Ion (Derstand) Bonding through Energy Level Diagrams

Turbov, Jane                  St. Viator High School
                              392-4050
                          

Objective(s):
Students will be able to:
1) Determine the number of valence electrons using energy level 
   diagrams
2) Explain why elements lose or gain electrons during ionic bonding   
   (in terms of noble gas stability)
3) Define an ion.
4) Write correct ion notation.
5) Describe some properties of ionic compounds
6) Develop a hypothesis which explains what happens when an ionic 
   compound dissolves.

Apparatus Needed:
2 movable energy level diagrams  
  2 poster boards (thin)           
  60 magnets (about 5 mm radius - available at Radio Shack)          
  2 pieces of (scrap) metal containing iron (about 55 cm x 65 cm)
1 conductivity tester
1 small table with legs of uneven length  (about 25 cm X 15 cm)
ionic compounds such as: NaCl  FeSO4  HCl(aq) CuCl2   K2CrO4   MgO   LiCl 
familiar compounds such as salt, sugar, iron supplements 

     Energy Level Diagram                     Unstable Table 
            - .___.__.                                              
             /        \ -                   ________________ 
         -  /  -.__.   \                   /______________ /|  
           /   /    \   \                 |_______________|/| 
          (   (  10+ )   ) -              | |             | |
        - (   (      )   )                |               | |
           \   \.__./-  /                 |                 |
            \          /                  |  
           - \.___.__./  -     Ne
+ = proton       -
- = electron

To make a movable energy level diagram, draw 3 concentric circles on 
poster board to represent the first 3 energy levels of any atom. (This 
demonstration is limited to the first 3 rows of the periodic table.)  
The magnets represent protons and electrons.   Spray-paint a set of 
magnets one color per charge and draw in +'s and -'s with a permanent 
marker.  All magnets are coordinated so that each of the same painted 
sides represent a particular pole, eg.  all protons on their painted 
sides are facing North.* 

Recommended Strategy:
Prepare solutions of the listed ionic compounds.  A few drops of HCl(aq)
will dissolve the MgO and LiCl.  Also included are distilled water and 
tap water.  The demonstration table is set up so that the two energy 
level diagrams are to one side and the solutions are to the other, 
along with familiar packages such as salt and sugar. 

The lesson (for a freshman physical science class) is developed as 
follows: the opener is the unstable table, which has difficulty 
standing up.  I ask for advice about how to rebuild the table.  The 
idea is for the students to get to the point that in its present state, 
the table is unstable.      

From the building of unstable tables, I move to the building of table 
salt.  The students are asked from what elements table salt is made, to 
determine the relative position of Na and Cl on the periodic table, and 
to determine the number of protons and electrons in each element using 
the periodic table.  A worksheet is passed out so that students can 
keep track of the gathered information.  We review energy levels of the 
atom, protons and electrons.  The movable energy level diagram is 
discussed. The repulsion of like charges and the attraction of opposite 
charges may be felt by students using the magnets (painted side only!)  
Placement of electrons at the correct energy levels of sodium and 
chlorine is done at this time by 2-4 volunteers.  When this is 
completed, it can be seen that sodium has 1 valence electron and sodium 
has 7.  Given that the elements will either gain or lose electrons, 
does it make sense for 7 electrons to move over to Na or for 1 electron 
to Cl?  (This can be compared to a group of friends sitting in the 
cafeteria at one table and another friend at a different table.  Who is 
more likely to move?)  Students are asked to name the elements which 
have the same number of electrons as the new "atoms."  Stability of 
noble gases and the tendency of elements to react in order to have a 
filled valence shell is brought out. 

Students are then asked to record starting and ending numbers of 
electrons and to determine the old and new charges.  The word "ion" is 
discussed and students then learn how to notate ions.  On the 
worksheet, a space for defining "ion" is provided.  Other examples, 
such as MgO or LiCl may be tried.  Students may construct their own 
energy diagrams out of felt, tacks, etc.  A more difficult example, 
such as MgCl2 may also be attempted.

The last part of the lesson moves to a property of ionic compounds in 
solution:  conductivity.  Students can see that a light bulb is lit 
when the circuit is completed with some piece of metal, such as a 
pliers with insulated handles. Then, various ionic solutions are tested 
to demonstrate that electricity is conducted.  Distilled water and tap 
water are compared.  Students are asked to predict the outcome.  Other 
solutions may be tested, such as sugar, to compare ionic and nonionic 
bonding.   After summarizing the lesson, the students are asked to form 
the hypothesis to the question, " What happens to an ionic compound 
when it dissolves?" 

* This setup is designed to also be used to demonstrate the movement of 
electrons which produces bright line spectra.  In this case, an energy 
diagram with 5 energy levels can be used.  A flannel board and felt 
circles may also be used instead of magnets and poster board.