How do crystalline solids melt compared with amorphous?

Sharply at a definite melting point

The smallest group of atoms that, repeated in 3D, builds the crystal

The set of all atoms in the crystal

Which is the definition of a space lattice?

A 3D array of points each with identical surroundings

Practice · 4 marks

Crystalline and amorphous solids

Short answer

Crystalline solids have long-range periodic atomic order, sharp melting points, and are anisotropic. Amorphous solids have only short-range order, soften over a temperature range, and are isotropic. A space lattice is an infinite 3D array of points with identical surroundings. The unit cell is the smallest repeating parallelepiped that builds the crystal.

Differentiate between crystalline and amorphous solids. Define space lattice and unit cell.

A crystalline solid has atoms arranged in a regular, repeating three-dimensional pattern with long-range order; an amorphous solid has only short-range order.

Basis	Crystalline	Amorphous
Atomic arrangement	Long-range, periodic	Random, short-range only
Melting	Sharp melting point	Softens over a temperature range
Properties	Anisotropic (single crystal)	Isotropic
Cooling curve	Shows arrest at freezing point	Smooth, no arrest
Examples	Metals, NaCl, quartz	Glass, many polymers, rubber
X-ray diffraction	Sharp peaks	Broad halo

Space lattice

A space lattice is an infinite three-dimensional array of points in which every point has identical surroundings. Each point is a lattice point; atoms are placed at or around these points.

Unit cell

The unit cell is the smallest group of atoms (smallest parallelepiped) that, when repeated in three dimensions, builds the whole crystal. It is described by the edge lengths $a, b, c$ and the angles $\alpha, \beta, \gamma$ between them. There are 7 crystal systems and 14 Bravais lattices; most metals crystallise as BCC, FCC or HCP.

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