order and disorder
/ OR-der and dis-OR-der /
Picture two rooms full of chairs. In one, the chairs stand in perfect rows like a theatre; in the other, they are scattered every which way after a party. Both rooms hold the same chairs, but one is ordered and one is disordered. Matter faces the same choice at the level of its atoms.
Order means the atoms sit in a regular, repeating, predictable arrangement — knowing where a few atoms are tells you where all the rest must be, as in a crystal. Disorder means there is no such pattern: the positions or orientations are jumbled and random, as in a glass or a liquid. Real materials fall along a spectrum between perfect order and complete randomness, and which way they lean is decided by a contest between the forces that favour neat packing and the heat that favours chaos.
This matters because order versus disorder controls how a material behaves: ordered crystals are often strong and conduct cleanly, while disordered materials can be transparent, flexible, or insulating in useful ways. A subtle point is that disorder is not the same as being broken or bad — many prized materials, from window glass to rubber, owe their special qualities precisely to their atomic disorder.
Table salt and window glass are both solids, but salt's atoms sit in a perfect cubic grid while glass's are frozen in a random tangle — that difference in order is why salt forms little cubic crystals and glass shatters into smooth, shapeless shards.
Salt is ordered, glass is disordered — both are solids.
Order can refer to more than position: atoms can also be ordered in how their magnetic spins point, so a magnet has magnetic order even when its atomic positions are perfectly crystalline anyway.