Which of the following is not true for isotopes?

Options

A. They have the same atomic number.
B. They have the same mass number.
C. They have the same electronic configuration.
D. They have the same chemical properties

Explanation

To determine which statement is not true for isotopes, we first need to understand the definition and properties of isotopes.

### Step-by-Step Solution:

1. **Definition of Isotopes**:
- Isotopes are atoms of the same element that have the same number of protons (atomic number) but different numbers of neutrons. This results in different mass numbers.

2. **Properties of Isotopes**:
- **Same Atomic Number**: Isotopes have the same atomic number, which means they have the same number of protons and, for neutral atoms, the same number of electrons.
- **Different Mass Numbers**: Since isotopes have different numbers of neutrons, they will have different mass numbers.
- **Same Chemical Properties**: Because they have the same electronic configuration (same number of electrons), isotopes exhibit similar chemical properties.
- **Different Physical Properties**: Due to the difference in mass numbers, isotopes may have different physical properties (like density, boiling point, etc.).

3. **Evaluating Statements**:
- Now, we need to evaluate the statements given in the question.
- We know that isotopes share the same atomic number but have different mass numbers. Therefore, any statement claiming that isotopes have the same mass number would be incorrect.

4. **Conclusion**:
- The statement that is not true for isotopes is that they have the same mass number.

### Final Answer:
The statement that is not true for isotopes is: **They have the same mass number.**

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