CHEMISTRY
TEST PAPER -2
1.Differentiate Tetrahedral voids and Octahedral voids(2)
2.Calculate the packing efficiency of face centered cubic structure(3)
3.Compare square close packing and hexagonal closepacking? (3)
4. Explain 3dimensional close packing from 2-D square close packing(2) an the following question correctly 20 points
Answers
Answer:
Tetrahedral void is a simple triangular void in a crystal and is surrounded by four spheres arranged tetrahedrally around it. On the other hand, an octahedral void is a double triangular void with one triangle vertex upwards and the other triangle vertex downwards and is surrounded by six spheres.
The packing efficiency of both types of close packed structure is 74%, i.e. 74% of the space in hcp and ccp is filled. The hcp and ccp structure are equally efficient; in terms of packing. The packing efficiency of simple cubic lattice is 52.4%. And the packing efficiency of body centered cubic lattice (bcc) is 68%.
Cubic close packing is an arrangement of spheres in a lattice; there are three layers of spheres placed one on the other, covering all the octahedral holes by a third layer of spheres. Hexagonal close packing has tetrahedral and octahedral holes.
CLOSE PACKING IN 3-D FROM 2-D SQUARE CLOSED PACKED LAYERS - DEFINITION
Three-dimensional close packing from two-dimensional square close packing: To build a three-dimensional structure, it is easier to stack two-dimensional square close packed planes one above the other. The spheres are aligned horizontally as well as vertically. If the arrangement of spheres in the first layer is considered to be of A type, then the arrangement of spheres in the subsequent layers is also of A type. This three-dimensional arrangement is referred to as AAA type packing.