The most obvious distinguishing feature of a solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. is its rigidity. On the microscopic level this corresponds to strong forces between the atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume., ions, or molecules relative to the degree of motion of those particles. The only movements within a solid crystal latticeAn orderly, repeating arrangement of points in 3-D space in which each p;oint has surroundings identical to every other point. A crystal's constituent atoms, molecules, and ions are arranged about each lattice point. are relatively restricted vibrations about an average position. Thus we often think and speak of crystalline solids as having atoms, ions, or molecules in fixed positions.