mass number

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).
The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. The mass number is different for each different isotope of a chemical element. This is not the same as the atomic number (Z) which denotes the number of protons in a nucleus, and thus uniquely identifies an element. Hence, the difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus: N=A−Z.

The mass number is written either after the element name or as a superscript to the left of an element's symbol. For example, the most common isotope of carbon is carbon-12, or , which has 6 protons and 6 neutrons. The full isotope symbol would also have the atomic number (Z) as a subscript to the left of the element symbol directly below the mass number: . This is technically redundant, as each element is defined by its atomic number, so it is often omitted.

For example, carbon-14 naturally decays by radioactive beta decay, whereby one neutron is transmuted into a proton with the emission of an electron and an anti-neutrino. Thus the atomic number increases by 1 (Z: 6→7) and the mass number remains the same (A = 14), while the number of neutrons decreases by 1 (n: 8→7). The resulting atom is nitrogen-14, with seven protons and seven neutrons:


Uranium-238, on the other hand, usually decays by alpha decay, where the nucleus loses two neutrons and two protons in the form of an alpha particle. Thus both the atomic number and the number of neutrons decrease by 2 (Z: 92→90, n: 146→144), which decreases the mass number by 4 (A = 238→234); the result is an atom of thorium-234 and an alpha particle ():


Nuclear isomers

A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons. A nuclear isomer occupies a higher energy state than the corresponding non-excited nucleus, called the ground state.

Metastable isomers of a particular isotope are usually designated with an "m" (or, in the case of isotopes with more than one isomer, m2, m3, and so on), which is placed after the mass number of the atom; for example, Co-58m, or . Increasing indices, m, m2, etc. correlate with increasing levels of excitation energy stored in each of the isomeric states (e.g., Hf-177m2 or ).

A different kind of metastable nuclear state (isomer) is the "fission isomer" or "shape isomer". In some actinide nuclei, quantum-mechanical states can exist in which the distribution of protons and neutrons is significantly non-spherical, so much so that de-excitation to the nuclear ground state is strongly hindered. Such fission isomers are usually denoted with a postscript or superscript "f" rather than "m", so that a fission isomer in e.g. plutonium 240 is denoted Pu-240f or .

Relative atomic mass

The mass number should not be confused with the relative atomic mass of an element, which is the average atomic mass number of the different isotopes of that element, weighted by abundance. For instance, there are two main isotopes of chlorine: chlorine-35 and chlorine-37. In any given sample of chlorine that has not been subject to mass separation there will be roughly 75% of chlorine atoms which are chlorine-35 and only 25% of chlorine atoms which are chlorine-37. This gives chlorine a relative atomic mass of 35.5 (actually 35.4527 g/mol).


Further reading

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