Crystals and glasses
Unlike a complex ion or molecule, which is a finite (often small) assembly of atoms, a solid has no fixed shape and size but can add atoms indefinitely. In a sample of uniform composition the bonding arrangements of atoms are expected to be similar as throughout. As defined: both glassy and crystalline forms of silica (SiO2) have structures with each Si surrounded by four oxygen atoms, and each O by two Si. However in crystalline solids it is possible to detect a unit cell containing a group of atoms that is repeated indefinitely in precise geometric fashion. In practice, all crystals have defects where this regularity is broken sometimes, but nevertheless crystals are different from glasses or non-crystalline solids, where there is no regular repetition.