Who proposed that atomic number is the fundamental property of an element?

Explanation

To answer the question, "Who proposed that atomic number is the fundamental property of an element?", we can follow these steps:

### Step 1: Understanding Atomic Number
- The atomic number of an element is defined as the number of protons in the nucleus of an atom of that element. It is a unique identifier for each element.

### Step 2: Historical Context
- Before the concept of atomic number was established, elements were arranged based on their atomic mass. However, this arrangement did not always reflect the true periodicity of the elements.

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