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Period: 370 to 460
Democritus ***actually 460-370 BC***
Saw the world and everything in it as endless motion of small, indivisible particles- “atomos”. Named the atom as the building block of matter. -
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Antoine Lavoisier
Lavoisier first outlined the Principle of Conservation of Mass. Said that one element could not change into another. Lavoisier’s concept of an element provided the foundation for Dalton’s atomism. -
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Joseph Louis Proust
Proust proposed the law of constant composition, which states that the ratio by mass of the elements in a chemical compound is always the same, regardless of the source of the compound. Helps in the development of universal laws. -
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John Dalton
Discovered the law of pressures, furthered the atomic theory, and discovered the law of stoichiometry. His Atomic Theory gave more evidence suggesting that atoms are the building blocks of matter because he had numbers to back it up. -
Law of Conservation of Mass
Says mass is an isolated system and will remain constant and that matter can neither be created nor destroyed. Lavoisier's concept of an element provided the foundation for Dalton's atomic theory. -
Law of Definite Proportions
States that a given chemical compound always contains the same proportion by mass of its constituent elements. Discovered by Proust. Helps in the development of universal laws. -
Dalton's Atomic Theory
States that elements consist of tiny particles called atoms. All atoms of an element are the same and have the same mass. Provided further reason to believe that atoms are the building block of matter by providing the weight of careful chemical measurements. -
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Dmitiri Mendeleev
He is known as the “Father of the Periodic Table” because he was the first to organize the elements into one chart. By noticing that every 8 elements had similar features, this set up further reason to look into the properties of each atom such as the electron shells. -
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Eugene Goldstein
Discovered the proton by using a tube filled with hydrogen gas. Fixed the confusion as to why the atom is neutrally charged with negatively charged electrons inside of it. -
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J.J Thompson
After many cathode ray observation, he concluded that the rays of light are actually streams of n egatively charged particles. Now knew that the atom actually wasn't indivisible, and there were particles smaller than it- “subatomic particles”. -
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Marie Curie
Curie discovered the element radium. It opened the door to deep changes in the way scientists think about matter and energy. -
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Robert Millikan
Measured the charge of an electron using an oil drop. Helped determine the mass of the electron because of the mass: charge ratio. -
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Ernest Rutherford
Conducted the gold foil experiment that showed the existence of a nucleus instead of the plum-pudding model. The experiment shot beams of alpha particles through a piece of gold foil to see what direction the beams would be reflected off at. -
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Lise Meitner
Proved that alpha particles did indeed scatter. The fact that particles scattered was a breakthrough in discovering the nucleus. -
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Niehls Bohr
Proposed a theory based on the line spectra. His theory shows that atoms cannot emit energy continuously but only in precise steps. This led to the idea of electron shells, or concentric circles around the nucleus. -
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Henry Moseley
Developed the application of X-Ray spectra to study atomic structure. This allowed more information to be found out about each element allowing for more accurate positioning on the periodic table. -
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James Chadwick
Discovered the neutron. It was the missing puzzle piece. Because the mass of the proton and electron were already known, but there were unknown masses in the neutron whenever it was being studied. -
Cathode Ray Tube
Tubes with all the air sucked out of them would give off a florescent light when high voltage was pumped into them. Led to the discovery of the electron by J.J Thompson. -
Gold Foil Experiment
Rutherford fired Alpha particles at a sheet of gold foil. He found that most passes through the foil but some bounced off. Proved Thomson’s raison bread experiment wrong and helped discover the nucleus. -
Rutherford Model
Conducted an experiment intending to determine the angles at which beams of alpha particles would scatter after passing through thin gold foil. The nucleus was discovered and added to the model. -
Plum Pudding Atomic Model
Thomson's model- depicted the electrons mixed with positively charged particles that made up a "pudding". Explained the neutral charge of the atom. -
Bohr Line Spectra
Proposed a theory based on the line spectra. Showed that atoms cannot give off energy continuously. Proposes electron shells surrounding nucleus. -
Bohr Planetary Model
Rutherford's theory stated that electrons orbited the nucleus, but didn't cover how they kept energy. But this new model had particular orbits which the electrons stayed in an kept energy, so they did not spin into the nucleus. This was the beginning of quantum mechanics. Helped develop the energy levels that are crucial to the function of atoms. -
Quantum Mechanical Model
The region in space around the nucleus in which an electron is most probably located is what can be predicted for each electron in an atom. Helps in finding the location of electrons around the atom.
