Mass Number

mass number, often represented by the symbol A, the total number of nucleons (neutrons and protons) in the nucleus of an atom. All atoms of a chemical element have the same atomic number (number of protons in the nucleus) but may have different mass numbers (from having different numbers of neutrons in the nucleus). Atoms of an element with the same mass number make up an isotope of the element. Different isotopes of the same element cannot have the same mass number, but isotopes of different elements often do have the same mass number, e.g., carbon-14 (6 protons and 8 neutrons) and nitrogen-14 (7 protons and 7 neutrons).

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mass number

the total number of neutrons and protons in the nucleus of a particular atom.

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mass number [′mas ‚nəm·bər]

(nuclear physics)

The sum of the numbers of protons and neutrons in the nucleus of an atom or nuclide. Also known as nuclear number; nucleon number.

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Mass number

The mass number A of an atom is the total number of its nuclear constituents, or nucleons, as the protons and neutrons are collectively called. The mass number is placed before and above the elemental symbol, thus ²³⁸U. The mass number gives a useful rough figure for the atomic mass; for example, ¹H = 1.00814 atomic mass units (amu), ²³⁸U = 238.124 amu, and so on. See Atomic number

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Mass Number 

the number of nucleons (protons and neutrons) in the atomic nucleus; designated by the letter A and usually indicated as a superscript to the left of the symbol of an element. The mass number and charge of a nucleus Z, expressed in units of elementary electric charge, determine the composition of the atomic nucleus: Z protons and (A — Z) neutrons. The mass of any atom, expressed in atomic mass units and rounded to the nearest whole number, is equal to its mass number.