Kent R. Logan George C. Marshall High School 7731 Leesburg Pike Falls Church, Virginia 22043
I
In the beginning stages of chemistry, the task of familiarizing oneself with the characteristics of the manv elements wouid indeed seem formidable were it not for the existence of chemical periodicity. This concept, primarily an outgrowth of Dimitri Mendeleev's work, lent order to understanding the elements by placing them in rows (periods) according to their atomic mass and in columns (families) by similarities. A fashionable yet scientific approach in accomplishing greater understanding of chemical periodicity is the utilization of laboratory facilities to (1)provide active participation and controlled interaction with selected variables and (2) offer first-hand experience from the physical world. One way to introduce Mendeleev's Periodic Law and simultaneously provide experience in examining pure elem e n t s - ~ e r h a ~ son a first time basis for some-is to trv the following Iatr)rator,v exercise or one similar to it. An exnmple used hs ms classes included six chemical families and six or o p t i o n i ~ ~ i s e v unknowns en depending on the degree of difficultv desired. ~ i e m e n t s both , known and unknown, were examined in sealed glass vials and their appropriate data recorded off 3 X 5 index cards. For efficiency, samples were divided into seven areas, and students rotated from station to station. A reeular periodic table was then consulted to accurately place the known elements in their correct position by atomic number. Data of the knowns was studied to see thesimilarities as well as trends in behavior. At this point, students were ready to place the unknowns, according to their properties, in the correct family. After further scrutiny, students then went on to locate the unknowns exactly within the perspective family. From this exoeriment. students gained some familiaritv with certain elements, but mure importmtly, recognized the existetiae of ordcr and loeic to the ~eri~gdic tahle as exem~lified by Mendeleev's law. he experiment was designed tocreate a "feeling" of the work and contribution of Mendeleev as well.
Mendeleev's Periodic Law A laboratory exercise
Sample data listings can he referred to in the tahle. Examples of the identifications follow. Unknown 5: Group IB is the easiest to solve since gold is the only element missing from the family. Color, conductivity, and density correlation lead to quick identification. Unknown 3: Solubility places #3 immediately in group I. Physical state eliminates hydrogen. From the periodic tahle, lack of radioactivity would rule out Fr. Unknown 3 is either Rb or Cs. Thesequenceof densities fail toestablishan accurate prediction. Examination of the melting points place 3 as Cs. The difference between Li and Na melting points is 82'C. The difference between Na and K melting points is 35%. The difference between K and 3 is 34°C. These differences areat least positive eorrelations except that the expected difference for the next element in group I should be around 20%. The next difference between Rh and Cs would be expected to be around 10°C.Since twodifferences are required (20 + 10) to approximate the 3 4 T difference, two elements must be between K and the unknown. Unknown 2: As a gas, group VII or VlII is mnsidered. Color suggests prohibiting this unknown from group VIII. Mp comelates well as do conductivities. Unknown 4: Colorless gas is highly suggestive ofgroup VIII. Melting points lead to Kr. Unknown 6: The physical state offers the first hint of a family I1 chloride salt. The densities offer some help, hut the known CaCIz is slightly lower in density whereas it would be expected to be slightly higher. Even so, unknown six's density fits well between CaC12 and BaCI2. A look at the melting points correlates very well, with an increase. Unknowns 1 These are the only ones with conductivities and 7 rated "fair to poor". Semiconductors are suspected. For bath unknowns in lump form, their chunky appearance resembles coal except for a slight shiny almost metallic 11wklngsurface for I with n m w e pronounced shmr on 7. A smilnrity in crysrnl chunk form ro lumpv rarhon calls for further pnhng. Vnknuwn 1 ' 9 m.p. could in.
