How many atoms are in a simple cubic unit cell?

Exactly 1 atom. Each of the 8 corners contributes 1/8 of an atom to the unit cell: 8 × (1/8) = 1.

Why is simple cubic packing so rare?

At only 52.4% packing efficiency, nearly half the volume is empty space. Most metals prefer tighter arrangements (BCC at 68% or FCC at 74%) to minimize energy.

What is the relationship between edge length and atomic radius?

In simple cubic packing, atoms touch along the cube edge, so the edge length a = 2r, where r is the atomic radius.

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Simple Cubic Packing

Explore the simplest crystal packing structure. Understand AA-layer stacking, the 1/8 corner atom trick, and why only Polonium uses this arrangement.

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AA-AAA Layer Stacking

Atoms stack directly on top of each other with no offset between layers, creating the simplest possible 3D arrangement.

Coordination Number 6

Each atom touches exactly 6 neighbors: one above, one below, and four in the same plane.

1/8 Corner Atom Rule

Each corner atom is shared by 8 unit cells, contributing only 1/8 to each. Total: 8 × 1/8 = 1 atom per unit cell.

Understanding Simple Cubic Packing

**Simple Cubic Packing (SC)** is the most fundamental crystal packing arrangement. Atoms are positioned at the eight corners of a cube, creating an open, highly symmetrical structure with the lowest packing efficiency among common crystal types.

The structure follows an **AA-AAA stacking sequence** — each layer sits directly on top of the previous one with zero offset. This results in a **coordination number of 6** and a **packing efficiency of only 52.4%**.

Despite its simplicity, SC packing is extremely rare in nature. Only **Polonium (Po)** crystallizes in this form at standard conditions, because the large voids make it energetically unfavorable compared to denser arrangements.