Periodic Trends for the Entropy of Elements Travis Thorns 2301 Shady Willow Lane, Brentwood, CA 94513 lntroductlon The periodic table organizes the chemical and physical properties of the elements. Through its use, students can see how atomic radii, electronegativity, electron d i t y , and many other properties of matter change in a periodic fashion. However, entropy, a driving force for many reactions. has been irmored. This DaDer exdores the ~eriodic trends in the entropy of elements, using S%ZS values as cited from the CRC Handbook ( I ) . This article focuses on concepts understandable to high school and beginning chemists.
value. vibrational frequencies that contribute to the Sozgs Moving across the periodic table, the higher the coordination number of the element, the tighter the packing, and the lower the entropy. For this reason s2elements have the lowest entropy values of the s block elements. The same applies to s'd5 elements, and probably p3 elements. The exceptions to these rules are the Group VII elements. The s2d5electron configuration increases the entropy to unexpected levels. Since all orbitals are occupied (although not filled), they are not be as tightly packed as their nearest neighbors.
Data A graphical representation of entropy values organized by groups and periods is shown in the figure a s an aid to visualize the trends. Polyatomic and uon-solid elements are not included as their translational and rotational modes (which are more temperature-sensitive than lattice vibrations) would mask any periodic trend. Lanthanides and actinides were not included due to lack of data.
Litewrature Cited
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1.Weast. R. C. CRC Handbook of Chemlsiry and Physus, 60th ed.; CRC Resa: Boea Raton,FL, 1919-1960: pp D62-63
Entropy by Group and Period
Results and Conclusion Entropy values increase with period, and generally decrease towards the center of a period. Such a generalization may be suitable for a high school chemistry class. A description more suitable for college students of the trend within a period is that the entropy values of the elements reach a minimum at half-filed d and p orbitals, that is, s1d5and p3. This trend in the p block elements cannot be conclusively confirmed due to the number of p orbital polyatomic elements. The reasons for these trends has much to do with how the electronic configurations and atomic sizes allow the atoms to bind together. The farther down the periodic table, the greater the atomic radius, the heavier the atoms, and thus the lower the Plot of entropy by group and period
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Journal of Chemical Education
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