Chemistry 1 Marks Question

Packing efficiency of:

1. Simple cubic = 52.4%
2. Body-centred cubic = 68%
3. Hexagonal close-packed = 74%

Hcp lattice has the highest packing efficiency.

Impurity from group 15 should be added to get n-type semiconductor.

The free electrons trapped in the anion vacancies are called F-centres.

Dispersion forces, dipole–dipole interactions and hydrogen bonds.

Silicon doped with Al forms p-type semiconductors, while silicon doped with P produces n-type semiconductors.

In 2D, square close-packed layer, an atom touches 4 nearest neighbouring atoms. Hence, its CN=4.

In solid state, the ions cannot move, they are held by strong electrostatic forces of attraction. So, ionic solids do not conduct electricity in solid state. However, in the molten state, they dissociate to give tree ions and hence conduct electricity.

Similar to liquids, glass has a tendency to flow, though very slowly. Therefore, glass is considered as a super cooled liquid. This is the reason that glass windows and doors are slightly thicker at the bottom than at the top.

Electrical conductivity of semiconductors increases with increase in temperature as more electrons can jump from valence band to conduction band.

Coordination no. of A = 8, Coordination no. of B = 4.

A face-centred cubic structure has one atom at each corner and one atom at each face centre.

p-type semiconductor.

There are 14 (8 from the corners + 6 from the faces) lattice points in face-centred cubic.

n-type of semiconductor is obtained.

When some ions (usually cations) are missing from the lattice sites and they occupy the interstitial sites so that electrical neutrality as well as stoichiometry is maintained, it is called Frenkel defect.

Ferromagnetism.

The constituent particles of a solid have fixed positions and are not free to move about, i.e., they possess rigidity. That is why they have definite volume.

Glass is not a true solid but a supercooled liquid of high viscosity (called pseudo-solid). It has the property to flow.

Because of the Frenkel defect, no ion is missing from the crystal, therefore there is no change in density.

Explanation:
n-type semiconductors are prepared by doping perfect crystal of silicon (Si) with pentavalent elements of group 15 (viz. P and As). Some of the lattice sites in silicon crystal are occupied by fifth electron which is left over as extra electron and couldn't form a bond with tetravalent Si. The images (i) and (iii) exhibits this situation representing n-type semiconductor.

Impurity defect.

Imperfection refers to the irregularities in the arrangement of atoms, ions or molecules in the structure of crystalline substances.