gases‖. This allows one to predict the charge the ion of an atom will

Use the atom’s position on the Periodic Table to predict the charge the ion will take when
forming a compound.

       Mg  __________               K ________
       S  ___________               N ________

6. Molecular, Empirical and Structural Formulas
       –   Molecular - A symbol for the molecule of a compound, consisting of the symbols of
           the atoms found in the molecule. Atoms present in numbers greater than 1 have the
           numbers indicated by a subscript. The TRUE atom ratio in a compound.
       –   Empirical - the SIMPLEST atom ratio in a compound.
       –   Structural - a formula which shows the TRUE atom ratio AND the
           ARRANGEMENT of atoms in a compound.

       –    Although many substances are molecular, others are composed of ions.

Ch 121 Ch 2,--F08—rt                                                               pg. 4/4