In: Chemistry
When spheres of radius r are packed in a simple cubic arrangement, they occupy 52.0 % of the available volume. Use the fraction of occupied volume to calculate the value of a, the length of the edge of the cube in terms of r.
In a simple cubic arrangement, the vertices of the cube coincides with the center of the spheres.
There are 8 spheres on the 8 vertices of the cube. Hence, only 1/8 th of each sphere lies inside the cube.
Volume of each sphere of radius r is .
Hence, 1/8 the of the volume of 8 spheres that lies inside the cube must equal the volume of 1 complete sphere.
Hence, the volume of all the spheres that lies inside the cube must be .
Now, the volume of the cube with edge length a is .
It is given that only 52% of the cube is occupied by the spheres.
Hence,
We can easily solve for the edge length a in terms of r from the above relation as shown below.
Hence, the relation between the edge length a and radius of sphere r is .