Question

a) Name the two isotopes of naturally occurring chlorine.

b) If ratio by weight of lighter isotope to heavier isotope is 3:1, find the average atomic weight of chlorine.

Answer 1

Part 1: Isotopes of chlorine

• Isotopes are atoms of the same chemical element with the same atomic number but different mass numbers.
• Chlorine has the atomic number 17.
• The two isotopes of chlorine are chlorine-35 and chlorine-37.
• Here one has a mass number 35 and the other has a mass number 37.

Part 2: Calculating the average atomic mass

• The two isotopes of chlorine are chlorine-35 and chlorine-37.
• Chlorine-35 is the lighter isotope and chlorine-37 is the heavier isotope.
• The ratio by weight of lighter isotope to heavier isotope is 3:1

$$\begin{gathered} \mathrm{Average}\mathrm{atomic}\mathrm{mass}=\frac{\mathrm{mass}\mathrm{number}\mathrm{of}\mathrm{lighter}\mathrm{isotope}\times \mathrm{fraction}\mathrm{of}\mathrm{its}\mathrm{natural}\mathrm{abundance}}{}+\frac{\mathrm{mass}\mathrm{number}\mathrm{of}\mathrm{heavier}\mathrm{isotope}\times \mathrm{fraction}\mathrm{of}\mathrm{its}\mathrm{natural}\mathrm{abundance}}{} \\ \mathrm{Average}\mathrm{atomic}\mathrm{mass}=\frac{35\times 3}{4}+\frac{37\times 1}{4} \\ \mathrm{Average}\mathrm{atomic}\mathrm{mass}=35.5 \end{gathered}$$