Why did mendeleev leave blank spots in his periodic table?

Electric charge is what ? a. kinetic energy b. radiation c. discovery d. electricity

Astudent is trying to create a table that compares hypotheses, theories, and laws. hypothesis theory law do scientific researchers formulate it? yes yes yes does it explain why things happen? yes yes no yes yes yes is it used to make predictions? no yes yes which of the following questions would most likely fill the blank in the table? is it an intelligent guess? is it newly formulated? is it based on observations? has it been proved?

Why is the melting of ice a physical change ?