a] Why are sodium and chlorine placed in the same period in the Modern Periodic Table?
b] Why do elements placed in the same group of the periodic table have similar chemical properties?
c] Who proposed the modern periodic law? On which fundamental property of an atom is it based?

Explanation

a) Since they have the same number of electron shells.
b) Since they have the same number of valence electrons, which decides the chemical properties of an element.
c) Moseley proposed the modern periodic law. It is based on the atomic number - the number of protons in an atom which decides its electronic configuration and hence its properties.