Polonium (Po) is unique in the periodic table — it is the only element known to naturally crystallize in a simple cubic (primitive cubic) structure at room temperature. Most metals prefer more efficient packing arrangements like Face-Centred Cubic (FCC) or Body-Centred Cubic (BCC). This topic is commonly tested in Class 12 Chemistry (Solid State chapter).
Polonium is the only element that exists in a simple cubic crystal structure at room temperature.
α-Polonium (below 36°C): simple cubic; β-Polonium (above 36°C): rhombohedral.
Simple cubic coordination number = 6.
Simple cubic packing efficiency = 52.4% (least efficient cubic structure).
Atoms per unit cell in simple cubic = 1.
FCC has the highest packing efficiency (74%); BCC has 68%; simple cubic has 52.4%.
In a simple cubic unit cell: • Atoms are present only at the 8 corners of a cube • Each corner atom is shared among 8 unit cells • Number of atoms per unit cell = 8 × (1/8) = 1 atom • Coordination number = 6 (each atom touches 6 neighbours — top, bottom, left, right, front, back) • Packing efficiency = 52.4% (least among cubic structures) • Bond angle between atoms = 90°
Polonium (α-Po) at room temperature is the only element with this structure.
Polonium has two allotropic forms:
α-Polonium (below 36°C): • Simple cubic (primitive cubic) crystal structure • This is the form at room temperature (since room temperature < 36°C) • Coordination number: 6
β-Polonium (above 36°C): • Rhombohedral crystal structure • Changes from simple cubic when temperature exceeds 36°C
Property | Simple Cubic | BCC | FCC Atoms per unit cell | 1 | 2 | 4 Coordination number | 6 | 8 | 12 Packing efficiency | 52.4% | 68% | 74% Example element | Polonium (Po) | Iron (Fe), Chromium (Cr) | Copper (Cu), Gold (Au), Silver (Ag) Radius relationship | r = a/2 | r = (√3/4)a | r = a/(2√2)
Simple cubic is the least efficient and rarest packing — polonium is its only natural elemental example.
Common exam questions: • Which element adopts a simple cubic structure? → Polonium (Po) • What is the coordination number of simple cubic? → 6 • What is the packing efficiency of simple cubic? → 52.4% • What fraction of space is occupied in simple cubic? → 52.4% (or π/6 ≈ 0.5236) • How many atoms per unit cell in simple cubic? → 1
This is a frequent one-mark or two-mark question in Class 12 Board exams and JEE.
Polonium (Po) is the only element that naturally adopts a simple cubic (primitive cubic) crystal structure at room temperature. Its α-form (below 36°C) is simple cubic with a coordination number of 6 and packing efficiency of 52.4%.
Coordination number of simple cubic = 6 (each atom is surrounded by 6 atoms). Packing efficiency = 52.4% (or π/6). Number of atoms per unit cell = 1. It is the least efficient cubic structure.
Polonium has two allotropes: α-Po (simple cubic structure, stable below 36°C) and β-Po (rhombohedral structure, stable above 36°C). At room temperature, α-polonium (simple cubic) is the stable form.
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