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.

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.

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Answer 2

Peyton Adams

Since the atomic numbers of elements indicate the number of protons in their nuclei, argon has an atomic number of 18 while potassium has a number of 19, meaning that argon is prior to potassium in the modern periodic table due to its lower atomic number.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.