Answer:
Question 1:
An element is a pure substance which cannot be broken down by chemical means.
Question 2:
A molecule is a group or cluster of two or more atoms held together by chemical bonds. A compound is a substance or material which is formed by two or more different types of elements which are combined chemically in a fixed proportion.
Question 3:
Electrons, Protons and Neutrons
Question 4:
Dmitri Mendeleev in 1869 developed the periodic table.