Question

modern periodic law:
properties of elements
when elements are arranged by

Explanation:

The Modern Periodic Law states that the properties of elements are periodic functions of their atomic numbers. So we need to fill in the blanks with the correct phrases. The first blank (after "Properties of elements") should be "are periodic functions" and the second blank (after "when elements are arranged by") should be "increasing atomic number".

Answer:

Properties of elements \underline{are periodic functions} when elements are arranged by \underline{increasing atomic number}.