Question

Explain why Chlorine(Cl) has relative atomic mass of about 35.5 u ?

Answer 1

Chlorine exists as two isotopes ${\mathrm{Cl}}_{17}^{35}\mathrm{and}{\mathrm{Cl}}_{17}^{37}$

These isotopes are available in a 3:1 ratio.

The relative atomic mass of an element = $\mathrm{Relative}\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{an}\mathrm{element}=\frac{(\mathrm{mass}\mathrm{of}1\mathrm{st}\mathrm{isotope}\mathrm{x}\mathrm{it}\text{'}\mathrm{s}\mathrm{abundance})+(\mathrm{mass}\mathrm{of}2\mathrm{nd}\mathrm{isotope}\mathrm{x}\mathrm{it}\text{'}\mathrm{s}\mathrm{abundance})}{\left(100\right)\mathrm{or}\left(\mathrm{ratio}\right)}$

Relative atomic mass of Chlorine(Cl) =$\frac{(35\mathrm{x}3)+\left(37\mathrm{x}1\right)}{4}=\frac{105+37}{4}=35.5$

Therefore the atomic mass of the Chlorine is fractional i.e 35.5 u