the interatomic force
Why do two atoms stop at a certain distance from each other instead of flying apart or crashing together? Because two opposite tendencies are always at play: at long range they attract, drawing together, but pushed too close they repel, shoving apart. The interatomic force is the net of these two, and where they exactly cancel is where the atoms happily rest.
The attraction comes from the bonding itself, opposite charges, shared electrons, the electron sea, and it strengthens as atoms approach, but only up to a point. The repulsion comes from the atoms' electron clouds and nuclei being forced to overlap; it is negligible far away but rises extremely steeply as the atoms get very close, far faster than the attraction. Add the two forces and the total is attractive when the atoms are far apart, zero at one special separation, and strongly repulsive if squeezed any closer. That crossover point is the equilibrium spacing, the natural bond length.
This tug-of-war is the mechanical foundation of every solid. Because the atoms sit at the bottom of the balance, stretching a material (pulling atoms apart) revives the attraction that pulls them back, and compressing it (pushing them together) revives the repulsion that pushes them back; that restoring behaviour is exactly why solids are springy and have a definite stiffness. The interatomic force is where mechanics, thermal expansion, and bonding all begin.
Imagine two atoms joined by a tiny spring at its relaxed length. Pull them apart and net attraction pulls back; push them together and net repulsion pushes back. The steepness of that spring near its rest length is, quite literally, the material's stiffness; its Young's modulus traces back to this single curve.
Attraction plus a steeper repulsion sets a resting distance, the bond length.
It is a common mistake to imagine only attraction holds a bond. Without the steep short-range repulsion the atoms would collapse into each other; the bond length exists precisely because attraction and repulsion balance, not because attraction alone stops somewhere.