Question

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

Answer 1

Because the Periodic Table is based on atomic number, `Z`, and not on atomic mass.

Explanation:

For potassium metal, `Z=19`; its mass is `39.10*g*mol^-1`. For argon gas, `Z=18`; its mass is `39.95*g*mol^-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.