Mass number is the sum of the number of protons and neutrons in an atom.

Answer

To determine whether the statement "Mass number is the sum of the number of protons and neutrons in an atom" is true or false, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Mass Number**:
- The mass number of an atom is defined as the total number of protons and neutrons present in its nucleus.

2. **Identifying Protons and Neutrons**:
- Protons are positively charged particles found in the nucleus of an atom.
- Neutrons are neutral particles also located in the nucleus.

3. **Mass Number Calculation**:
- The formula for calculating the mass number (A) is:
\[ A = Z + N \]
where \( Z \) is the number of protons and \( N \) is the number of neutrons.

4. **Example Calculation**:
- Let's take the example of Sodium (Na) with a mass number of 23 (written as \( ^{23}_{11}Na \)).
- Here, the atomic number (number of protons) is 11.
- To find the number of neutrons, we subtract the number of protons from the mass number:
\[ N = A - Z = 23 - 11 = 12 \]
- Therefore, Sodium has 11 protons and 12 neutrons.

5. **Conclusion**:
- Now, we can confirm that the mass number is indeed the sum of the number of protons and neutrons:
\[ A = Z + N = 11 + 12 = 23 \]
- Thus, the statement "Mass number is the sum of the number of protons and neutrons in an atom" is **true**.

### Final Answer:
The statement is **true**.