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. **Understanding Mass Number**:
- The mass number of an element is the total number of protons and neutrons in its nucleus. It is represented as:
\[
\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}
...