Question

What is the average atomic mass of chlorine?

• A. 35
• B. 35.5
• C. 40
• D. 35.2

Answer 1

The correct option is B 35.5

The atomic number of chlorine is 17 and the mass number is 35.5. Chlorine has two common isotopes, chlorine-35 and chlorine-37. In nature, chlorine-35 occurs about $75\%$ and $25\%$ of chlorine-37.
Relative Atomic Mass = $(\frac{75}{100}\times 35)+(\frac{25}{100}\times 37)=35.5$