Question

What is the reason for fractional atomic mass of some elements?

Answer 1

The reason for the fractional atomic mass of some elements is:

• Some elements are present in the form of Isotopes due to which they have different atomic masses.
• In the case of Isotopes, the average atomic mass of all the Isotopes is calculated on the basis of the percentage they are present in the atmosphere multiplied by the atomic mass of the isotopes of the element.

Example: Atomic mass calculation for chlorine

• The atomic mass of Chlorine (Cl) is 35.5 which is calculated on the basis that Chlorine has 2 Isotopes i.e${}_{17}{}^{35}\mathrm{Cl}\&{}_{17}{}^{37}\mathrm{Cl}$
• But the isotope having an atomic mass 35 is present 75% in the atmosphere while Chlorine having an atomic mass 37 is present only 25% in the atmosphere.
• So average atomic mass $=\left(\frac{35\times 75}{100}\right)+\left(\frac{37\times 25}{100}\right)=35.5\mathrm{u}$