mass number

American  

noun

Physics.

1. the integer nearest in value to the atomic weight of an atom and equal to the number of nucleons in the nucleus of the atom. A

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mass number British  

noun

1. Also called: nucleon number.   A.  the total number of neutrons and protons in the nucleus of a particular atom

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mass number Scientific  

1. The total number of protons and neutrons in the nucleus of an atom. For example, nitrogen has 7 protons and 7 neutrons in its nucleus, giving it a mass number of 14. Isotopes of elements are distinguished by their mass number; for example, carbon-12 and carbon-14 have mass numbers of 12 and 14 respectively.

2. Also called nucleon number

3. Compare atomic mass atomic weight

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Etymology

Origin of mass number

First recorded in 1920–25

Compare meaning

How does mass-number compare to similar and commonly confused words? Explore the most common comparisons:

• mass number vs. atomic mass
• atomic number vs. mass number
• atomic weight vs. mass number

Example Sentences

As an example, the masses and radii of all nuclei up to mass number 50 were calculated using this method.

From Science Daily • May 15, 2024

The mass number and atomic number of carbon-13 is 13 and 12 , while that of carbon-12 is 12 and 6 , respectively.

From Textbooks • Jun. 9, 2022

Since an element’s isotopes will have slightly different mass numbers, scientists also determine the atomic mass, which is the calculated mean of the mass number for its naturally occurring isotopes.

From Textbooks • Jun. 9, 2022

Note that the small contribution of mass from electrons is disregarded in calculating the mass number.

From Textbooks • Jun. 9, 2022

This gives off alpha particles at such a rate that half of it has decomposed in 150 days, leaving plutonium with atomic number 94 and mass number 238.

From A Brief History of Element Discovery, Synthesis, and Analysis by Watson, Glen W.