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.
Elements and Their Valency (Table for Class 9 & 10)
Find a complete table of elements and their valency. Learn valencies of Hydrogen, Oxygen, Nitrogen, Carbon, and 20+ common elements for Class 9 Chemistry.
How to Find the Empirical Formula of a Hydrated Salt
Learn how to calculate the empirical formula of a hydrated salt. Step-by-step guide on heating the salt, finding the mass of lost water, and calculating moles.
What is Enthalpy of Atomisation?
Learn the definition of Enthalpy of Atomisation. Understand why it is always positive (endothermic) and how it relates to bond strength and metallic bond energy.
Equilibria Involving Sparingly Soluble Salts — Ksp, Common Ion Effect Explained
Sparingly soluble salts establish ionic equilibria described by Ksp (solubility product). The common ion effect reduces solubility. Precipitation occurs when ionic product > Ksp.
What is the Formula for Equivalent Weight?
Learn how to calculate Equivalent Weight in chemistry. Discover the formulas for finding the equivalent mass of acids, bases, and salts using the n-factor.
Turn this guide into revision flashcards, a practice exam, or an AI-generated podcast — free, no signup required.