History of the atom

Leucippus, originally came up with the atomic theory, but it was then adopted by Democritus. The atomic theory stated that “The universe is composed of two elements: the atoms and the void in which they exist and move.” According to Democritus atoms were miniscule quantities of matter.

Law of conservation of mass...

Dalton's atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties.
Dalton based his theory on the law of conservation of mass and the law of constant composition.
The first part of his theory states that all matter is made of atoms, which he believed was a indestcructable solid sphere of uniform density.
The second part of the theory says all atoms of a given element are identical in mass and properties.
The third part says compounds are combinations of two or more different types of atoms.
The fourth part of the theory states that a chemical reaction is a rearrangement of atoms. That chemical reactions don't destroy or create atoms. They merely rearranged the atoms.

Dmitri Mendeleev creates the periodic table of elements

J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles later named electrons.
(Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.)
To test the properties of the particles, Thomson placed two oppositely-charged electric plates around the cathode ray. The cathode ray was deflected away from the negatively-charged electric plate and towards the positively-charged plate. This indicated that the cathode ray was composed of negatively-charged particles.
Thomson also placed two magnets on either side of the tube, and observed that this magnetic field also deflected the cathode ray. The results of these experiments helped Thomson determine the mass-to-charge ratio of the cathode ray particles, which led to a fascinating discovery-−minusthe mass of each particle was much, much smaller than that of any known atom. Thomson repeated his experiments using different metals as electrode materials, and found that the properties of the cathode ray remained constant no matter what cathode material they originated from. From this evidence, Thomson made the following conclusions: the cathode ray is composed of negatively-charged particles, the particles must exist as part of the atom, since the mass of each particle is only approx. one-two thousandth the mass of a hydrogen atom, and these subatomic particles can be found within atoms of all elements.

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. (its description is very similar to plum pudding, a popular English dessert)
Thomson's plum pudding model of the atom had negatively-charged electrons embedded within a positively-charged "soup."

Bohr's model of hydrogen explains atomic emission spectra