Atomic Theory

Democritus 460 – 370 B.C. There are various basic elements from which all matter is made Everything is composed of small atoms moving in a void Some atoms are round, pointy, oily, have hooks, etc. to account for their properties Ideas lacked experimental evidence Ideas rejected by leading philosophers because void = no existence

First Concept of an Atom

Lavoisier The “Father of Modern Chemistry.” In 1774, he showed/developed the Law of Conservation of Mass: Matter cannot be created nor destroyed, only changed in form.

John Dalton 1766-1844 Introduced his ideas in 1803 Each element is composed of extremely small particles called atoms All the atoms of a given element are identical, but they differ from those of any other element Atoms are neither created nor destroyed in any chemical reaction (Repeating Lavoisier) A given compound always has the same relative numbers and kinds of atoms

Dalton’s Model

J.J. Thomson 1856-1940 Cathode Ray tubes were developed in1879 by William Crookes and fine tuned over the next 20 years. Discovered electron 1897 – Cathode Ray Experiment Plum Pudding model 1904 All sources (elements) could produce cathode rays so electrons in all atoms. All atoms are neutral so there also had to be a positive charge in the atoms Electrons in a soup of positive charges Discovered isotopes in1913

Cathode Ray Tube Experiment

Plum Pudding Model

In 1909 Robert Millikan measured the charge on an electron in his Oil Drop Experiment. Using the charge to mass ratio he concluded that: the mass of an electron is 1840 times smaller than that of the Proton the charge on an electron is -1.

Ernest Rutherford (That’s Sir to you) 1871-1937 Becquerel discovered radioactivity around 1895 but Rutherford separated and defined the three types. Figured out alpha particles were helium nucleii using atomic spectra. Worked with Gieger to develop an Alpha particle counter.

Ernest Rutherford 1871-1937 Nucleus Theory 1911 Alpha particle gold foil experiment Most particles went straight through so an atom is mostly empty space Some particles were deflected so the nucleus (center of the atom) must have a positive charge An atom’s mass is mostly in the nucleus Electrons in fixed orbit

Rutherford to Chadwick Rutherford was convinced the nucleus, because of it’s mass, contained another particle other than protons. He thought the other particle was a “neutral couplet” Chadwick discovered the neutron by bombarding atoms with alpha particles which caused a release of neutrons from the nucleus.

Rutherford Model

Niels Bohr 1885-1962 Planetary Model 1913 Nucleus surrounded by orbiting electrons at different energy levels Electrons have definite orbits These orbits are at different distances from the nucleus. These orbits, or energy levels, are not evenly spaced. Utilized Planck’s Quantum Energy theory Worked on the Manhattan Project (US atomic bomb)

Bohr’s Model

Bohr Model for Nitrogen

Werner Heisenberg 1901-1976 Ernst Schrödinger 1887-1961 Quantum Mechanical Model 1926 Electrons are in probability zones called “orbitals”, not orbits and the location cannot be pinpointed Electrons are particles and waves at the same time Developed quantum numbers based on theories of Einstein and Planck

Orbitals

Questions How did the atomic theory come about? Was it one person? Did each person start anew? Are we sure the current theory is correct? What technological developments might lead to changes in the current theory? What would cause one to change this or any theory?