Final answer:
The mass number of a neon (Ne) atom with 11 neutrons is 21.
Step-by-step explanation:
The mass number of an atom is the sum of its protons and neutrons. Since a neon (Ne) atom contains 11 neutrons, we can calculate its mass number by adding the number of protons (10) and neutrons (11):
Mass number = Protons + Neutrons = 10 + 11 = 21