Explain fractional atomic mass. What is the fractional mass of chlorine?

Fractional atomic mass:

• When the value of atomic mass exists as a fraction instead of a whole number, it is called fractional atomic mass.
• Fractional atomic mass occurs for those elements in which atoms of the same elements have more than one value of atomic mass. Example: chlorine (Cl). Some atoms of chlorine have an atomic mass of 35 u and others have an atomic mass of 37 u.
• The fractional mass of chlorine is 35.5 u.

Calculation of fractional atomic mass of Chlorine (Cl):

Step-1: Calculate the fraction of the isotopes present

• ${}_{17}{}^{35}\mathrm{Cl}$ and ${}_{17}{}^{37}\mathrm{Cl}$ exist in the ratio of 3:1 which means for a given sample, atoms of Chlorine-35 are present 3 times more than the atoms of the Chlorine-37.
• Fraction of ${}_{17}{}^{35}\mathrm{Cl}$= $\frac{3}{4}$
• Fraction of ${}_{17}{}^{37}\mathrm{Cl}$= $\frac{1}{4}$

Step-2: Calculate the fractional atomic mass

• Fractional atomic mass = fraction of ${}_{17}{}^{35}\mathrm{Cl}$× atomic weight of ${}_{17}{}^{35}\mathrm{Cl}$+ fraction of ${}_{17}{}^{37}\mathrm{Cl}$× atomic weight of ${}_{17}{}^{37}\mathrm{Cl}$
• Fractional atomic mass= $\frac{3}{4}\times 35+\frac{1}{4}\times 37$
• Fractional atomic mass= $26.25+9.25$
• Fractional atomic mass= $35.5$ u