State Atomic hypothesis.

Explanation

### Step-by-Step Solution:

1. **Definition of Atomic Hypothesis**: The atomic hypothesis is a scientific theory proposed by John Dalton in the early 19th century. It serves as a foundational concept in chemistry and physics.

2. **Composition of Elements**: Dalton stated that all elements are composed of small indivisible particles called atoms. This means that atoms are the fundamental building blocks of matter.

3. **Identical Atoms in Elements**: According to Dalton, all atoms of a given element are identical in mass and properties. However, atoms of different elements are different from each other. This distinction is crucial in understanding the nature of elements.

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