State Atomic hypothesis.

Explanation

### Step-by-Step Solution:

1. **Definition of Atomic Hypothesis**: The atomic hypothesis is a fundamental concept in chemistry and physics that describes the nature of matter. It states that matter is composed of small, indivisible particles called atoms.

2. **Composition of Elements**: Each element is made up of extremely small particles known as atoms. This means that all matter can be broken down into these basic units.

3. **Identical Atoms of an Element**: All atoms of a given element are identical in mass and properties. This implies that every atom of, for example, carbon, has the same characteristics as every other carbon atom.

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