Question

Why is the atomic mass of chlorine taken as $35.5\mathrm{u}$ and not a whole number like $36\mathrm{u}$ or $35\mathrm{u}$? Explain.

Answer 1

Isotopes

• Isotopes are varieties of a certain element that share the same number of protons but differ in the number of neutrons in the atom.
• An example is a chlorine atom has two isotopes: chlorine$-35$ and chlorine$-37$.
• There are 17 protons and 18 neutrons in the Chlorine$-35$. It occurs about $75\%$ of the time in nature.
• There are 17 protons and 20 neutrons in the Chlorine$-37$. It occurs about $25\%$ of the time in nature.

Atomic mass of chlorine

• Due to the existence of isotopes, the atomic mass of chlorine is $35.5\mathrm{u}$.
• The average mass of an element's naturally occurring isotopes compared to the mass of an atom of $12\mathrm{C}$ is defined as its relative atomic mass.
• Relative atomic mass of chlorine is as follows:

$\begin{array}{rcl}\mathrm{Relative}\mathrm{atomic}\mathrm{mass}& =& \frac{75}{100}\times 35+\frac{25}{100}\times 37\\ & =& 35.4\mathrm{u}\end{array}$

• As a result, the atomic mass of chlorine is $35.5\mathrm{u}$ rather than a full number like $36\mathrm{u}$ or $35\mathrm{u}$.