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Democritus 442BC
In 442 BC Democritus said that matter is composed of empty space through which atoms move. He also said atoms are solid, homogemous, indestructible and invisible. Democritus also said different kinds of atoms have different sizes and shapes, and that apparent changes in matter result from changes in the groupings of atoms and not from changes in the atoms themselves. -
Antoine Lavoisier-Law of Conservation of Mass
In 1785 Antoine Lavoisier proposed The Law of Conservation of Mass. He set the stage for the foundation of chemistry and demonstrated that substances could combine to form new materials. -
John Dalton-Dalton's Atomic Theory
In 1803 Dalton proposed a theory known as Dalton's Atomic Theory that consisted of five basic parts. 1. All matter is composed of extremely small particles called atoms. 2. All atoms of a given element are identical, having the same size, mass, and chemical properties. 3. Atoms cannot be created, divided into smaller particles, or d estroyed.4. Different atoms combine in simple whole number ratios to form compounds. 5. In a chemical reaction, atoms are seperated, combined, or rearranged. -
Dmitri Mendeleev
Dmitri Mendeleev arranged the 63 known elements into a Periodic Table based on atomic mass, which he published in Principles of Chemistry in 1869. -
J.J. Thompson-cathode ray tube-Plum Pudding Atomic Model
In 1897, J.J. Thomson discovered the electron by experimenting with a cathode ray, tube. He proposed the Plum Pudding Atomic Model. The negative electrons represented the raisins in the pudding and the dough contained the positive charge. -
Robert Millikan
Robert Millikan determined the unit charge of the electron in 1909 with his oil drop experiment. This allowed for the calculation of the mass of the electron and the positively charged atoms. -
Ernest Rutherford-Gold Foil Experiment-Rutherford Model
In 1911, Ernest Rutherford, a former student of J.J. Thomson, proved Thomson's plum pudding structure incorrect. Through the Gold Foil Experiment, he came up with the Rutherford Model. Rutherford concluded that the atom consisted of a small, dense, positively charged nucleus in the center of the atom with negatively charged electrons surrounding it. The discovery of the nucleus is considered to be Rutherford's greatest scientific work. -
Neils Bohr-The Bohr Planetary Model
In 1913, Neils Bohr developed a new model of the atom-the Bohr Planetary Model. He proposed that electrons are arranged in concentric circular orbits around the nucleus. This model is patterned on the solar system. -
Henry Moseley
In 1913, Henry Moseley's discoveries resulted in a more accurate positioning of elements in the Periodic Table by closer determination of atomic numbers. When atoms were arranged according to increasing atomic number, the few problems with Mendeleev's periodic table had disappeared. Because of Moseley's work, the modern periodic table is based on the atomic numbers of the elements. -
Erwin Schrödinger-Quantum Mechanical Model
In 1926 Erwin Schrödinger took the Bohr atom model one step further. used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the Quantum Mechanical Model of the atom. Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron. -
Electron Cloud Model
In 1926 Erwin Schrodinger wondered why electrons had to be limited to specific orbitals. He proposed a new model-The Electron Cloud Model-in which electrons move in cloudlike orbits rather than fixed ones. He said it is impossible to know the exact location of electrons. The best we can do is know where the electron probably is.The Electron Cloud Model was a product of the Quantum Mechanical Model. -
James Chadwick
Until 1932, the atom was believed to be composed of a positively charged nucleus surrounded by negatively charged electrons. In 1932, James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron. This allowed a better model of the atom to be made.
