Contributor Contribution Democritus According to Democritus' atomic theory, the universe and all matter obey the following principles: Everything is composed of “atoms”, which are physically, but not geometrically, indivisible. Between atoms, there lies empty space. Atoms are indestructible. Antoine Lavoisier Lavoisier hypothesized that no loss of mass should be true of all chemical changes, and further experiments showed that he was right. This principle is now called the law of conservation of mass. John Dalton* J.J. Thomson* As Lavoisier continued his experiments with oxygen, he noticed something else. Although oxygen combined with many other substances, it never behaved as though it were itself a combination of other substances. Lavoisier was able to decompose the red calx into mercury and oxygen, but he could find no way to break down oxygen into two or more new substances. Because of this he suggested that oxygen must be an element—an ultimately simple substance which could not be decomposed by chemical changes. Lavoisier did, however, produce the first table of the elements which contained a large number of substances that modern chemists would agree should be classifies as elements. Dalton’s Atomic Theory The main points of Dalton’s atomic theory are: 1. Everything is composed of atoms, which are the indivisible building blocks of matter and cannot be destroyed. 2. All atoms of an element are identical. 3. The atoms of different elements vary in size and mass. 4. Compounds are produced through different whole-number combinations of atoms. 5. A chemical reaction results in the rearrangement of atoms in the reactant and product compounds. In 1897, J.J. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He demonstrated that cathode rays were negatively charged. In addition, he also studied positively charged particles in neon gas. J. J. Thomson, who discovered the electron in 1897, proposed the plum pudding model of the atom in 1904 before the discovery of the atomic nucleus in order to include the electron in the atomic model. In Thomson's model, the atom was a sphere embedded with electrons. The sphere had positive charges throughout it and negatively charged particles were also scattered across this model. Some nicknames for this model are "plum pudding" and "raisin bread" Ernest Rutherford* James Chadwick Niels Bohr* Erwin Schrodinger* Robert Millikan Henry Moseley Rutherford overturned Thomson's model in 1911 with his well-known gold foil experiment in which he demonstrated that the atom has a tiny and heavy nucleus. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. James Chadwick played a vital role in the atomic theory, as he discovered the Neutron in atoms. Neutrons are located in the center of an atom, in the nucleus along with the protons. They have neither a positive nor negative charge, but contribute to the atomic weight with the same effect as a proton. In 1913, Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. The Bohr Model explains to us that electrons or negative charges orbit around the atom's nucleus in energy levels. It also describes that electrons can change energy levels. For example, an electron could change from a high energy level to a lower one. In 1926 Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one step further. Schrödinger 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. Using quantum mechanics, chemists can use the electron cloud model to assign electrons to different atomic orbitals. These atomic orbitals are not all spheres. Atomic orbitals also explain the patterns in the periodic table. The electron cloud model was developed in 1926 by Erwin Schrödinger and Werner Heisenberg. American physicist Robert Millikan. His oil drop experiment helped to quantify the charge of an electron, which contributed greatly to our understanding of the structure of the atom and atomic theory. Henry Moseley was an English physicist who contributed to atomic theory and lived from 1887 to 1915. ... In his research he coined Moseley's Law, which is: the frequency of x-ray radiation has a precise mathematical relationship to an element's atomic number. Moseley's law advanced atomic physics, nuclear physics and quantum physics by providing the first experimental evidence in favor of Niels Bohr's theory, aside from the hydrogen atom spectrum which the Bohr theory was designed to reproduce. When Moseley arranged the elements in the periodic table by their number of protons rather than their atomic weights, the flaws in the periodic table that had been making scientists uncomfortable for decades simply disappeared.
