The Crystal Structure of Molybdenite

[Ed note: This is the first in a six part series of posts featuring models of crystal structures solved by Linus Pauling during the early years of his career.  The models were built by two undergraduate students working in the OSU Libraries Special Collections.  More about their work can be found on page 10 of this PDF link.]

Molybdenite model, side view.

Molybdenite, MoS2 (molybdenum = pink; sulfur = yellow).  A hexagonal crystal system constructed of molybdenum ions bonded to two layers of sulfur atoms through ionic bonding. The sulfur layers do not present strong bonds with other sulfur layers, which creates perfect cleavage.

After moving to Pasadena to begin his graduate studies in 1922, it was decided that Linus Pauling would begin working as Roscoe Dickinson’s sole graduate student. Dickinson received the first doctorate from the California Institute of Technology in 1920, having joined the staff there after completing his undergraduate studies at M.I.T. in 1917.

In late September of 1922, Pauling and Dickinson were working on structure determinations of crystals with the use of X-ray diffraction, and Pauling was initially encouraged to determine the structure of the lithium hydride crystal. However, after three weeks of synthesizing the crystals and setting up his photographic apparatus, he was forced to abandon his work upon receiving word that the structure had recently been determined in Holland.

Pauling then created crystals of fifteen different organic substances. He subjected some of them to preliminary stages of X-ray analysis, but none of the samples proved sufficient for the work. After these disappointments, Dickinson helped Pauling work through the process of determining the complete structure of the molybdenite crystal.

Molybdenite, or molybdenum sulfide, is a very soft metallic mineral. Its properties include a bluish-silver color and a greasy feel that can leave marks on fingers. It has a very high melting point, so it is often alloyed with steel to make it stronger and more heat resistant. It is also an important material for the chemical and lubricant industries, and can be used as a catalyst in some chemical applications.

Molybdenite model, top view

Molybdenite crystals bend easily but are not elastic, making X-ray spectral photo analysis, the method used by Pauling and Dickinson, somewhat difficult. Despite these issues, Pauling and Dickinson managed to take a suitable photograph and were able to determine the crystal’s structure. Molybdenite was expected to have an octahedral atomic structure. Pauling and Dickinson discovered that it was instead comprised of six atoms surrounding the corners of a trigonal prism. This surprise demonstrated, among other things, the potential for unpredictability and excitement in chemical experimentation.

Fresh off their laboratory success, Pauling co-authored his first scientific paper with Dickinson. It appeared in a 1923 issue of the Journal of the American Chemical Society under the title “The Crystal Structure of Molybdenite.” The success renewed Pauling’s confidence in his capacity to carry out professional scientific analysis, and instilled in him an understanding of the value of well-planned experimentation. The process likewise provided him with a valuable framework that he began shaping and developing to fit his particular temperament. Throughout the rest of the 1920s and 1930s, he would use this method to write and publish multiple crystal structure papers nearly every year.

Pauling notebook entry documenting work on molybdenite, 1922.

Much of Pauling’s early work on the structure of molybdenite is detailed in his Research Notebook 2.  For more on his amazing achievements in structural chemistry, see the website Linus Pauling and the Nature of the Chemical Bond: A Documentary History.

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