---
title: "Crystalline and amorphous solids"
subject: "Material Science"
chapter: 2
key: "semester_3/material_science:crystalline-vs-amorphous-solids"
source: https://www.bctengineer.com/q/semester_3/material_science/crystalline-vs-amorphous-solids
site: BCT Engineering
---

# Crystalline and amorphous solids

Material Science · Chapter 2: Atomic Structure, arrangement of atoms

- Asked in: year not given, 4 marks
- Video: [Crystalline Solids and Amorphous Solids](https://www.youtube.com/watch?v=4NZV0ZvDm5c) — Andrey K
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/material_science/crystalline-vs-amorphous-solids
- Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-2

## Question

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

## 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.

## Answer

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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which describes an amorphous solid?
   A. Long-range periodic order
   B. Only short-range order
   C. Sharp X-ray peaks
   D. Sharp melting point
   Answer: B. Amorphous solids have only short-range order and give a broad XRD halo. (Key point: Order)

2. How do crystalline solids melt compared with amorphous?
   A. Over a range of temperature
   B. They never melt
   C. Sharply at a definite melting point
   D. Both melt identically
   Answer: C. Crystalline solids show a sharp melting point and an arrest on the cooling curve. (Key point: Melting behaviour)

3. What is a unit cell?
   A. An infinite array of points
   B. The smallest group of atoms that, repeated in 3D, builds the crystal
   C. The set of all atoms in the crystal
   D. A single lattice point
   Answer: B. Repeating the unit cell in three dimensions generates the whole crystal. (Key point: Unit cell)

4. Which is the definition of a space lattice?
   A. A 3D array of points each with identical surroundings
   B. A random array of atoms
   C. A cell with edges a, b, c
   D. A set of 14 atoms
   Answer: A. In a space lattice every point has identical surroundings. (Key point: Space lattice)

## Related in Material Science

- [BCC, FCC and HCP crystal structures](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Abcc-fcc-hcp-structures) — Material Science, chapter 2

## Related in other subjects

- [BCC unit cell: lattice constant and packing](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Abcc-lattice-constant-packing) — Electrical Engineering Material, chapter 1, asked 2×
- [FCC unit cell: body diagonal and packing density](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Afcc-body-diagonal-packing) — Electrical Engineering Material, chapter 1, asked 3×
- [Lattice, basis and FCC packing density](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Alattice-basis-fcc) — Electrical Engineering Material, chapter 1, asked 3×
- [Lattice constant of a simple cubic structure](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Asimple-cubic-lattice-constant) — Electrical Engineering Material, chapter 1