In 1897, J.J. Thomson discovered the electron through the use of his cathode ray experiment. He also concluded that they are a part of every atom.

In 1913, through one of his radioactive experiments, Frederick Soddy discovered the different types of atoms in different parts of the periodic table.

Margeret Todd made up the term isotopes for Soddy's discovery on atoms. This later led to the discovery of stable isotopes.

In 1913, Niels Bohr came up with the first model of the hydrogen atom. He also stated that electrons could jump from their individual spheres to others.

In 1938, chemist Otto Hahn, through his chemical experiments, discovered the element barium. This was later verified as the first example of nuclear fission.

In 1803, John Dalton used atoms in a theory to explain how different types of chemical reactions occurred. This caused him to be called the father of modern atom theory.

In 1789, Antoine Lavoisier created the law of conservation of mass and discovered that elements can't be broken down further.

In 1905, Albert Einstein was the first person to analyze Robert Brown's "Brownian Motion" using mathematics.


Sources:
http://en.wikipedia.org/wiki/Atom