A _________ is the smallest particle of an element or a compound which is capable of free existence.

Explanation

To answer the question, "A _________ is the smallest particle of an element or a compound which is capable of free existence," we need to identify the correct term that fits in the blank.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the smallest particle of an element or compound that can exist independently.

2. **Identifying Key Terms**:
- "Smallest particle" indicates we are looking for a fundamental unit.
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