Mass Number

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. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion. 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 12C, 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: 12
6C. This is technically redundant, as each element is defined by its atomic number, so it is often omitted.

Read more about Mass Number:  Mass Number Changes in Radioactive Decay, Mass Number and Isotopic Mass, Atomic Mass of An Element

Other articles related to "mass number, mass, number, numbers":

Mass Number - Atomic Mass of An Element
... The mass number should also not be confused with the relative atomic mass (also called atomic weight) of an element, which is the ratio of the average atomic ... 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) ...
Spontaneous Fission
... Because the nuclear binding energy of the elements reaches its maximum at an atomic mass number greater than about 58 atomic mass units (u), spontaneous breakdown into ... possible) for some atomic nuclei with atomic masses greater than 92 atomic mass units (a.m.u.), with the probability of spontaneous fission increasing as the atomic mass ... fission are the synthetic high-atomic-number actinide elements with odd atomic numbers mendelevium and lawrencium, and also some of the transactinide very-heavy ...
Mass (mass Spectrometry) - Mass Number
... The mass number, also called the nucleon number, is the number of protons and neutrons in an atomic nucleus ... The mass number is unique for each isotope of an element and is written either after the element name or as a superscript to the left of an element's symbol ...
Radium Series - Types
... decay processes, only alpha decay changes the atomic mass number (A) of the nucleus, and always decreases it by four ... Because of this, almost any decay will result in a nucleus whose atomic mass number has the same residue mod 4, dividing all nuclides into four classes ... The mass number of every isotope in these chains can be represented as A = 4n, A = 4n + 2, and A = 4n + 3, respectively ...
Atom - Properties - Mass
... The large majority of an atom's mass comes from the protons and neutrons that make it up ... The total number of these particles (called "nucleons") in a given atom is called the mass number ... The mass number is a simple whole number, and has units of "nucleons." An example of use of a mass number is "carbon-12," which has 12 nucleons (six protons ...

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