442 BC Thinking about matter 
Democritus, a Greek philosopher, co-originated the thought (with his teacher, Leucippus) that all matter is composed of indivisible elements.

1803 John Dalton and atoms 
John Dalton, a British chemist and physicist, developed a theory that matter is simply composed of atoms of different weights and is combined in ratios by weight. Also proposed that these atoms are spherical, and are in motion. 

1870 Cathode ray tube and TV (1870's) 
Sir William Crookes constructed a primitive cathode ray tube, which later became the basis for television.

1896 X-rays developed 
Wilhelm Rontgen discovered that certain chemicals glowed when exposed to cathode rays. These rays weren't deflected by a magnetic field produced in the cathode ray tube. He named these X-rays.

1898 Radiation, energy, and the atom 
Pierre and Marie Curie theorized that radioactive particles cause atoms to break down, then releasing radiation that takes the form of energy and subatomic particles.

1898 Electrons discovered 
JJ Thomson discovers the electron, using properties of cathode rays.

1900 Early quantum theory developed 
Max Planck introduced what would be known as quantum theory, stating that electromagnetic energy could only be emitted in quantized form. His quanta are now called photons by physicists.

1905 Einstein and the nature of light 
Albert Einstein, creates special and general theories of relativity, and hypothesizes about the particle nature of light. This was the basis of nuclear energy.

1908 Charge of an electron measured (1908-1917) 
Robert Millikan measured the charge of a single electron. This is known as the elementary charge, one of the fundamental physical constants.

1909-Structure of an atom developed (1909-1911) 
Ernest Rutherford known as the father of nuclear physics, developed the theory for the structure of the atom. He used a gold foil experiment, observing the scattering of alpha particles, and demonstrated for the first time the existence of the atomic nucleus.

1913 Bohr improves the atomic model 
Neils Bohr developed the Bohr atomic model, with electrons travelling in orbits around the nucleus, and chemical properties being determined by how many electrons are in the outer orbits. He also integrated the Planck quantum theory, stating that when electrons change orbits they emit a quantum of discrete energy.

1926 Math describes electron changes 
Erwin Shrodinger described how electrons move in wave form, and developed the Schrodinger equation which describes how the quantum state of a system changes with time.

1931 The neutron is discovered 
James Chadwick discovers the neutron component of the atomic nucleus, explaining the nuclear fission of uranium 235. This also made it possible to produce elements heavier than uranium in the lab.

1938 Nuclear fission developed 
Otto Hahn, regarded as the father of nuclear chemistry, discovers nuclear fission, along with Lise Meitner.

1951 Nuclear medicine and I-131 
Glenn Seaborg, many discoveries of the transuranium elements, as well as many advances in nuclear medicine, including the development of I-131 for thyroid disease.

1964 Elementary particles smaller than the atom Murray Gell 
Mann proposes the quark model (independently George Zweig does as well), which describes elementary particles that have no substructure (and therefore can't be split).