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DEMOCRITUS (CIRCA 470 B.C)Democritus said that all matter is made

up of tiny, indivisible particles called atoms. These atoms were completely solid and were eternally in motion.

ARISTOTLE (384 BC)Aristotle said that atoms were composed

of the four elements: fire, water, earth, and air. Each of these atoms had certain properties depending on what they were made of: hotness, dryness, wetness, or coldness. This contradicted the work of Democritus and put a second theory in use for almost 2000 years (before it was disproved). Aristotle admired Democritus as a role model.

JOSEPH-LOUIS PROUST (1793)Proust proved that the relative quantities

of any given pure chemical compounds’s constituent elements remain invariant, regardless of the compound’s source. This is known as Proust’s law, or the law of definite proportions.

ANTOINE LAVOISIER (LATE 1800’S) Antoine Lavoisier formulated the theory

of conservation of mass. This showed that matter couldn’t be created or destroyed, which supported the theory of Democritus. Lavoisier also showed that compounds are simply combinations of different types of atoms, further supporting the idea that atoms are indivisible. Lavoisier provided crucial information for John Dalton’s model.

JOHN DALTON (1803)Dalton compounded all the work of

Lavoisier and Democritus into one, single atomic theory. This theory stated that all matter is made of indivisible and indestructible atoms, all atoms of a given element are identical, compounds are combinations of elements (atoms), and a chemical reaction is merely a rearrangement of atoms.

MICHAEL FARADAY (1832)Faraday Studied the effect of electricity

on solutions, "electrolysis" as a splitting of molecules with electricity, developed laws of electrolysis.

HENRI BECQUEREL (1896)Becquerel discovered radioactivity in his

research of uranium and other materials. This helped to show what happened when atoms were affected and observed. Becquerel shared a Nobel Prize with the Curies.

J.J. THOMSON (1898)Thomson conducted an experiment with a

cathode ray tube, and bent the path of the rays with magnets. He interpreted this as evidence of much smaller particles, which he called electrons. Using this, he formulated the plum pudding model, which added negatively charged electrons into the positively charged solid part of the atom. Thomson was the teacher of Ernest Rutherford.

MARIE AND PIERRE CURIE (1898)The Curies enhanced the understanding

of radioactivity through their research of radioactive materials (discovering radium and polonium). This helped to show what happened when atoms were affected and observed. The Curies shared a Nobel Prize with Henri Becquerel.

MAX PLANK (1900)Max Planck created the quantum theory,

which was later used by Niels Bohr to create his model of the atom.

ALBERT EINSTEIN (1905)Einstein best known for his Theory of

Relativity and his mass energy equivalence formula E = mc2. He is also famous for creating the first Atom Bomb.

ROBERT MILLIKAN (1908)Robert Millikan discovered the exact

charge of the electron in his “Oil Drop Experiment”, enhancing the model of J.J. Thomson. He created this experiment with the aid of Harvey Fletcher.

ERNEST RUTHERFORD (1909)In Rutherford’s experiment he shot alpha

particles at gold foil, expecting them to pass right through. Some bounced back, however, and he formulated the planetary model, which showed that most of the mass of an atom was concentrated in the center (the nucleus). Ernest Rutherford was the student of J.J. Thomson (he was trying to prove Thomson’s model with his experiment, but ended up disproving it).

NIELS BOHR (1913)Niels Bohr said that electrons had

quantified amounts of energy using Planck’s Quantum theory. He proposed a new model of the atom where the electrons travel around the nucleus in orbits determined by their energy levels.

JAMES CHADWICK (1931)James Chadwick discovered the neutrally

charged neutron, the discovery that ultimately led to the atomic bomb. He enhanced Bohr’s model by adding neutrons to the nucleus.

http://www.timetoast.com/timelines/the-history-of-the-atom--4