The elements are identified by their atomic numbers and not by their mass numbers. Justify.

Explanation

### Step-by-Step Solution:

1. **Understanding Atomic Number and Mass Number**:
- The atomic number of an element is defined as the number of protons present in the nucleus of an atom. It is unique for each element.
- The mass number is the total number of protons and neutrons in the nucleus of an atom. It can vary for different isotopes of the same element.

2. **Uniqueness of Atomic Number**:
- Each element has a unique atomic number. For example, hydrogen has an atomic number of 1, helium has an atomic number of 2, and so on. No two elements can have the same atomic number.
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