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# Trigonal Hole

The display below shows three atoms packed together in a closest-packed arrangement. In the very center of this structure is a hole. This hole is said to be a trigonal hole, because the hole is surrounded by three atoms (the coordination number is three).

Take note that the size of this hole is quite small. If an atom is sufficiently small, it can fit into this hole. Click on the "Show Atom in Hole" to insert a small atom (colored red) in the hole.

What is the largest atom that can fit into the hole without pushing the outer (blue) atoms apart?

The radius of the outer atoms is designated r. The radius of the hole (this is the radius of the largest sphere that can fit in the hole) is represented by rhole.

What is the ratio rhole/r?

Derive this ratio using the triangular geometry of the outer (blue) atoms and the laws of trigonometry.

Hint: Begin by drawing an equilateral triangle with sides of length 2r. Recall that the sum of the angles around a specific point, such as the center of the hole site, equals 360o. Finally, remember the definitions of the various trigonmetric functions.

This virtual reality display requires Java3D. If the display is not visible, consult the Java3D FAQ. Dragging with the left mouse button rotates the display.

Holes in Closest-Packed Structures:     Trigonal     Tetrahedral     Octahedral     Cubic