Question

Why argon (at. mass 39.94) has been placed before potassium (at. mass 39.10) in the periodic table?​

Answer 1

Answer:

Argon (atomic mass=39.94) has been placed before potassium (atomic mass=39.10) in the periodic table but still it is placed before potassium because periodic table is based on atomic number. Atomic number of Ar is 18 and that of K is 19 so, K comes after than Ar.

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Answer 2

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In the modern periodic table, elements have been placed in order of their increasing atomic numbers. The atomic number of argon is 18 and that of potassium is 19. 

Thus, argon has been placed before potassium.