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Standard XII
Chemistry
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In an arrange...
Question

In an arrangement of type ABABA ....., identical atoms of first layer A and third layer A are joined by a line passing through their centres.
Choose the correct statement from the following.

A
No void is found on the line
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B
Only tetrahedral voids are found on the line
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C
Only octahedral voids are found on the line
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D
Equal number of tetrahedral and octahedral voids are found on the line
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Solution

The correct option is B Only tetrahedral voids are found on the line
3-D hexagonal close packing is formed from 2-D hexagonal closed packing. Here, the second row of spheres is placed in the interstices or depressions of the first-row spheres of 2-D hexagonal close packing. Only 50% void of one layer can be occupied by sphere of other layer (II layer). Then, sphere of third layer occupy those voids of II layer under which there are sphere of first layer. This leads to hexagonal closed packing.
This follows the pattern of ABABAB.....
The ABABAB.. arrangement results in hexagonal lattice.
In such an arrangment, spheres of third layer A occupy the tetrahedral void of the B layer and is identical to the first layer A. So the line joining AA layers passing through the centre of spheres will go through tetrahedral void.

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Q.

Which of the following statement is not true about the hexagonal close packing ?

(a) The coordination number is 12

(b) It has 74% packing efficiency

(c) Tetrahedral voids of the second layer are covered by the spheres of the third layer

(d) In this arrangement spheres of the fourth layer are exactly aligned with those of the first layer

Q. In hexagonal systems of crystals, a frequently encountered arrangement of atoms is described as a hexagonal prism. Here, the top and bottom of the cell are regular hexagons and three atoms are sandwiched in between them. A space-filling model of this structure, called hexagonal close-packed (HCP), is constituted of a sphere on a flat surface surrounded in the same plane by six identical spheres as closely as possible. Three spheres are then placed over the first layer so that they touch each other and represent the second layer. Each one of these three spheres touches three spheres of the bottom layer. Finally, the second layer is covered with third layer that is identical to the bottom layer in relative position. Assume radius of every sphere to be 'r'.

The empty space in this HCP unit cell is?
Q. In hexagonal system of crystals, a frequently encountered arrangement of atoms is described as a hexagonal prism. Here, the top and bottom of the cell arc regular hexagons and three atoms are sandwiched between them. A space-filling model of this structure, called hexagonal close packed (hcp), is constituted of a sphere on a flat surface surrounded in the same plane by six identical spheres as closely as possible. Three spheres are then placed over the first layer so that they touch each other and represent the second layer. Each one of these three spheres touches three spheres of the bottom layer. Finally, the second layer is covered with a third layer that is identical to the bottom layer in relative position. Assume radius of every sphere to be 'r'.

The empty space in this hcp unit cell is:
Q.
In the given sphere, the black spheres forms the first layer and red sphere forms the second layer and blue sphere forms the third layer.
The correct representation of the given arrangement of spheres is
Q. Three spheres of the first layer and three of the second layer enclose a site at the centre in a closest packing arrangement, this site is called:
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