Chemistry · Liquids and solids. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
In a crystalline solid the particles are arranged in a regular, repeating three-dimensional pattern. This gives a sharp melting point, flat faces at fixed angles, cleavage along definite planes and anisotropy (some properties differ with direction). An amorphous solid, such as glass, has no regular long-range order and softens over a range of temperature.
The pattern of points in space is the crystal lattice. Its smallest repeating part is the unit cell, described by three edge lengths (a, b, c) and three angles (α, β, γ). These give seven crystal systems.
Crystals are also classed by their particles and forces: ionic, covalent, molecular and metallic. The forces decide hardness, melting point and conductivity.
Students mix up isomorphism and polymorphism. Isomorphism is many substances sharing one crystal form; polymorphism is one substance having many crystal forms (called allotropy when the substance is an element).
Answer: X is a covalent (network) solid, like diamond or silicon carbide.
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