Answer 1

ln an ionic solid, the actions are positioned at alternate lattice points. Generally, cations are smaller than anyone in size. in a crysta/,thr number of anions surrounding a cation is called as Ligancy.
Ligancy 3:     Triangular configuration
The arrangement is triangular since a triangle is formed if the centres of the neighbouring anions are joined. In critical condition all the neighbouring anions and the central cation are in contact with each other, as shown in figure,
Here, BC=r_A and OC=r_A+rc and ∠ BC0=30°

In the Δ OBC, we get
BC /CO=cos3°
or r_Arc+r_A = √3/2
or rc+r_A√r_A = 2/√3
The critical radius ratio is thus
rc / r_A = 0.155