Definition of "mass number" []

• The sum of the number of neutrons and protons in an atomic nucleus. Also called nucleon number. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Nitrogen is the commonest gas in the atmosphere and its mass number is 14, the same as carbon-14’s."