Crystalline arrangement in elemental semiconductors Silicon and germanium atoms join in an orderly, repeating arrangement of atoms in three dimensions. This repeating geometric pattern is called a:

Introduction / Context:
Elemental semiconductors such as silicon and germanium form solids with long-range atomic order. Describing that order correctly is foundational to understanding band structure, defects, and device properties.

Given Data / Assumptions:

• Atoms arranged periodically in three dimensions.
• Each atom is bonded tetrahedrally by covalent bonds (diamond cubic lattice).
• Macroscopic solid extends over many unit cells.

Concept / Approach:
A crystal is a solid in which atoms are arranged in a periodic lattice. The smallest repeating building block is the unit cell; for Si and Ge it is the diamond cubic structure (two interpenetrating face-centered cubic lattices). While the bonds are covalent, the ordered pattern itself is the crystal lattice, not the bond.

Step-by-Step Solution:
Identify the repeating arrangement → that defines a crystal lattice.Recognize that “covalent bond” describes the bonding type, not the pattern.A “molecule” is a finite grouping, not an extended periodic solid.“Valence bond” describes bonding theory, not the geometric repetition.

Verification / Alternative check:
X-ray diffraction patterns exhibit sharp Bragg peaks only in crystals with long-range order, confirming periodicity.

Why Other Options Are Wrong:
(b) and (d) concern bonding, not the spatial pattern; (c) refers to discrete entities rather than infinite periodic solids.

Common Pitfalls:
Interchanging “lattice” and “unit cell” terminology; forgetting that glasses lack such long-range periodic order.

Final Answer:
crystal