The mass number of neon is 20.
Mass number is defined as the total number of protons and neutrons in an atomic nucleus. For neon, which is a noble gas, it has 10 protons in its nucleus. The most abundant isotope of neon, Neon-20, also contains 10 neutrons. Therefore, when you add the number of protons (10) to the number of neutrons (10), you arrive at a mass number of 20. This mass number helps in identifying the isotope of the element and understanding its stability and behavior in various chemical reactions.