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500 BCE
Greek Era Discoveries
Empedocles proposed that matter was composed of four basic elements: earth, water, fire, and air. Meanwhile, Democritus originated the thought that all matter is composed of indivisible elements. -
Dalton's Discoveries
John Dalton theorized that matter is composed of indivisible particles called atoms. He then comes up with an "atomic theory", saying that the atoms are different weights, spherical, and are in motion. -
Mendeleev it to Dmitri
Dmitri Mendeleev arranged the elements into 7 groups based on their similar properties. He discovered that the properties of elements were periodic functions of their atomic weights, now known as the Periodic Law. -
Radiation and Energy in the Atom
Marie Currie theorized that radioactive particles caused atoms to break down, then releasing radiation that takes the form of energy and subatomic particles. -
Electrons are Discovered
JJ Thomson discovers the electron, using properties of cathode rays. -
Early Quantum Theory
Max Planck introduced the quantum theory, stating that electromagnetic energy could only be emitted in quantized form. His quanta are now known as photons. -
Einstein Sheds Some Light on the Subject
Albert Einstein creates special and general theories of relativity, and hypothesizes about the particle nature of light. This later becomes the basis of nuclear energy. -
Period: to
Electron Charge is Measured
Robert Millikan measured the charge of a single electron. This is known as the elementary charge, one of the fundamental physical constants. -
Period: to
Atomic Structure is Developed
Ernest Rutherford developed that the theory for the structure of the atom. He used a gold foil experiment and demonstrated for the first time the existence of the atomic nucleus. -
Bohr's Improvement
Neil Bohr developed the Bohr atomic model, with electrons traveling in orbits around the nucleus, and chemical properties being determined by how many electrons are in the outer orbits. -
The Neutron is Discovered
James Chadwick discovers the neutron component of the atomic nucleus, making it possible to produce elements heavier than uranium.