Scarrow structures

Rob Scarrow's Web Pages about Crystal Structures

Graphical introduction to unit cells in crystals

This page uses animated gif files to teach about the translational symmetry found in crystals, and to explain the meaning of "unit cell" and how to determine the elemental composition of the unit cell.

Structures of selected ionic and metallic solids (requires CHIME plug-in; for more info click here)

Click on the title above to go to a web page allowing you to visuallize the structure of iron (body-centered cubic), sodium chloride, cesium chloride, and some more complicated ionic structures. Each page contains buttons allowing you to select various pre-set views including showing the unit cell. You must click on the first button on the right side of each page before the structure will show up.

Structures of molecular solids (requires Rasmol [see below] or CHIME [for more info click here])

It is common for chemists and biologists to show pictures of molecules in ball and stick or wireframe views based on structures obtained from X-ray crystallography. It is less common for them to show pictures of how the molecules pack together to form a semi-infinite lattice of repeating unit cells. Clicking on the title above will allow you to compare views of a coordination complex showing a single molecule with another view showing roughly 50 molecules packed together in the semi-infinite lattice.

Structures of elements (requires CHIME plug-in; for more info click here)

I recommend the WebElements.com site. You can browse and compare the structures and properties of the different elements. Here is a page I use with quick links to show my Gen Chem class (but if the links are outdated, just go to the WebElements.com site).

Relevant to experiments 17-19 from the Chem 101 lab manual, here are several different 3-D structures of the [CoCl₂(en)₂]⁺ cation. These may be viewed with using the Chime plug-in (for more info click here) or the stand-alone program Rasmol (see below).

The cation of trans-[CoCl₂(en)₂]Cl•2 H₂O • HCl, taken from a neutron diffraction study. Neutron diffraction is similar to X-ray diffraction, but is better able to locate hydrogen atoms. The structure was found in the Cambridge Structural Database (click here for information on how to access this database if you are a Haverford student or staff). The journal reference is Roziere, J.; Williams, J. M. Inorg.Chem., 1976, 15, 1174.
A crystal structure of a different salt of the same cation but with a different anion; this salt is trans-[CoCl₂(en)₂]₂[CoCl₄]. The reference for this structure (also taken from the Cambridge Structural Database) is Schubert,U.; Zimmer-Gasser, B.; Dash, K. C.; Chaudhury, G. R. Cryst.Struct.Commun., 1981, 10, 239.
Lamda and Delta enantiomers of the cis-[CoCl₂(en)₂] cation. (Entries CENCOC and CLECOC in the Cambridge Stuctural Database).

Here are a few structure other files maintained locally (may be viewed with Chime or Rasmol (see below)):

The iron site of nitrile hydratase, the above-mentioned enzyme. The hydrogen atoms are omitted for clarity.
A couple of extended lattices. Because the distances between atoms in these are more than the sum of covalent radii, RASMOL does not find bonds. Thus you will see a blank screen upon loading the file. Change to Ball and Stick view in order to see the atoms as small balls. Simple cubic polonium and cesium chloride are available. Each view shows a 3 x 3 x 3 array of unit cells.