Democritus's Atomic Theory everything is composed of "atoms", which are physically, but not geometrically, indivisible; that between atoms, there lies empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size. Of the mass of atoms, Democritus said "The more any indivisible exceeds, the heavier it is." But his exact position on weight of atoms is disputed. Most people did not believe Democritus because he didn't have any proof.
Dalton's Atomic Theory 1) All matter is made of atoms. Atoms are indivisible and indestructible. 2) All atoms of a given element are identical in mass and properties 3) Compounds are formed by a combination of two or more different kinds of atoms. 4) A chemical reaction is a rearrangement of atoms.
John Dalton's model was almost identical to Democritus's. John Dalton pretty much copied Democritus, but everyone had already forgotten about Democritus.
Thomson Atomic Theory In 1904, 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. Thomson realized that the accepted model of an atom did not account for negatively or positively charged particles.
Rutherford Atomic Theory Rutherford's new model for the atom, based on the experimental results, contained the new features of a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom and with this central volume also containing the bulk of the atomic mass of the atom.
Bohr's Atomic Theory In 1913, Neils Bohr, a student of Rutherford's, developed a new model of the atom. He proposed that electrons are arranged in concentric circular orbits around the nucleus.
Chadwick's Atomic Theory The neutron was discovered in 1932 by James Chadwick. With the discovery of the neutron, all models and scientific discoveries what altered.
Electron Cloud Model Since John Dalton breathed life into the atomic model in the early 1800s, scientists have been laboring to understand the intricacies of atomic structure. In the mid-1920s, research supporting the electron cloud model began to gain momentum as classical physics failed to explain such phenomena as how electrons could seemingly be everywhere at once, or why electrons did not crash into the nucleus when they gained or lost energy.
Quantum Mechanical Model of an Atom It provides a neat and familiar picture of electrons orbiting a central nucleus like planets around the Sun. Because of this, it can be challenging to replace this picture with one that more accurately represents the quantum-mechanical model used by modern physicists: an atom with a tiny nucleus with probability waves instead of sharp orbits to describe the distribution of electrons, which have fuzzy positions but definite energy levels. Nevertheless, students should get a glimpse of this more modern model.