How do you account for the fact that, although potassium is placed after argon in the periodic table, it has a lower relative atomic mass?

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List 1 S6 and Sg 1⁹F and ³¹p isotope of hydrogen ¹²℃ and ¹³℃ hydrogen ion group 1 elements same energy negatively charged particle spin-paired electrons electron, proton and neutron group 15 elements Cr, Mn, Fe List 2 electron proton pnictogens d-block elements protium fundamental particles m₁ = ±1/ allotropes degenerate monotopic elements alkali metals isotopes of an element