Crystal structure – atomic arrangement in crystalline solids In crystalline solids, how are atoms arranged at the microscopic scale?

Introduction / Context:
Materials are broadly categorized as crystalline or amorphous based on their atomic arrangements. This distinction governs mechanical, thermal, electrical, and optical properties. Recognizing the defining attribute of crystalline solids is foundational in materials science and solid-state physics.

Given Data / Assumptions:

• Crystalline material in equilibrium (no catastrophic defects dominating).
• Presence of thermal vibrations but with average atomic positions well defined.
• Comparative reference: amorphous solids and liquids lack long-range periodicity.

Concept / Approach:

A crystalline solid exhibits long-range periodic order: atoms occupy lattice sites forming a repeating three-dimensional pattern characterized by a Bravais lattice and a basis. Thermal motion causes small oscillations about lattice sites but does not destroy long-range order except near melting. Diffraction experiments (X-ray, neutron, electron) reveal sharp Bragg peaks that are direct signatures of periodic order in crystalline materials.

Step-by-Step Solution:

Verification / Alternative check:

Bragg's law analysis of X-ray diffraction patterns from metals, semiconductors, and ionic crystals consistently shows discrete peaks, confirming periodic order in crystals.

Why Other Options Are Wrong:

(b) describes amorphous materials, not crystalline. (c) and (d) are incorrect: while temperature affects vibrations, long-range order persists until melting and does not vanish at any nonzero temperature below melting.

Common Pitfalls:

Confusing imperfections (dislocations, vacancies) with total loss of periodic order; crystals sustain defects while retaining long-range order.

Final Answer:

True – atoms occupy a regular, periodic lattice arrangement