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Chemistry Review Worksheet
ANSWER PAGE

    First, let us review the Rules of Thumb that I gave in class.
        1.    The atomic number is the number of protons in the atom.
        2.    The atomic mass number is the number of protons + the number of neutrons
               in the atom.
        3.    The charge on the ion is = the number of protons - the number of electrons.
                Remember that this can be a - or a +, therefore you must perform the
                math as it is written.
        4.    Rule of Eight, or the Octet Rule.  Subtract 2, and if necessary subtract 8
                repeatedly until the answer is equal to or less than 8.
                This gives you the number of electrons in the outermost orbital (shell).
        5.    If there are 1-3 electrons in the outermost orbital, the atom will give them
                up.
        6.    If there are 5-7 electrons in the outermost orbital, the atom will take enough
               to fill its outer orbital to a total of 8 electrons.

    Now we can proceed.  Refer back to these Rules of Thumb as needed.

1. The atomic number of an element is the number of protons.

    Does this number ever change for atoms of the same element?
    Never!!!  An atom of a specific element will always have the
    same number of protons.

2. The atomic mass number of an element is the number of protons
    plus the number of neutrons.

    If this number changes for an atom of a specific element you have an
    isotope.   Has the number of protons or neutrons changed?
   

3. When an element exists in nature by itself, it must have a charge of
    zero.  It must be neutral without an electrical charge.

    When that charge changes to become either plus or minus, you have
    changed the number of electrons.

    If the charge becomes minus, you have more or less electrons than
    protons?

4. An atom has 7 protons, 7 electrons, and 8 neutrons. Fill in the blanks below for this atom:

    Atomic Number 7   Atomic mass: 15  Charge: 7 - 7 = 0

    This atom belongs to which element?  Nitrogen

5. An ion of oxygen has a charge of –2. Home many protons and electrons does the ion have?

  •     Protons the atomic number is 8 therefore there are 8 protons Electrons charge = # protons - # electrons

    •             -2 = 8 - X     X = 10

    6. An ion of potassium has a charge of +1. How many protons and electrons does this ion have?

  •     Protons  the atomic number is 19, therefore there are 19 protons
            Electrons  Charge = # protons - # electrons  +1 = 19 - X  X =18

    • 7. The mineral fluorite is made of fluorine and calcium. Use the periodic table of the elements to determine what charge each of these elements will take in order to form a compound.

  •     Charge on fluorine
        Atomic number is 9
  •         number of protons     = 9
  •         number of electrons  = 9
  •                                             - 2
                                                leaves  7
            therefore fluorine will take 1 electron giving it a new total of 9 + 1 = 10
  •         Charge = # protons - # electrons = 9 - 10 = - 1
  •  
  •  Charge on calcium 
        Atomic number is 20
            number of protons    = 20
  •         number of electrons = 20
  •                                           - 2
                                              - 8
                                              - 8
                                            leaves 2
            therefore calcium will give up 2 electrons giving it a new total of 20 - 2 = 18
            Charge = # protons - # electrons = 20 - 18 = +2

    •     Using this information, write a chemical formula for the compound, which makes up fluorite:

    How can we combine a +2 with  - 1 in order to get zero?  We must use two - 1's to equal + 2.  By convention, a standard used so that everyone understands, we list the positively charged ion first.

    CaF2

    8. The element oxygen will always take a charge (valence) of –2 in naturally occurring rocks and minerals. The chemical formula for the compound, which makes up the mineral quartz, is SiO2. What is the valence (charge) of silicon in this compound?

    There are two methods of solving this question.

    1.     Since oxygen always has a - 2 charge, then the the two oxygen ions will have a combined charge
            of - 4.
            Since quartz is stable in our environment, it must be electrically neutral.  Therefore Si has a 
            charge of = + 4.

    2.    The atomic number for Si is 14
                                 # protons        = 14
                                 # electrons     = 14
                                                        - 2
                                                        - 8
                                leaves us with 4 electrons in the outer orbital or a charge of 4.

    9. In some cases several atoms may combine to forma structure in which the electrical charge remains unbalanced. Such a structure is called a complex ion. One common complex ion is the carbonate ion, which consists of one carbon plus three oxygen atoms. Write the formula for the carbonate ion below, including the charge in the upper right:

    The combined three oxygen ions have a total charge of - 6    or [O]-6

    The atomic number of carbon is 6
                                            # protons        = 6
                                            # electrons     = 6
                                                                    - 2
                                            leaves us with 4 electrons in the outer orbital or a + 4 charge

    Therefore the charge on this complex ion is
        [C]+4 [O3]-6  =  [CO3]-2

    10. An atom has 6 protons, 7 neutrons, and 6 electrons. Fill in the blanks below for this atom:

                                 Atomic number  6  Atomic Mass  13  Charge  6 - 6 = 0

        This atom belongs to which element?  Carbon

    11. A calcium ion has a charge of +2. How many protons and electrons does it have?

  • Protons the atomic number is 20 therefore the # protons = 20
    Electrons  Charge = # protons - # electrons   +2 = 20 - X   X = 18

    • 12. Sulfur is an element, which may remain neutral, take on a positive charge, or take on a negative charge.

        If no oxygen is present, sulfur will remain neutral or take on a negative charge. If it takes on a negative charge, it becomes a sulfide ion, and fills its outer electron orbital (shell) with additional electrons. Use the periodic table to determine what the charge should be on this ion:

    The atomic number of sulfur is 16
                                    # protons       = 16
                                    # electrons    = 16
                                                          - 2
                                                          - 8
                                that leaves us with 6 electrons in the outer orbital   this ion needs 2 electrons
                                Charge = # protons - # electrons   = 16 - 18  = -2

        If oxygen is present, the sulfur will take a positive charge, and combine with the oxygen to produce a sulfate ion. If sulfur looses all of the electrons in its outer orbital (shell), what charge will it have?

    If it has 6 electrons in its outer orbital and loses all 6 then the charge is + 6

        The sulfur will combine with 4 oxygen atoms. Each oxygen atom will take on a charge of –2. What is the total charge on all of the oxygen atoms put together?

    Each oxygen ion has a charge of - 2.  The total of 4 ions together have a charge of - 8

        Now write the formula for the sulfate ion as a whole, with the net charge on the complex ion (radical) in the upper right:

                        [SO4]-2