Classification – Chemical elements: choose the odd one out. Three are well-known radioactive elements; one is a common alkali metal not typically cited as radioactive. Which option does not belong? Options: Radium, Thorium, Sodium, Uranium.

Introduction / Context:
Chemical GK classifications often contrast radioactive elements with ordinary metals. Radium, thorium, and uranium are classic radioactive elements; sodium is an alkali metal commonly discussed without reference to radioactivity.

Given Data / Assumptions:

• Radium (Ra): radioactive alkaline earth metal.
• Thorium (Th): radioactive actinide.
• Uranium (U): radioactive actinide.
• Sodium (Na): alkali metal; standard stable isotope is non-radioactive in common contexts.

Concept / Approach:
Group by radioactivity status commonly taught in GK.

Step-by-Step Solution:
Tag Ra/Th/U as radioactive elements.Tag Na as a typical non-radioactive alkali metal in GK context.Hence Sodium is the odd one out.

Verification / Alternative check:
Introductory periodic-table summaries consistently list Th and U among radioactive actinides and Ra as radioactive; Na is not grouped there.

Why Other Options Are Wrong:
Radium/Thorium/Uranium: Each is classically identified as radioactive in general knowledge.

Common Pitfalls:
Avoid niche isotope discussions; the intended GK contrast is clear.

Final Answer:
Sodium