Suppose you attempt to break a cubic salt crystal, 1.75 mm on a side, by applying

Suppose you attempt to break a cubic salt crystal, 1.75 mm on a side, by applying forces f and −f perpendicular to opposite sides of the cube, trying to pull it apart. how great would f have to be to overcome the attractive forces of all the ions in a 3.0625 mm2 plane of the crystal? (be sure to type in units! hint: how many ions are in one side? a plane? how much force does each ion apply to each other? )