Assertion : Mass number of an atom is equal to total number of nucleons present in the nucleus
Reason :Mass number defines the identity of an atom

Options

A. If both assertion and reason are true and the reason is the correct explanation of the assertion
B. If both assertion and reason are true but reason is not the correct explanantion of the assertion
C. If assertion is true but reason is false
D. If the assertion and reason both are false

Explanation

To solve the question, we will analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Mass number of an atom is equal to total number of nucleons present in the nucleus."
- Nucleons are the particles found in the nucleus of an atom, which include protons and neutrons.
- Therefore, the mass number (A) is calculated as:
\[ A = \text{Number of Protons (Z)} + \text{Number of Neutrons (N)} \]
- This means the assertion is correct.

2. **Understanding the Reason**:
- The reason states that "Mass number defines the identity of an atom."
- The identity of an atom is actually defined by its atomic number (Z), which is the number of protons in the nucleus.
- While the mass number gives information about the total number of nucleons, it does not uniquely identify the atom as different isotopes of the same element can have different mass numbers but the same atomic number.
- Thus, the reason is incorrect.

3. **Conclusion**:
- Since the assertion is correct and the reason is incorrect, we conclude that the correct relationship is:
- Assertion is true, but reason is false.

### Final Answer:
- The correct option is: Assertion is true but reason is false (Option C).