the interatomic force

Why do two atoms settle at a definite distance apart instead of collapsing together or drifting away? Because they feel two opposing forces at once, like two magnets on a spring. There is an attraction that pulls them together and a repulsion that pushes them apart, and the net interatomic force is the tug-of-war between them. Where the two exactly cancel, the atoms sit still — that is a chemical bond.

The attraction (call it FA) comes from the bonding itself — the pull of opposite charges, or the sharing of electrons — and it reaches out over a fair distance. The repulsion (FR) comes from the inner electron clouds and nuclei refusing to overlap; it is negligible far away but rises very steeply once the atoms get close, because you cannot squeeze two full electron shells into the same space. The net force is FN = FA + FR. Far apart, attraction wins and pulls the atoms inward; too close, repulsion wins and shoves them back. At one special separation the two balance, FN = 0, and the atoms rest.

This simple balance is the seed of nearly everything mechanical about a solid. The distance where the force is zero fixes the bond length; how stiffly the force resists being pulled apart from that point governs elastic stiffness; and how much energy it takes to pull the atoms fully apart sets the bond energy that controls melting. Every property in the rest of this field grows out of this one tug-of-war.