The force of attraction between oppositely charged particles is directly proportional to the product of the charges on the two objects (q1 and q2) and inversely proportional to the square of the distance between the objects (r2). The strength of the bond between the ions of the opposite charge in an ionic compound therefore depends on the charges on the ions and the distance between the centers of the ions when they pack to form a crystal. An estimate of the strength of the bonds in an ionic compound can be obtained by measuring the lattice energy of the compound, which is the energy given off when oppositely charged ions in the gas phase come together to form a solid.