Volume 53.Number 10, October 1976 / 847
Sample Data Listing
PhyIical rtate density hardness CondUCtiVitY melting point solubility (H,O) Color physical state density hardnePI Conductivity melting point IOlubilitY (H*Ol
physical rtate density hardness COndUCtiYitY melting point solubility (H,O) Color physical state density hardness conductivity melting paint rolubility (H,Ol Color
~i(in mineral oil) (observe) 0.534 g/cc soft, claylike good
laoOc
exploder (observe) Na (in oil) (observe) 0.971 glee soft, ciaylike good
98°C exploder (observe) K (in oil)
(observe)
0.86 glcc
soft, ciaylike good
2.32-2.33
CU (obrervel 8.96 glmi somewhat roft excellent
He (Observe) 0.18 911
708'~
1083°C none
-269'~ none
M9Ci2 (obrervel
glee brittle eseentialiy none
good (observe)
CaCll (obrervel 2.15 gjml brittle
none 772°C good (observe) BaCI, (obrerve) 3-85 g l m l brittle
925-c
exploder (obrerve) unknown 1 (observe) 2.33 g/mi brittle intermediate t o poor
good (observe) unknown 2 gas 1.70 g l l
1410CC none (obrerve) unknown 6 lobrerve) 3.05 glml brittle
(obrervel
yeilowlgreen
A9 (obrerve) 10.49 glml somewhat Soft excellent-belt known
7.30 glml
Br2 lobrerve) 3.12 glml
somewhat soft good
Yew p o r
961'~
232'~
-7.2
none
negligible (observe)
none or negligible (observe)
none
64'~
...
very POOI
...
very poor
(observe)
(obrerve) Pb (observe) 11.36 glml Iomewhat r o f t good
327'~ none unknown 3 solid 1.90 g/ml soft good
-219.6'~
29°C
$light pale yellow unknown 7 (observe) 5.36 glml fairly brittle fair to poor
exploder violently gray
... C
11 (obrerve) glml soft very poor
4.94
113.7'~
very poor
negligible (obrerve) unknown 5 (observe) 19.32 g l m l somewhat soft excellent
-157.3'~ none
1063'~ none
(observe) unknown 4 (obrerve)
..
(observe)
(obrerve) Ne (obrerve) 0.90 911
...
very poor -248'~
none (obrerve) A, (observe) 1.78 911
.. .
very poor
- 1 ~ 9 . 4 ~ ~ none (obrerve)
xe (obrerve) 5.85 911
. ..
very w o r
-111.9~~ none lobrerve)
960'~
none
dicate a position of either Si or Ge, but its density places its position next to csrhan. Unknown 7's density and mp place it solidly as Ge.
This experiment is not without its limitations. Used early in chemistry, the experiment had the shortcoming of excluding chemical properties with which students would later become familiar (e.g., electronegativity, ionization energy, oxidation numbers, etc.). For safety reasons, hazardous chemicals such as fluorine. cesium, and chlorine were not used alldata was provided. FO; noble gas samples, air was used as a convenience. Furthermore, the difficulty of working with the properties of group I1 metals was avoided by using group I1 chlorides. In addition, ground rules were needed and conveyed such as: (1) Each family has a t least one unknown. (2) No radioactive elements are used. (3) Consultation of outside information is not in the spirit of simulating Mendeleev's ac-
648 / Journal of Chemical Education
complishment. (4) If more than one laboratory session is required, all recorded data remains until the next session. T o increase student involvement, many samples called for properties marked with "observe," Color and physical state observations built student confidence; whereas, conductivity measurements and accompanying ratings as "fair," "good," or "excellent" needed some explanations along with instructions on the correct usage of a volt-ohm meter. For instance, one might set a VOM on a 1-ohm scale and use the needle deflection as a crude conductance measurement. If the needle moves midway, use a rating as "fair" for conductivity. Three-frths movement could mean "good"; pinning the needle could stand for "excellent." Generally in spite of these drawbacks, I would recommend this type of experiment. My justification depends not upon educational henefit alone, hut rests on the enthusiasm and "element of excitement" this lahoratory approach has generated for students of chemistry.
