If we see the atomic mass versus atomic volume curve, we will notice the following features:

• Alkali metals such as $\text{Na,}\text{K,}\text{Rb}$ which have similar properties occur as peaks of the curve.
• Halogen elements like $\text{F,}\text{Cl,}\text{Br,}$ which have similar properties occur at the rising or the ascending part of the curve.
• Noble gases such as $\text{Ne,}\text{Ar,}\text{Kr}$ which have similar properties occur just before the alkali elements.
• Hydrogen and Helium seem to be exception to the rule.