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Unformatted text preview: Contact # Atoms/Unit Cell Coord. No. Relation between r and a Simple cubic Edge 1 6 1 2 r a = Body-Centered Body Diagonal 2 8 3 4 r a = Face-Centered Face Diagonal 4 12 1 8 r a = solid mass of unit cell volume of unit cell d = . The mass of the unit cell will be equal to the number of atoms in the unit cell times the mass of a single atom. We saw earlier that simple cubic structures have 1 atom per unit cell; body-centered have 2 atoms per unit cell and face-centered have 4 atoms per unit cell. The mass of a single atom can be determined from the atomic weight of the atom. Recall that the atomic weight is the weight in grams of 1 mole of the substance and that 1 mole of a substance contains 6.022 10 23 atoms. Putting these ideas together, we get the following formula for the density of a solid: ( 29 23 solid 3 # atoms per unit cell AW/6.022 10 d a p = ....
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- Spring '07