Classification — chemical elements: select the item that is NOT a metal.

Introduction / Context:
Chemistry classification tests whether you can distinguish metals from nonmetals. Three listed are classic metals; one is a nonmetal gas under standard conditions.

Given Data / Assumptions:

• Silver (Ag) → metal.
• Iron (Fe) → metal.
• Gold (Au) → metal.
• Hydrogen (H) → nonmetal; diatomic gas H2 at STP.

Concept / Approach:
Apply periodic table categories and physical properties at standard conditions.

Step-by-Step Solution:
1) Ag/Fe/Au → metallic luster, conductivity, malleability.2) H → nonmetal gas; lacks metallic properties.3) Therefore “Hydrogen” is the outlier.

Verification / Alternative check:
Check position in periodic table: H is placed with nonmetals, distinct from transition/precious metals.

Why Other Options Are Wrong:
They are all metals with typical metallic characteristics.

Common Pitfalls:
Associating “hydrogen” with metallic hydrogen in extreme conditions—irrelevant to everyday classification.

Final Answer:
Hydrogen