Mass Number: Why Can It Be a Fraction?

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Mass number, defined as the total number of protons and neutrons in an atom, is typically a whole number, but can appear as a fraction due to the existence of isotopes. Isotopes are variants of elements that have the same number of protons but different numbers of neutrons, leading to variations in atomic mass. The mass of an atom is calculated based on the weighted average of all its isotopes, which can result in non-integer values. This distinction between mass number and atomic weight clarifies why mass numbers can be fractional in certain contexts. Understanding isotopes is essential to grasping the concept of mass in atomic physics.
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Homework Statement


My instructor give me a question:
Mass number (number of protons and number of neutrons in an atom) is supposed to be a whole number for a particular atom, in reality IT IS NOT. Why is it so?

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The Attempt at a Solution


Here, I am very sure that this question ask for mass number (nucleon number) not atomic weight. However, how can only a part of a proton or neutron exist in an atom if mass number are not a whole number?
 
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Hint: Check out isotopes.
 
you are absolutely correct. Mass number is the NUMBER of neutrons and protons (nucleons)
This does not mean that the MASS is a whole number.
Like steamking I suggest you look at isotopes to get the idea of mass
 

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