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428 BCE
Plato
Plato came up with the theory that the world is not as real as timeless or absolute ideas. -
400 BCE
Democritus
Democritus believed that atoms were uniformed, incompressible, and indestructible solids that moved in infinite numbers in empty space. -
330 BCE
Aristotle
Aristotle believed that there is a source of all beings that is separate and unchanging. -
500
The Alchemists
The Alchemists came up with the theory that there were two principals (mercury and sulfur) that formed all metals. -
Lavoisier
Lavoisier created the theory that water was not an element but a combination of oxygen and hydrogen. -
John Dalton
John Dalton had the theory that all matter was made of up a composition of atoms all with different masses and properties. -
Newland's Law of Octaves
J.A.R Newland's created the generalization chemical elements can be arranged according to atomic weight and those with similar physical and chemical properties can align. -
Mendeleev's Periodic Table
Mendeleev came to the realization that the atomic mass of elements were related to their physical and chemical properties so he arranged them periodically with the similar properties of the atoms falling in the same column. -
Photoelectric Effect
Photoelectric Effect is the phenomenon in which charged particles are released from a material and it absorbs electromagnetic radiation. -
Discovery of Radioactivity
In on of Henri Becquerel's most famous accidental discoveries, he opened a drawer and discovered radioactivity. -
Discovery of the Electron
J.J. Thomson was experimenting with a Crookes, or cathode ray tube when he discovered the electron. He demonstrated that cathode rays were negatively charged. In addition, he also studied positively charged particles in neon gas. -
Plum Pudding Model
The plum pudding model is a model of the atom with negatively charged "plums" and positively charged "pudding". -
Planck's Quantum Theory of Light
Planck's theory was different atoms and molecules can absorb energy in discrete quantities only. the smallest amount of energy that can be absorbed is called the quantum. -
Charge of the Electron
R. Millikan measured the charge of an electron by using negatively charged oil droplets. -
Robert Millikan
Robert Millikan demonstrated the change carried by an electron using the falling-drop method. -
Rutherford's Gold Foil Experiment
Particles hit a piece of gold foil, which have a positive charge. Most of the atoms went right through proving electrons have a negative charge. -
Bohr's Planetary Model
Bohr's model represents the theory that electrons circle the nucleus and when the is in one of the orbits the energy is fixed. -
Mosley's Atomic Number
Moseley had published a paper claiming that the atomic number is also the number of positive charges within the element. -
Discovery of the Proton
Ernest Rutherford's research led to the nuclear reaction in first the splitting of, in which he discovered protons. -
Schrodinger Equation
Erwin Schrödinger formulated a wave equation that calculated the energy levels of electrons in atoms. -
Heisenberg Uncertainty principal
His principal he came up with is that we cannot know the position and speed of a particle, like a proton or electron, with exact accuracy. -
Discovery of the Neutron
James Chadwick had made the discovery that there was an atom with a neutral charge, called the neutron.
