Hello, See question 7.4 from the link. http://books.google.com/books?id=0N...CfU3U1qlbfJCRXsRow1xzu7hxEWOjYCow&w=685&w=800 "Assuming that atoms are hard elastic spheres, show that Poisson's ratio for a close-packed array of spheres is 1/3" I am having trouble explaining the proof for this. I know the that the volume modulus, K, = E(elastic modulus) / ((3(1-2[itex]\upsilon[/itex])) where [itex]\upsilon[/itex] is the elastic modulus. K = E / (3(1-2[itex]\upsilon[/itex])) When [itex]\upsilon[/itex] = 1/3, K=E. I'm thinking that since for a hexagonal close packed structure, HCP, the angles between lattice sites is 120°, or 1/3 of the plane of a full crystal structure. Refer to: http://www.science.uwaterloo.ca/~cchieh/cact/fig/hcp.gif http://www.chem.ufl.edu/~itl/2045/lectures/h1.GIF [Broken] Therefore the elastic properties for a given volume is split in thirds? It seems like a misleading argument, but I can't find a way to explain it with math!