The Solid State

The correct answer is A. C & D only.

Statement A is correct: In an ionic solid, ions are the constituent particles.

Statement C is correct: Ionic solids are electrical insulators in the solid state. The ions are held in a rigid lattice structure, and there are no freely moving charged particles to conduct electricity.

Statement D is correct: Ionic solids can conduct electricity in the molten state or when dissolved in water. In these states, the ions are free to move and carry electric charge.

Therefore, the correct answer is A. C & D only.

To determine the formula of the compound formed by elements A and B, we need to consider the ratio of the atoms in the compound based on their arrangement in the crystal lattice.

Given that the atoms of element B form a hexagonal close-packed (hcp) lattice, the closest packing ratio for this lattice is 2:1. This means that for every two atoms of element B, there is one atom of element A occupying the tetrahedral voids.

Since the atoms of element A occupy 2/3 of the tetrahedral voids, we can deduce that for every 2 atoms of element B, there will be 2/3 atoms of element A. Simplifying this ratio, we get:

2 atoms B : 2/3 atoms A

To simplify the ratio further, we can multiply it by 3 to eliminate the fraction:

(2 atoms B) x (3 atoms A / 2 atoms B) = 3 atoms A

Therefore, the formula of the compound formed by elements A and B is A3B2.