Assertion. The sum of the radii of Na^+ and Cl^- ions in NaCl crystal is 281 pm. Hence, edge of the unit cell is 281 pm Reason. Edge of the unit cell is the distance between the centres of Na^+ and Cl^- ions touching each other .

Assertion. The sum of the radii of Na^+ and Cl^- ions in NaCl crystal

1.	Assertion. The sum of the radii of Na^+ and Cl^- ions in NaCl crystal is 281 pm. Hence, edge of the unit cell is 281 pm Reason. Edge of the unit cell is the distance between the centres of Na^+ and Cl^- ions touching each other .
Answer» If both assertion and reason are true, and reason is the true explanation of the assertion. If both assertion and reason are true, but reason is not the true explanation of the assertion. If assertion is true , but reason is false If both assertion and reason are false. Solution :Correct A . The edge of NACL UNIT CELL is the distance from `Na^+` to NEXT `Na^+` ion or from `Cl^-` ion to next `Cl^-` ion. Correct R. Edge of the unit cell of NaCl is DOUBLE the distance between the centre of `Na^+` and `Cl^-` ions touching each other.