The mass number of an isotope is calculated by adding the number of protons and neutrons together.
In this case, the isotope of sodium has:
- 11 protons
- 7 neutrons
So, the mass number is calculated as follows:
Mass number = Number of protons + Number of neutrons
Mass number = 11 + 7 = 18
Therefore, the mass number of this isotope is 18.