Odd One Out — Hydrogen, Oxygen, and Nitrogen are diatomic gases at room temperature; Iodine is a solid halogen. Pick the odd term and justify.

Introduction / Context:
Chemical classification can hinge on physical state and group behavior. Three listed elements are gaseous diatomic molecules at room conditions; one is a solid halogen.

Given Data / Assumptions:

• Hydrogen (H2): diatomic gas.
• Oxygen (O2): diatomic gas.
• Nitrogen (N2): diatomic gas.
• Iodine (I2): solid at room temperature, subliming to a violet vapor on heating.

Concept / Approach:
Compare physical state under standard conditions (approximately 25°C, 1 atm). Select the only solid.

Step-by-Step Solution:
Hydrogen → gas.Oxygen → gas.Nitrogen → gas.Iodine → solid.

Verification / Alternative check:
Iodine also differs by group chemistry: it is a halogen (Group 17), while N2 and O2 are non-halogen p-block gases, and H2 is unique among elements but gaseous in state like N2 and O2.

Why Other Options Are Wrong:

• Hydrogen, Oxygen, Nitrogen: share gaseous state at room temperature.

Common Pitfalls:
Assuming all halogens are gases at room temperature (only fluorine and chlorine are; bromine is liquid; iodine is solid).

Final Answer:
lodine