Mass number

Mass number (A), in nuclear physics and chemistry, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is approximately equivalent to the atomic (also known as isotopic) mass of the atom expressed in atomic mass units. 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.

Isotopes

Isotopes are forms of an element with different numbers of neutrons. The atomic number of an element determines its chemical properties, so isotopes of an element behave the same way in chemical reactions. However, because of their different numbers of neutrons, isotopes of an element have different masses, which leads to their use in chemical analysis and other applications.

See also

• Atomic number
• Isotope
• Nuclear physics
• Chemistry

References

Mass number Resources
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