Solid is a state of matter in which atoms are closely packed and are difficult to move past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree of resistance being dependent upon the specific material under consideration. Solids also always possess the least kinetic energy per atom/molecule relative to other phases or, equivalently stated, solids are formed when matter in the liquid / gas phase is cooled below a certain temperature. This temperature is called the melting point of the substance and is an intrinsic property; i.e. independent of how much of the matter there is. The vast majority of substances, when in the solid state, can be arranged in one of a few ubiquitous structures. Solids are characterized by structural rigidity and resistance to applied external forces and pressure. Unlike liquids, solids do not flow to take on the shape of their container, nor do they expand to fill the entire available volume like a gas. Much like the other three fundamental phases, solids also expand when heated, the thermal energy put into increasing the distance and reducing the potential energy between atoms. However, solids do this to a much lesser extent. When heated to their melting point or sublimation point, solids melt into a liquid or sublimate directly into a gas, respectively. For solids that directly sublimate into a gas, the melting point is replaced by the sublimation point. As a rule of thumb, melting will occur if the subjected pressure is higher than the substance's triple point pressure, and sublimation will occur otherwise. Melting and melting points refer exclusively to transitions between solids and liquids. Melting occurs across a great extent of temperatures, ranging from 0.10 K for helium-3 under 30 bars (3 MPa) of pressure, to around 4,100 K at 1 atm for the composite refractory material hafnium carbonitride. The atoms in a solid are tightly bound to each other in one of two ways: regular geometric lattices called crystalline solids (e.g. metals, water ice), or irregular arrangements called amorphous solids (e.g. glass, plastic). Molecules and atoms forming crystalline lattices usually organize themselves in a few well-characterized packing structures, such as body-centered cubic. The adopted structure can and will vary between various pressures and temperatures, as can be seen in phase diagrams of the material (e.g. that of water, see left and upper). When the material is composed of a single species of atom/molecule, the phases are designated as allotropes for atoms (e.g. diamond / graphite for carbon), and as polymorphs (e.g. calcite / aragonite for calcium carbonate) for molecules. Non-porous solids invariably strongly resist any amount of compression that would otherwise result in a decrease of total volume regardless of temperature, owing to the mutual-repulsion of neighboring electron clouds among its constituent atoms. In contrast to solids, gases are very easily compressed as the molecules in a gas are far apart with few intermolecular interactions. Some solids, especially metallic alloys, can be deformed or pulled apart with enough force. The degree to which this solid resists deformation in differing directions and axes are quantified by the elastic modulus, tensile strength, specific strength, as well as other measurable quantities. For the vast majority of substances, the solid phases have the highest density, moderately higher than that of the liquid phase (if there exists one), and solid blocks of these materials will sink below their liquids. Exceptions include water (icebergs), gallium, and plutonium. All naturally occurring elements on the periodic table have a melting point at standard atmospheric pressure, with three exceptions: the noble gas helium, which remains a liquid even at absolute zero owing to zero-point energy; the metalloid arsenic, sublimating around 900 K; and the life-forming element carbon, which sublimates around 3,950 K. When applied pressure is released, solids will (very) rapidly re-expand and release the stored energy in the process in a manner somewhat similar to those of gases. An example of this is the (oft-attempted) confinement of freezing water in an inflexible container (of steel, for example). The gradual freezing results in an increase in volume, as ice is less dense than water. With no additional volume to expand into, water ice subjects the interior to intense pressures, causing the container to explode with great force. Solids' properties on a macroscopic scale can also depend on whether it is contiguous or not. Contiguous (non-aggregate) solids are characterized by structural rigidity (as in rigid bodies) and strong resistance to applied forces. For solids aggregates (e.g. gravel, sand, dust on lunar surface), solid particles can easily slip past one another, though changes of individual particles (quartz particles for sand) will still be greatly hindered. This leads to a perceived softness and ease of compression by operators. An illustrating example is the non-firmness of coastal sand and of the lunar regolith. The branch of physics that deals with solids is called solid-state physics, and is a major branch of condensed matter physics (which includes liquids). Materials science, also one of its numerous branches, is primarily concerned with the way in which a solid's composition and its properties are intertwined.
Microscopic description
The atoms, molecules or ions that make up solids may be arranged in an orderly repeating pattern, or irregularly. Materials whose constituents are arranged in a regular pattern are known as crystals. In some cases, the regular ordering can continue unbroken over a large scale, for example diamonds, where each diamond is a single crystal. Solid objects that are large enough to see and handle are rarely composed of a single crystal, but instead are made of a large number of single crystals, known as crystallites, whose size can vary from a few nanometers to several meters. Such materials are called polycrystalline. Almost all common metals, and many ceramics, are polycrystalline.
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