Question

what is the main idea in the periodic law?
the periodic law showed that the elements with close number of neutrons in atom have the similar properties.
the periodic law showed that while the elements have unique properties, there is a periodic repetition of similar properties.
the periodic law showed that the elements with close number of electrons in atom have the similar properties.
the periodic law showed that the elements with close number of protons have the similar properties.

Explanation:

The periodic law states that when elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties. It is not about neutrons, electrons in a general sense (valence electrons are more relevant for properties in a group - but the law is about periodic repetition), or just protons (atomic number which is number of protons is the basis for arrangement). The key is the periodic repetition of similar properties.

Answer:

The periodic law showed that while the elements have unique properties, there is a periodic repetition of similar properties.