Pauling give method to calculate univalent ion radii by assuming that (i) In ionic crystal (let M^(+)X^(-)) cations and anions are is contact of eac other and sum of their radii is equal to interionic distance, i.e., d_((M^(+)-X^(-))) = r_(M^(+) +r_(X^(-)) (ii) The radius of an ion having noble gas configuration is inversely proportional to the effective nuclear charge felt at the periphery of the ion, i.e., r_((M^(+))) = (C)/(Z_(eff(M^(+)))) and r_((X^(-))) = (C)/(Z_(eff(X^(-)))) Here C is constant of proportionality whose value depends on electronic configuration of ion. Thus, d_((M^(+)-X^(-))) =(C)/(Z_(eff(M^(+)))) +(C)/(Z_(eff(X^(-)))) pm Z_(eff) is the effective nuclear charge whose value can be calculated by the formula: Z_(eff) = Z - sigma. Here sigma is shielding constant and for neon, the value of sigma when calculate by Slater's rule is found to be 4.5 The value of constant C for NaF crystals is [given that interionic distance of NaF = 231 pm]: