.................. are the smallest units that take part in chemical reactions.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding the Question**: The question asks for the smallest units that participate in chemical reactions.

2. **Identifying the Key Concept**: In chemistry, the fundamental units that engage in chemical reactions are atoms. Atoms are the basic building blocks of matter.

3. **Defining an Atom**: An atom can be defined as the smallest particle of an element that retains the properties of that element. This means that even if an atom is broken down into smaller parts, it will no longer represent the original element.

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