An "unbalanced force" means there is a difference in strength or direction between two opposing forces acting on an object, causing it to move or change its motion. Full definition
This presentation includes: Balanced and unbalanced forces Resultant force Changing atmospheric pressure Exam style question with mark scheme (tes.com)
Conversely, if the atoms should be further apart than this equilibrium internuclear distance, then there would be an unbalanced force (primarily due to valence - shell interactions) which would tend to decrease the interatomic distance. (religion-online.org)
If, for any reason, the distance between the two iodine - atom nuclei should be less than that distance, there will be an unbalanced force (mainly arising from interactions of the filled shells) which will tend to increase the distance between the atomic centers. (religion-online.org)