The force of attraction (F) is given by the formula:
F = (1/4π∈r²)(Zc*e)(Za*e)---------(1)
where:
∈ = permittivity of free space = 8.85*10^-15 F/m
Zc = charge on the cation = +2
Zc = charge on the anion = -2
e = charge on an electron = 1.602*10^-19 C
r = interionic distance = rc + ra
where rc and ra are the radius of cation and anion respectively
F = 1.91 * 10^-8 N
therefore based on equation (1) we have:
1.91 *10^-8 = [1/4π(8.85*10^-15)r²](2*1.602*10^-19)²
r² = 0.04832 * 10^-15
r = 6.951 nm
Now: r = rc + ra
where rc = 0.074 nm
thus, ra = r - rc = 6.951 - 0.074 = 6.877 nm
