NCERT Solutions for Class 12 Chemistry Packing and Packing efficiency in Solids
NCERT Chemistry Class 12 – Packing and Packing efficiency in Solids
- Matters exist in the solid state because of close packing of their constituent particles.
- The constituent particles try to get closely packed together so that the maximum closely packed structure is attained.
- The maximum closely packed structure is such that the minimum empty space is left.
- Due to this arrangement, a maximum possible density of the crystal is achieved.
- Close packing imparts stability to the crystal.
We know that there are 7 crystal systems and 14 types of unit cells that occur in nature. The solid states of the majority of metallic elements, some covalent compounds, and many ionic compounds occur as cubic lattices. There are three types of cubic unit cells within the cubic system.
Let’s pack the three cubic unit cells and the hexagonal unit cell and also determine their packing efficiency. The packing efficiency is the percentage of the total volume occupied by the spheres themselves. We already know that unit cells result from the way the atoms pack together in a crystal lattice.
- The simple cubic unit cell: When we arrange the first layer of spheres (imagine particles of the crystals to be identical spheres) in vertical and horizontal rows, then large diamond-shaped spaces are formed. If we place the second layer of spheres directly above the first layer, we obtain the arrangement based on the simple cubic unit cell. The spheres occupy only 52% of the unit cell volume, so 48% of the empty space is left. Hence it is a very inefficient way of packing of particles.
- The body-centered cubic unit cell (bcc): If we place the second layer of spheres over the diamond-shaped spaces created, rather than directly above the first layer, than the packing will be more efficient. The third layer is placed on the diamond-shaped space formed in the second layer. Thus the first and the third layer line up vertically. This arrangement gives body-centered cubic unit cells. This gives the packing efficiency of 68%, which is higher than the simple cubic unit cell arrangement. Several metallic elements, including chromium, iron, and all the elements of Group 1A of Periodic Table, have this arrangement.
CBSE sample papers Class 12 are available, at Takshilalearning.
- Face-centered unit cells and Hexagonal unit cells: The spheres are arranged most efficiently in these cells. The spheres in the first layer are arranged such that triangular-shaped spaces are formed, which are smaller than the diamond-shaped Then we place the second layer over these spaces. In the second layer, some spaces lie above the spheres in the first layer, whereas some spaces lie above the spaces in the first layer. The third layer can be arranged in either of the two ways-
- Hexagonal unit cells: If the third layer of spheres is placed on to the spaces of the second layer that lies just above the spheres of the first layer, then the spheres of the third layer are exactly over the spheres in the first layer. The third layer has the same arrangement of spheres as the first layer. Since the structure repeats itself after every two layers, the stacking is described as (abab…) layering pattern, and we get Hexagonal closed packing(hcp) which is based on the hexagonal unit cell. In this, the atoms occupy 74% of space.
- Face Centered Unit Cells: If we place the third layer of spheres, over the spaces of the second layer that lies just above the spaces of the first layer, then the stacking of spheres in the third layer will be different from the first layer. Thus the pattern obtained will be abcabc… This is the cubic closed packing (ccp), and it is based on the face-centered cubic unit cell.
The packing efficiency of both hexagonal and cubic closest/ closed packing is 74% and the coordination number of both is 12. Most metallic elements crystallize in either of these two arrangements only. Magnesium, Titanium, and Zinc adopt the hexagonal structure; while Nickel, Copper and Lead adopt the Cubic structure. Many ionic compounds and other such as CO2, CH4(methane) and most noble gases also adopt cubic structure.
For more Chemistry notes for Class 12
Visit our website www.takshilalearning for CBSE guide, CBSE exam pattern, CBSE sample papers, and Chemistry notes of Class 12.
Takshila Learning offers live classes and offline classes for School courses. Online/offline classes for Professional courses and Skill development courses are also available. Kindly visit www.takshilalearning.com
Follow us on a Social media
Call us: 8800999280/83/84 or fill the form for any other details: