Structure of Solids

Fluorspar and cotunnite crystallize in fluorite structure.

Copper and nickel both crystallize in cubic close-packed or face centered cubic (fcc) arrangement.

In fluorspar or fluorite structure, cations are arranged into cubic close packing and the anions occupy all the tetrahedral voids.

In halite or rock-salt structure, anions occupy the lattice points of expanded face centred cubic lattice and the cations occupy all the octahedral holes. The limiting radius ratio is in between 0.414 and 0.732. The co-ordination numbers for both cation and anion is 6:6. The general formula of ionic compounds having this structure is AB. Examples: NaCl, KCl, MgO, CaO etc.

Wurtzite, cadmoselite and mercury telluride are examples of group 12 – 16 compounds.

Hexagonal crystal system is present in graphite, ZnO and CdS.

The CCP lattice is formed by the element Q. The number of octahedral voids generated would be equal to the number of atoms of A present in it. Since all the octahedral voids are occupied by the atoms of P, their number would also be equal to that of the element Q. Thus, the atoms of elements P and Q are present in equal numbers or 1:1 ratio. Therefore, the formula of the compound is PQ.

Germanium doped with phosphorus is an example of n-type semiconductor.
Silicon and germanium are group 14 elements having four valence electrons. In their crystals, each atom forms four covalent bonds with its neighbours. When doped with a group 15 element like P or As, which contains five valence electrons, they occupy some of the lattice sites in silicon or germanium crystal. Four out of five electrons are used in the formation of four covalent bonds with the four neighbouring silicon atoms. The fifth electron is extra and becomes delocalised. These delocalised electrons increase the conductivity of doped silicon or germanium. Here, the increase in conductivity is due to the negatively charged electron, hence silicon or germanium doped with electron-rich impurity are called n-type semiconductors.

Correct answer.
Fullerenes is an example of molecular solid.
Kickerite is an example of ionic solid.
Copper is an example of metallic solid.
Silicic oxide is an example of covalent network solid.
 

Tetragonal-Wulfenite
Orthorhombic-Aragonite
Hexagonal-Hanksite
Trigonal-Dolomite
Thus, this option is correct.
 

Packing efficiency in HCP lattice is 74%.
Packing efficiency of BCC lattice structure is 68%.
HCP and BCC lattice structures have different packing efficiency. Thus, this option is correct.
 

The number of tetrahedral voids formed is equal to twice the number of atoms of element B and only 2/3rd of these are occupied by the atoms of element A. Hence, the ratio of the number of atoms of A and B is 2 × (2/3):1 or 4:3 and the formula of the compound is A₄B₃.

Rock-salt structure: Anions occupy the lattice points of expanded face centred cubic lattice and the cations occupy all the octahedral holes. The limiting radius ratio is in between 0.414 and 0.732. The co-ordination numbers of cation and anion are (6:6). The general formula of ionic compounds having this structure is AB. Examples: NaCl, KCl, MgO, CaO etc.
Fluorite structure: The cations are arranged into cubic close packing and the anions occupy all the tetrahedral voids. Thus, there are four cations and eight anions per a unit cell. The formula of ionic compound having this structure is A₄B₈ or AB₂. The co-ordination numbers of cation and anion are (8:4). Examples: CaF₂, ZrO₂, ThO₂, BaF₂, BaCl₂, SrCl₂, PbCl₂ etc.
Anti-fluorite structure: Anions are arranged in cubic closest packing and cations occupy all the tetrahedral voids. There are four anions and eight cations per unit cell of this structure and hence, the general formula of an ionic compound is A8B4 or A2B. The ideal radius ratio is between 0.225 - 0.414 (but this ratio is not always maintained). The co-ordination numbers of cation and anion are (4:8). Examples: Na₂O, K₂O, Li₂O, Rb₂O, K₂S, Cl₂O, Na₂S etc.
Zinc-blende or sphalerite structure: Anions occupy the face centred cubic lattice points and cations occupy half of the tetrahedral holes. The ideal radius ratio is in between 0.225 to 0.414. There are four anions and four cations in the unit cell. Therefore, the formula is A₄B₄ or AB. The co-ordination numbers of cation and anion are (4:4). Examples: ZnS, BeO etc.

Crystal systems are differentiated on the basis of their edge lengths and axial angles. So, option 2 classifies them on this basis.
Monoclinic a ≠ b ≠ c, α = γ = 90°, β ≠ 90°
Tetragonal a = b ≠ c, α = β = γ = 90°
Orthorhombic a ≠ b ≠ c, α = β = γ = 90°
Trigonal a = b = c, α = β = γ ≠ 90°