Developed the atomic theory of the universe, the idea that everything is made of small, indivisible particles (atoms) moving through the void. Democritus’ ideas were purely theoretical and based on reason and logic, not experimental science. He didn’t perform physical experiments like we do today; instead, he used philosophical argumentation and observation of the world around him

Law of conservation of matter: He states that “Mass is neither created nor destroyed in chemical reactions”. He tested this by measuring the mass of the reactants before and after the chemical reaction and compare them. (1774)  

Other discoveries

• He gave a name to every element existing in the moment of the periodic table, which helped communication and future experiments. 

• He also managed to discover the elements inside H2O and air. 

• That respiration and combustion are caused by a chemical reaction 

• He helped by giving an explanation of elements and matter components. 

• He discovered new elements, organized and described chemical elements and made a visual representation. His periodic table, was the most similar to the current one, he used geological terms and ions and compounds. Then introduced to the Russian chemical society. 1869 

  
  

To prove it he needed a way to detect the alpha rays (Alpha rays are two protons and two neutrons together in a particle and they are “stopped when collide with other particles”) to help him discover the actual structure of the atom and prove it. He did an experiment that consisted of a gold foil sheet surrounded by a Zinc Sulphide coated screen and radioactive source screen that would flash every time an alpha ray hit it. And because some of them were deflected at more than 90 degrees assuming the emptiness of the atom. 

Sir James Chadwick discovered the neutron, a fundamental subatomic particle with no electric charge. It explained the existence of isotopes, atoms with the same number of protons but different masses. The discovery completed the basic model of the atom, with protons and neutrons in the nucleus and electrons orbiting around it.

He observed that protons were ejected from the paraffin with high energy.

 Neils Bohr proposed the Bohr Model of the atom in 1913 to explain the structure of the hydrogen atom. This theory introduced the idea that the electrons orbit in the nucleus (specific energy levels). 

The particle wave duality is that quantum objects can exhibit both wave and particle properties (depends on how you observe it)