He proposed that matter could not be divided into smaller pieces forever. He said that matter was made of small, hard particles that he called “atoms”. 

He created the very first atomic theory. 
All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. Atoms are invisible.
Atoms join together to make new substances (molecules).

He discovered negatively charged particles inside the atom (electrons). 

He proposed that in the center of the atom is positively charged part called the nucleus. He discovered the protons and nucleus.