Question

If the distance between na+ and cl–ions in sodium chloride crystal is x pm, the length of the edge of the unit cell is

Answer 1

In case of NaCl , two Cl⁻ ions present at corner of NaCl lattice and one Na⁺ ion present at edge of NaCl lattice. So, the edge length is the sum of two radius of Cl⁻ ion and two radius of Na⁺ ion as you can see in attachment.

∴ 2 × radius of Cl⁻ ion + 2 × radius of Na⁺ ion = edge length
Given, distance between Na⁺ and Cl⁻ = x pm
means , (radius of Na⁺ + radius of Cl⁻) = x pm
∴ 2(radius of Na⁺ + radius of Cl⁻) = edge length
⇒2x pm = edge length

Answer 2

2x pm the length of cell edge