We determine the Coulombic force using the Coulomb's Law:
F = kQ₁Q₂/d²
where
k is equal to 9×10⁹ N·m²/C²
Q₁ and Q₂ are the charges of the two particles
d is the distance between them
For this problem, Q₁ = +2(0.16×10⁻¹⁸ C) and Q₂ = -2(0.16×10⁻¹⁸ C), because a single electron or proton has a charge of 0.16×10⁻¹⁸ C. The distance between the two ions is the sum of their radii.
d = radius of Ca²⁺ + radius of O²⁻
d = 0.106×10⁻⁹ m + 0.132×10⁻⁹ m
d = 0.238×10⁻⁹ m
F = (9×10⁹ N·m²/C²)(+2(0.16×10⁻¹⁸ C))(-2(0.16×10⁻¹⁸ C))/(0.238×10⁻⁹ m)²
F = 1.017×10¹¹ N
