Ion (Derstand) Bonding through Energy Level Diagrams

Turbov, Jane St. Viator High School

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.
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