What is the mass number minus the atomic number?
The mass number minus the atomic number is the number of neutrons in an atom. The atomic number is both protons and electrons.
Boron, atomic number 5, has two naturally occuring isotopes with mass numbers of 10 and 11, 10B, 11B. The number of neutrons in each is 5 and 6 respectively. The percentage occurence is roughly 20%, 80% respectively. This gives rise to the fractional atomic mass.