edited by JAMES P. BlRK Arizona State University Tempe. AZ 85287
Designing a Periodic Table A Laboratory Approach Mary E. Irons 4623 Huron Court, Midland. MI 48640 In a first-vear chemistrv course it is difficult to find a laboratory enercise that r o k p o n d s well with a unit on the periodic table. In our course we have used both the Holt text Modern Chemistry and Basic Chemistry by Seese and Daube. In both of these texts, the chapter on the periodic table follows atomic structure and precedes chemical bonding. Thus, the material on the periodic table occurs fairly earlv in the course. before the students have ~roficientlaboratury skills or sufficient chemiral knowledge to rarry out a so~histicatedlaboratorv exercise. What follows is a lahoratoiy approach to help-students gain some insight to the relationships of the elements on the table and also to help them review the scientific method. The students are shown 12 elements labeled with letters of the alphabet and are given information regarding atomic radii and ionization energies. Depending on the time and equipment available, information on electrical conductivity may be determined by the studentsor given to them directly. (See table.) The elements that we have used include, but are by no means limited to, sodium, potassium, magnesium, calcium, iron, copper, zinc, sulfur, iodine, tin, lead, and carbon. I t should be noted that the information on atomic
radii and ionization energies is slightly distorted to discourage the students from identifying the elements from a textbook. The purpose of this procedure is not to identify the elements, but to organize them based on their physical and chemical properties. The first step in the organizing process is for the student t o distinguish the metals from the nonmetals. I t is emphasized that metals must conduct electricity and have a metallic luster. Students are encouraged t o clean the hard elements with a piece of steel wool and t o cut the soft ones to expose the metallic surface. Metalloids are grouped with the nonmetals in this procedure, and once the nonmetals have been identified they are no longer considered in the experiment. Each metal is then tested for reactivity with water. I t is noted for the student on the chart that magnesium reacts withsteamand should beincluded as a water-reactive metal. The instructor must not permit the students actually to test sodium and potassium, as these highly reactive metals could cause student injury. Students may also try to react all of the metals with acid later in the experiment, and i t would be very dangerous to react sodium and potassium with acid. At
Information and Data Sheet for Students
Element Appearance
A
B
(c
Ca
C Cu
D Fe
E K
MQ
G Na
H Sn
F
I S
J Zn
K I
L Pb)
+
+
+
+
+
+
+
t
-
+
-
t
Atomic Radius (Angstroms)
0.8
1.7
1.2
1.2
2.0
1.4
1.5
1.4
1.0
1.3
1.3
1.3
Ionization Energy (Kcellmol)
260
140
180
180
100
180
120
170
240
220
240
170
Conductivity
Water Rxn
N&P04 Rxn HCI Rxn
Volume 66 Number 2
February 1989
155
this point, there are three groups of elements: nonmetals, metals reacting with water, and metals not reacting with water. Students are next instructed t o obtain salt solutions of those metals that reacted with water. The instructor must prepare these solutions, using a soluble salt of the metal. Solutions concentrations of 0.1 M are sufficient for this procedure. Solution bottles should be labeled with the letter of the corresoondine metal. The solutions are tested with a 0.1 M solution of s o h m phosphate. The group I solutions willnot produce aprecipitate, whereas the group I1 solutions will. This procedure will enable the students to divide further the metals that reacted with water. At this point, if the 12 elements listed aboveareused, then the nonmetals would include carbon, iodine, and sulfur: the metals reacting with water and producing a phosphateprec i ~ i t a t ewould include mamesium and calcium: and sodium add potassium would fall &to the category of reacting with water but not with the sodium phosphate solution. The remaining five metals are then tested with 6 M hydrochloric acid. All but the copper will react; however, the tin and the lead may have to be gently heated to observe the reactions. Concentrated acid could be used for more readily ohservable reactions. Aluminum or nickel could also be used in place of tin or lead. I t should also be noted here that metal e oxides will also react with the acid and mav ~ r o d u c confusing results. I t is therefore important t o st& with untarnished pieces. After the laboratory information has been gathered, students are instructed to organize the elements based on the physical and chemical properties they have observed in the laboratory exercise. They are encouraged to develop their own design, grouping elements with similar physical and chemical properties. Consistency as far as the ordering of elements according to radii and ionization energies is strongly encouraged. Students eniov this exercise. as it ekes them a chance actually to seeand work with &any of ihe elements that are discussed in class. As a follow-up activity, it is interesting to
156
Journal of Chemical Education
have the students try to identify the 12 unknowns and to compare their designs to an actual periodic table. I t is very challenging for first-year students to organize and tabulate such a large amount of experimental data. Further to insure success with this experiment, it must he emphasized that it is very important to follow the directions carefully. Sometimes students will attempt to test all of the elements with water and all of the metals with acid. Not only does this lead to confusing data, it has the potential to be dangerous. Sodium and potassium do react violently with acid. The following flow chart should help to eliminate any confusion.
No Rxn with N ~ ~ P \ O I In the tahle. the actual elements are indicated to heln the reader; this information would not be given to the experb menter. Usine the electromotive serles and asolubilitv chart, this type of experiment could be designed around-any 12 elements. Although more elements could also be used, that would require more than the 90 minutes this exercise generally takes.
