The arrangements of molecules, atoms or ions are called crystal structures . The packing of spheres is often used to model metal crystal structures. Atoms, molecules, and ions are too small, so we use balls and sticks to represent them. These models enable us to see the invisible world of atoms and molecules.
There are two types of closest packing, the hexagonal and the face-centered closest packing, and they are called hcp and fcc (or ccp) respectively. The hcp packing has an ABABAB... two-layer sequence, where A and B represent the location of the layers. In other word, the third layer is exactly above the first layer in hcp. The fcc packing has a three-layer sequence, ABCABC... rather than the two-layer sequence of the hcp. The two types of packing are shown below:
Since these structures have been discussed in Solids, work on the questions below.
Draw diagrams of closest packing and draw the polyhedra of neighbouring atoms. One each for the fcc and hcp type.
The covalent radius is half the covalent bond length between two atoms of the same element.
The volume of a sphere with radius r is 4/3 p r3. The chemical literature uses the unit Angstrom or Å (= 10-10 m). 1 Å = 100 pm.
You can almost guess Avogadro's number, but make sure you know how to calculate it from the given data.
Find the relationship: 2 * sqrt(2) * radius of Ca = edge.
mass = 4 * 40.08 /6.023e23 g
volume = (557e-10 cm)3
density = mass / volume.
Volume of cell = mass / density, and there are 4 atoms per unit cell.
Calculate the number of atoms per unit cell from the density and a cell edge of 408 pm.
Mass = 2 * 55.85/6.022e23 = 1.855e-22 gm
Volume = 2.36e-23 cm3
Density = ?
Iron transforms from a bcc structure to fcc structure at 910 C. The volume shrinks as the structure transforms from bcc to fcc, explain!