Answer: 11
Step-by-step explanation:
Mass number is defined as the sum of number of protons and neutrons that are present in an atom.
Mass number = Number of protons + Number of neutrons
Atomic number is defined as the number of protons or number of electrons that are present in an atom.
Atomic number = Number of electrons = Number of protons
Hence, number of electrons is equal to the atomic number which is 11.