a free surface

Every solid object stops somewhere, at the place where it meets air or vacuum. The atoms deep inside a crystal are surrounded on all sides by neighbours, bonded snugly like a brick in the middle of a wall. But an atom right at the outer skin is different: on one side it has neighbours, and on the other side it has nothing, so its bonds point out into empty space and dangle unfulfilled. That outermost layer, where the crystal simply ends and the world begins, is a free surface.

A free surface is a two-dimensional (planar) defect, meaning it is a fault spread over a surface rather than a point or a line. It exists because the periodic stacking of atoms is abruptly cut off. A surface atom has fewer neighbours than a bulk atom: for example, in an FCC metal a bulk atom has 12 nearest neighbours (coordination number 12), but an atom on a close-packed {111} surface has only 9, so three of its bonds are broken, or dangling. Those unsatisfied bonds cost energy, which is why every free surface carries an extra energy per unit area called the surface energy.

Free surfaces are where almost all chemistry happens: corrosion, catalysis, oxidation, adhesion, and crystal growth. Because breaking bonds costs energy, materials try to minimise surface area, which is why small liquid droplets are spherical and why fine powders sinter together when heated. Honest note: a free surface is an idealisation. In real air a surface is instantly covered by adsorbed gas, water, or an oxide film within moments, so a truly clean free surface only exists in ultra-high vacuum.