Atomic Theory Timeline

Alchemy

500 BCE

Is the art of understanding, deconstructing, and reconstruction of matter. It is composed of the four basic elements. (fire, earth, water, and air)

Democritus

400 BCE

Everything is composed of 'atoms', which are physically but not geometrically indivisible.

Aristotle

400 BCE

Disagreed with Democritus.
Everything is a combination of the four elements: earth, fire, water, and air.
Theory that a mass of incomprehensible size was everywhere: he called this 'hyle' (there was no separate particles for each material, it was all one.)

Leucippus

400 BCE

Theory of atom-ism. The world is composed of an infinite number of solid, indestructible, indivisible atoms that interact with one another. What separates the atoms is the void, which is empty space.

George Bauer

1500

Presented the first scientific classification of minerals (based on their physical properties) and described many new minerals: their occurrence and mutual relationship.

Paracelsus

1540

Paracelsus Theory: combination of Aristotelian Theory of Matter, alchemical correspondences, chemical therapy in medicine

Robert Boyle

1688

Wrote a book that overturned Aristotle's conception of the four elements, and replaced it with the modern idea of an element, namely that an element is a substance that cannot be separated into simpler components by chemical methods.

Isaac Newton

1704

Theorized a mechanical universe with small solid masses in motion.

Jons Jakob Berzelius

1779

Accepted John Dalton's atomic theory and made a new system of chemical symbols.

LaVoisier

1788

Law of Conservation of Mass: matter cannot be created or destroyed, only rearranged.
Discovered that water is made of Hydrogen and Oxygen.

Proust

1794

Law of Definite Proportions: A pure substance will always contain the same elements, combined in the same proportion by mass.

Gay-Lussac

1804

Law of Combining Volumes: At a constant temperature and pressure, volumes of reacting gases and gaseous products are in the ratio of small numbers.

John Dalton

1810

Law of multiple proportions: same two elements can combine in different compound; ratio of first element is fixed, while the second element varies.
Proposed first atomic theory, all matter is made up of tiny indivisible atoms, atoms of the same element are the same, atoms of different elements are different, atoms combine in while number ratios to form compounds.

Amedeo Avogadro

1811

Avogardo's hypothesis: equal volumes of gases under the same conditions that have the same number of particles.

Dmitri Mendeleev

1863

Formulated the table of elements listing them by their atomic weight and grouping them into families with similar characteristics, 'periodic law'.

Wilhelm Roentgen

1895

Discovered x-rays.

Henri Bequerel

1896

Discovered that uranium ores emit radiation, resembling x rays because it exposed film.

J. J. Thomson

1897

Used cathode rays to discover the electron, measured the bending of the rays to determine the charge to mass ratio of the electron.
Modified the cathode ray tubes to soon discover the positively charged proton.

Max Planck

1900

Quantum theory; energy is not emitted constantly, but in small plackets called quantum.

Albert Einstein

1905

Photoelectric effect; when light of a certain frequency shines on a metal, electrons are emitted.

Robert Millikan

1911

Performed the oil drop experiment which determined the charge of electrons as negative.

Ernest Rutherford

1911

Studied substances, especially alpha particles. Conducted the gold foil experiment; he bombarded gold foil with alpha particles, most particles passed through the foil, but some were deflected. He concluded that the atom is mostly empty space, but also discovered the very small nucleus core.

Niels Bohr

1913

Improved upon the atomic model by trying to answer the question;
Why aren't electrons pulled into the nucleus?
Electrons travel in definite energy levels without radiating energy
Electrons in each orbit have a certain amount of energy
Energy increases as the distance from nucleus increases.
Energy loses energy by dropping energy levels.

Henry Mosely

1913

Used x ray experiments to discover the atomic number of an element.

Arthur Compton

1923

Discovery of the Compton effect, which demonstrated the particle nature of electromagnetic radiation

Louis de Broglie

1923

Debroglie's hypothesis; if waves can behave like particles, then particles can behave like waves.

Erwin Shrodinger

1926

Schrodinger's wave equation:
Treat the electron like a wave
Developed an equation used to determine the probability of finding the electron in any given place around the nucleus.
Solutions to the equation are quantum numbers.

Werner Heisenberg

1927

Heisenberg's uncertainty principle: it's impossible to know both position and velocity of an electron at the same time.

James Chadwick

1932

Discovered the neutron.

Murray Gell-Mann

1969

Description and classification of subatomic particles.

Carlo Rubbin

1984

Discovery of W and V particles.

Peter Higgs

2013

Higgs Boson, a subatomic particle; the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles