• 400 BCE

# Democritus

Democritus was the first to introduce the Atomic Theory stating that all matter is made up of small indestructible units called atoms.
• Period: 400 BCE to

Timeline
• # Isaac Newton

Isaac Newton theorized theorized a mechanical universe with small, solid masses in motion.
The first law states that every object in the state of uniform motion tends to remain in the same state of motion unless an external force is applied to it
The second law states that the net force of an object is proportional to the time rate of change to its linear momentum.
The third law states that for every action there is an equal and opposite reaction.
• # John Dalton

Proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together.

Michael Faraday developed the two laws of electrochemistry.
m=(Q/F)*(M/z)
m is the mass of the substance liberated at an electrode in gms
Q is the total electric charge passed through the substance in coulombs
F = 96500 C mol−1 is the Faraday constant
M is the molar mass of the substance in grams per mol
z is the valence number of ions of the substance
n=(It/F)*(1/z)
n is the amount of substance ("number of moles") liberated: n = m/M
t is the total time the constant current was applied.
• # J. Plucker

J. Plucker built one of the first cathode-ray tubes.
• # Dmitri Meneleev

Dmitri Mendeleev created the periodic table. He formulated the Periodic Law, created a farsighted version of the periodic table of elements, and used it to correct the properties of some already discovered elements and also to predict the properties of eight elements yet to be discovered. He arranged the periodic table in order from smallest to largest mass size.
• # James Clerk Maxwell

James Clerk Maxwell proposed the theory of electromagnetism and made the connection between light and electromagnetic waves.
• # G.J. Stoney

G.J. Stoney theorized that electricity was comprised of negative particles he called electrons
• # E. Goldstein

E. Goldstein discovered canal rays, which have a positive charge equal to an electron.
• # Wilhelm Roentgen

Wilhelm Roentgen discovered x-rays
• # J.J. Thomson

J.J. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He demonstrated that cathode rays were negatively charged.
• # Ernest Rutherford

Rutherford discovered alpha, beta, and gamma rays in radiation in his gold foil experiment in which he demonstrated that the atom has a tiny and heavy nucleus.
• # Maie Sklodowska Curie

Marie Sklodowska Curie discovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium
• # Max Planck

Max Planck proposed the idea of quantization to explain how a hot, glowing object emitted light.
• # Frederick Soddy

Frederick Soddy came up with the term "isotope" to explain the unintentional breakdown of radioactive elements.
• # Hantaro Nagaoka

Hantaro Nagaoka proposed an atomic model called the Saturnian Model to describe the structure of an atom.
• # Richard Abegg

Richard Abegg found that inert gases have a “stable electron configuration.”
• # Hans Geiger

Hans Geiger invented a device that could detect alpha particles.
• # H.G.J. Moseley

H.G.J. Moseley discovered that the number of protons in an element determines its atomic number
• # Francis William Aston

Francis William Aston used a mass spectrograph to identify 212 isotopes
• # Niels Bohr

Niels Bohr proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.
• # Louis de Broglie

Louis de Broglie proposed that electrons have a wave/particle duality
• # Cockcroft and Walton

Cockcroft / Walton created the first nuclear reaction, producing alpha particles
• # Paul Dirac

Paul Dirac proposed the existence of anti-particles