Q: The Ca2+ and F– are located in CaF2 crystal, respectively at face centred cubic lattice points and in

Explanation:

Calcium Fluoride (CaF₂) Crystal Structure

In the calcium fluoride (CaF₂) crystal structure, calcium ions (Ca²⁺) and fluoride ions (F⁻) are arranged based on their sizes and charges.

Calcium ions are larger and have a double positive charge (Ca²⁺). They occupy the face-centered cubic (FCC) lattice points, where each unit cell has atoms at the corners and the centers of all the faces of the cube. This arrangement allows for dense packing of the larger calcium ions.

Fluoride ions are smaller and carry a single negative charge (F⁻). They fit into the spaces between the calcium ions, specifically in the tetrahedral voids. A tetrahedral void is a space in the crystal lattice surrounded by four atoms arranged in a tetrahedron shape. In the FCC lattice of calcium ions, there are twice as many tetrahedral voids as there are lattice points. For every calcium ion, there are two possible tetrahedral sites, but in CaF₂, each calcium ion is surrounded by eight fluoride ions, effectively filling the tetrahedral positions.

This arrangement maximizes the electrostatic forces between the positively charged calcium ions and the negatively charged fluoride ions, leading to a stable and tightly bound crystal structure.

In summary, in CaF₂ crystals, the F⁻ ions are located in the tetrahedral voids of the FCC lattice formed by the Ca²⁺ ions.