In Mendeleev's atomic-mass arrangement, argon (39.95 amu) was placed before potassium (39.10 amu). What did this anomaly reveal about the true basis of periodicity?

1. By mass, argon is heavier than potassium, yet argon (unreactive) cannot sit with reactive sodium and lithium. 2. This showed atomic mass is not the true ordering property. 3. The fundamental property turned out to be the number of protons, i.e. atomic number. 4. So periodicity is governed by atomic number, leading to the modern periodic law. Option B is exactly the assumption the anomaly disproved. _Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 8 Periodic Classification of Elements "Modern Periodic Law", p.114_