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Question

Why is the atomic mass of an element usually fractional?


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Solution

Atomic mass

  • An element's atomic mass is the average mass of its atoms measured in atomic mass units.
  • The atomic mass of an element is usually fractional.
  • The formula to calculate atomic mass is as follows:

Atomicmass=Massofisotopes×Percentageofabundanceofisotopes100

Atomic mass of an element is usually fractional

  • The majority of elements exist as a mixture of different mass isotopes.
  • Fractional atomic masses result from this combination.
  • The atomic mass of an element is the average of its isotope's atomic masses.

Example

  • An example is a chlorine atom has two isotopes: chlorine-35 and chlorine-37.
  • The percentage of abondance of chlorine-35 is 75.8 and chlorine-37 is 24.2.
  • Mass of chlorine-35 is 35u and chlorine-37 is 37u.
  • The atomic mass is as follows:

Atomicmass=Massofisotopes×Percentageofabundanceofisotopes100=35u×75.8+37u×24.2100=3548.4100=35.485u

  • Due to the existence of isotopes, the atomic mass of chlorine is 35.485u.

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