MediumChemistryHigh School (9–10)GeneralThe Periodic Table › Development of the Periodic TableConceptual
Argon has an atomic mass of 39.9 u and potassium 39.1 u.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Argon has an atomic mass of and potassium . Suppose the table were ordered strictly by atomic mass. Into which group would each of them land, and why would that be absurd? How does the modern table settle it?
Need a reminder? The key ideaShow
Mendeleev arranged the elements by atomic mass, in groups of similar properties. He left gaps and predicted the missing elements, such as eka-aluminium (gallium) and eka-silicon (germanium). Error of a prediction .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. By mass. Potassium, being lighter, would come before argon. Potassium would land in group 18, and argon in group 1.
Step 2. Absurd. A soft, violently reactive metal would sit among the unreactive noble gases, and an unreactive gas among the alkali metals.
Step 3. The fix. By atomic number, argon is and potassium , so each sits with its own family.
Answer: by mass they swap families, which makes no sense; atomic number puts them right.
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