Two different elements may have same mass number but not the same __________________.

Explanation

To solve the question "Two different elements may have the same mass number but not the same __________________," we need to understand the concepts of mass number and atomic number.

### Step-by-Step Solution:

1. **Define Mass Number**:
- The mass number of an element is defined as the total number of protons and neutrons in the nucleus of an atom. It is represented by the symbol 'A'.
- Formula: Mass Number (A) = Number of Protons (Z) + Number of Neutrons (N).

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