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Plato

430 BCE

Plato theorized that solid forms of matter are composed of indivisible elements shaped like triangles.

Democritus

400 BCE

Democritus believed that atoms were uniform, solid, hard, incompressible, and indestructible and that they moved in infinite numbers through empty space until stopped.

Aristotle

332 BCE

Aristotle did not believe in the atomic theory, but that at all materials on Earth were made of the four elements, Earth, Fire, Water, and Air.

The Alchemists

850

The alchemists believed that all metals were formed from two principles. Those two principles would be mercury and sulfur. The mercury, with its essential property of fluidity and fusibility, gave rise to the malleability of metals. The sulphur, with its essential property of combustibility, contributed body and rusting.

Robert Boyle

1662

He believed that everything was composed of very tiny particles, an idea known as atomism. In this early atomic theory of Boyle, he referred to these tiny particles as corpuscles.

John Dalton

1804

John Dalton discovered that all matter is composed of extremely small particles called atoms, and that atoms cannot be subdivided, created, or destroyed

Newland's Law of Octaves (J.A.R. Newlands)

1865

when chemical elements are arranged according to increasing atomic weight, those with similar physical and chemical properties occur after each interval of seven elements

Mendeleev's Pd. Table (Dmitri Mendeleev)

1869

Mendeleev's chart allotted spaces for elements that were yet to be discovered. For some of these missing pieces, he predicted what their atomic masses and other chemical properties would be.

Discovery of Radioactivity (Henri Becquerel)

1896

While investigating the newly discovered X-rays in 1896, it led to studies of how uranium salts are affected by light. This lead to Henri accidentally discovering that uranium salts spontaneously emit a penetrating radiation that can be registered on a photographic plate.

Planck's Quantum Theory of Light

1900

Planck proposed that the energy of light is proportional to frequency, and Planck's constant (h) is the constant that relates them.

Plum Pudding Model (J.J. Thomson)

1904

In the plum pudding model, the electrons were embedded in a uniform sphere of positive charge, looking like plums in plum pudding. The positive matter was thought to be jelly-like, or similar to a thick soup.

Photoelectric Effect (Albert Einstein)

1905

The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material.

Robert Millikan

1910

Millikan's work demonstrated that electrons did have a discrete, quantifiable charge. Thomson had also already calculated the charge-to-mass ratio of an electron, so once Millikan was able to measure the charge of an electron, the mass of an electron could also be calculated.

Discovery of the Proton (Ernest Rutherford)

1911

Ernest Rutherford discovered that at the core of every atom is a nucleus. He stated that the atomic nuclei consist of electrically positive protons and electrically neutral neutrons.

Rutherford's Gold Foil Experiment (Ernest Rutherford)

1911

In Rutherford's experiment alpha particles were observed to scatter backwards from a gold foil. Rutherford's explanation was that the scattering was caused by a hard, dense core at the center of the atom–the nucleus.

Bohr's Planetary Model (Neils Bohr)

1913

This model shows that the electrons encircle the nucleus of the atom in specific allowable paths called orbits and that an electron's energy is fixed when in orbit.

Mosley's Atomic Numbers (Henry Mosley)

1914

Known as Moseley's law, this fundamental discovery concerning atomic numbers was a milestone in advancing the knowledge of the atom. Moseley came to the conclusion that the atomic number should be the number of positive charges in the atomic nucleus.

Schrodinger Equation (Erwin Schrödinger)

1925

The Schrodinger equation is a linear partial differential equation that governs the wave function of a quantum-mechanical system.

Heisenberg Uncertainty Principle (Werner Heisenberg)

1927

The uncertainty principle states that we cannot know both the position and speed of a particle and that the more we nail down the particle's position, the less we know about its speed and vice versa.

Discovery of the Neutron (James Chadwick)

1932

In May of 1932, James Chadwick announced his discoveries, that the core also contained a new uncharged particle, which he called the neutron.