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Democritus (460-360 B.C.)
Democritus found the atom.All atoms are indivisble, Atom derives from atomos. Atomos is greek for indivisible. Based on logical reasoning, he theorized that all matter was made of indivisble particles. This was based off logical reasoning and experiments weren’t considered necessary. -
Antoine Lavoisier
The French chemist, Lavoisier, developed the Law of conservation of mass by studying several chemical reactions. The law stated that the mass of a system must remain constant over time as for it cannot change quantity if it is not added or removed. The law also suggested that mass can neither be created nor destroyed, but it may be rearranged in space or change form. Lavoisier found that mass is conserved in chemical reactions, as well. -
Joseph Proust
Proust's law of definite proportions stated that a chemical compound always contains exactly the same proportion of elements by mass. Proust's formulation of the law occurred after several experiments that concerned inorganic binary compounds. He thought that most metals created two distinct oxides at constant proportions and these were capable of producing two separate series of compounds. Proust was the first scientist to accumulate conclusive evidence for this law. -
John Dalton
First to form an Atomic Theory. Believed atoms composed of tiny indivisible and indestructible tiny particles called “atoms”. Atoms of the same element are identical and compounds have the same relative number and types of atom. Chemical reactions involve only a rearrangement of atoms. -
Spherical Atomic Model
In 1808, Dalton formed the Spherical Atomic model of the atom. The discovery of subatomic particles proved atoms were divisible. The discovery of isotopes proved atoms of the same element could have different masses. Overall, his atomic theory led him to develop the Spherical model. The model stated that one sphere could not be broken into smaller parts. -
Eugen Goldstein
Eugene Goldstein, along with Rutherford, discovered the proton in cells. He made this discovery by using a hydrogen gas-filled tube, which was similar to Thompson's tube in 1897.Goldstein was a German physicist who observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He called these rays the canal rays and concluded that they were composed of positively charged subatomic particles, called protons. -
Quantum Theory
Theory based on using the concept of the quantum unit to describe the dynamic properties of subatomic particles and the interactions of matter and radiation. It was postulated By Max Planck, that energy can be emitted or absorbed by matter only in small, discrete units called quanta. -
J.J. Thomson
J.J. Thompson carried out a cathode ray experiment. by sending electricty through a cathode ray, he proved that these small cathode ray particles must be negative since they were attracted to the positive pole. He called these particles electrons. At the time, electrons were neutral subatomic particles embedded within the nucleus. Thompson resembled this is his Plum Pudding Model. -
Ernest Rutherford
Ernest Rutherford conducted his famous experiment known as the gold foil experiment. Their test used alpha particles at a sheet of gold foil to discover that most passed through while some where deflected. he discovered the nucleus, a small, dense central point of an atom. He also proposed his nuclear model, stating that electrons were distributed around the nucleus. -
Niels Bohr
Niels Bohr predicted that the electrons moved in specific circular paths, or orbits, at different levels of different energy amounts. He also said that the electrons can jump from one level to another. -
Louis DeBroglie
The French physicist made significant contributions to the quantum theory with his wave-particle duality concept. It stated the wave nature of electrons and suggested that all matter has wave properties. His theory set the basis of wave mechanics and said “with every particle of matter with mass and velocity a real wave must be associated”. -
Werner Heisenberg- Uncertainty Principle
With the help of Bohr, Heisenberg published results that closed the Quantum theory. It was a theory of matter and energy based on the concept of quanta, especially quantum mechanics. Their experiment used high-energy gamma rays for illumination. Heisenberg measured the position of an electron and he noticed a relationship between them. Werner Heisenberg proposed the theory that an electron's position and velocity cannot be simultaneously known. The Quantum theory is still used today. -
Erwin Schrodinger
Schrodinger took Bohr's model one step further using mathematical equations to describe the likelihood of finding an electron in a certain position. He provided an equation that related the distance an electron could be found. Erwin Schrodinger stated that the moving particles, electrons, were similar to waves. He developed the Quantum Mechanical Model which illustrated that electrons are found with a "blurry" cloud called the electron cloud. -
James Chadwick
James Chadwick discovered the neutron. He used scattering data to calculate the mass of his neutral particle. Chadwick showed that the physical properties of the radiation could be explained by invoking a neutral particle of one amu. He also explored the artificial radioactivity reason of Irene and Frederic Joliot-Curie by repeating their experiments and finding a neutral particle with the same mass of a proton, but with a neutral charge.
