The periodic law is a law relating the properties of the element with their
atomic numbers.
The element is <u>potassium, K</u>.
Reason:
By atomic mass, potassium is 39.10 g, while argon is 39.10 g.
Going by the atomic mass alone, potassium should come before argon in
the periodic table and therefore should be an halogen and not an alkali
metal.
However, the number of protons in the potassium is 19, and therefore larger
than the number of protons in argon, which is argon.
Therefore, the element is <u>potassium, K</u>.
Learn more about the periodic law here:
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