The mass number of carbon is the total number of protons and neutrons in its nucleus. For the most common isotope of carbon, carbon-12, the mass number is 12.
Understanding Mass Number
The mass number (A) is a whole number that represents the total count of nucleons (protons and neutrons) within an atom's nucleus. It is not the same as atomic mass, which is a weighted average of the masses of all isotopes of an element.
Calculating the Mass Number of Carbon
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Identify the Isotope: Carbon exists in different isotopic forms (Carbon-12, Carbon-13, Carbon-14). The question usually implies the most common isotope, Carbon-12.
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Determine Protons and Neutrons: Carbon always has 6 protons (this defines it as carbon). For Carbon-12, it also has 6 neutrons.
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Add Protons and Neutrons: The mass number is calculated by adding the number of protons and the number of neutrons.
Mass Number (A) = Number of Protons + Number of Neutrons
For Carbon-12: A = 6 + 6 = 12
Example: Carbon-14
For Carbon-14, which is used in radiocarbon dating:
- Number of protons = 6
- Number of neutrons = 8 (14 - 6 = 8)
- Mass number = 14
Summary
The mass number of an element is the sum of its protons and neutrons. For Carbon-12, the most abundant isotope, the mass number is 12. For other isotopes, such as Carbon-14, the mass number will differ based on the number of neutrons.
