The mass number, also known as an atomic mass number or nucleon number, refers to the total number of neutrons and protons (in a combination known as nucleons) in an atomic nucleus. It determines the atomic mass of atoms. However, while calculating the mass number, the small contribution of mass from electrons is ignored.
Mass number is equal to Number of protons + Number of neutrons
Mass number = 11 + 7 = 18.
Hence, the correct answer is 18.
