The mass number of an element represents the total number of protons and neutrons found within the nucleus of a single atom of that element.
In more detail:
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Definition: The mass number (symbol A) is a whole number that indicates the approximate mass of an atom, expressed in atomic mass units (amu). Since protons and neutrons each have a mass of approximately 1 amu, the mass number is essentially the sum of these particles.
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Calculation:
- Mass Number (A) = Number of Protons (Z) + Number of Neutrons (N)
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Location in Atomic Symbol: The mass number is typically written as a superscript to the left of the element's symbol. For example, carbon-12 is written as 12C.
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Isotopes: Elements can have different isotopes, which are atoms of the same element (same number of protons) but with different numbers of neutrons. Therefore, different isotopes of the same element will have different mass numbers. For example, carbon has two common isotopes: carbon-12 (12C) and carbon-14 (14C). Both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons.
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Distinction from Atomic Weight: It is important to distinguish the mass number from the atomic weight (also known as relative atomic mass), which is the weighted average of the masses of all the naturally occurring isotopes of an element. Atomic weight is usually a decimal number, reflecting the average isotopic abundance.
In summary, the mass number provides a quick way to determine the number of nucleons (protons and neutrons) in an atom's nucleus.
