# Why is argon placed before potassium in the modern periodic table?

Because the Periodic Table is based on atomic number, $Z$, and not on atomic mass.
For potassium metal, $Z = 19$; its mass is $39.10 \cdot g \cdot m o {l}^{-} 1$. For argon gas, $Z = 18$; its mass is $39.95 \cdot g \cdot m o {l}^{-} 1$. So even though most argon atoms are more massive than most potassium atoms, all potassium atoms have one more positively charged, massive nuclear particle than does argon. This results in a quite dramatic difference in reactivity.