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

1 Answer
Oct 30, 2016

Answer:

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.