.. .......... and not the ......... is the most fundamental property of an element.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question:** The question asks for two terms related to the properties of elements, where one is identified as the most fundamental property.

2. **Identifying the Fundamental Property:** The most fundamental property of an element is its atomic number. The atomic number is defined as the number of protons present in the nucleus of an atom.

3. **Comparing with Other Properties:** The question contrasts the atomic number with another property. The atomic mass, which is the total number of protons and neutrons in an atom, is often confused with atomic number but is not as fundamental for classification.

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