The images on this page show a form of copper oxide called cuprite ? Cu₂O. Created by researchers at Arizona State University, they’ve given us our clearest view yet of the electronic bonds that hold atoms together.

Images Courtesy of Jian-Min Zuo, Miyoung Kim, Michael O’Keefe, John Spence, and Arizona State University

What you see in the pictures aren’t the electrons themselves ? they’re too small and move much too quickly ? but electron clouds, or orbitals. These clouds represent the areas where electrons are most likely to occur. The orbitals take on a variety of shapes, including doughnuts, dumbbells, and butterfly wings; the exact size and shape depends on such properties as the energy level of the electrons. When atoms interact with each other and form bonds, subtle changes in the orbitals result.

Atoms may be held together by covalent bonds, in which electrons are closely shared between the two atoms, or ionic bonds, in which one atom loses electrons to another. Because cuprite has an unusual structure, some scientists had proposed that both covalent and ionic bonds occur in this compound. The new images provide the first clear evidence to support this controversial hypothesis. Even more surprising, the images show faint regions where covalent bonds occur between copper atoms. Previously, researchers had seen this bonding only between copper and oxygen. According to J.C.H. Spence, one of Arizona scientists, it is this discovery that is “likely to make them rewrite the chemistry textbooks.” (1)

Spence and his colleagues produced the images by a complicated process that combines two techniques: X-ray diffraction and electron diffraction. With X-ray diffraction, X rays are beamed toward atoms in a crystal lattice and scatter when they come in contact with the nuclei of the atoms. The atomic structure can be determined from the pattern of the scattered rays. Electron diffraction works much the same way, except that electron beams are scattered by orbiting electrons. By creating a composite of images using the two techniques, the researchers were able to show both the atoms and the bonds holding them together.

Why are scientists excited about this new imaging technique? It may help them understand the atomic structure a class of substances known as high-temperature superconductors. Most superconductors ? substances that can conduct electricity without resistance ? function only at extremely cold temperatures approaching absolute zero. Cooling substances to such low temperatures is difficult and expensive. But some become superconducting at temperatures up to 140°F higher. Among these compounds are cuprates, copper oxides very similar to the cuprite used in this study.

How high-temperature superconductors work has been one of the great mysteries in materials science. Understanding the nature of the atomic bonds in these compounds could help scientists solve this mystery. Cuprates and other high-temperature superconductors may become a cornerstone of 21st century technology, with potential for use in high-efficiency motors, transmission lines, and other applications. (2)

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