1. Democritus, 460-370BC(Greece) Democritus believed that all matter is made up of various imperishable, indivisible elements which he called atomamatterelements (sg. atomon) or "indivisible units", from which we get the English word atom.atom Honors ChemistryName _________________________ History of the Atom Date __________________________ John Dalton, 1766 -1844AD (England) Five main points of Dalton's Atomic Theory Elements are made of tiny particles called atoms. All atoms of a given element are identical. The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative weights. (now they’re identified by the # of protons) Atoms of one element can combine with atoms of other elements to form compounds; a given compound always has the same relative numbers of types of atoms. (now known as the Law of Definite Proportions) Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together. (now known as the Law of Conservation of Mass) Dalton’s model of the atom

2. J.J. Thomson, 1856-1940 (England) Plum Pudding Model of Atom In 1903, J. J. Thomson proposed a subatomic model of the atom. The model pictured a positively-charged atom containing negatively-charged electrons. Thomson visualized electrons in a homogeneous sphere of positive “stuff” in a way that was analogous to raisins in English plum pudding Thus, the Thomson proposal became popularly know as the plum pudding model of the atom. This model was not correct, but it was consistent with the evidence of the time. J.J. Thomson used results from cathode ray tube (commonly abbreviated CRT) experiments to discover the electron. Cathode ray tube (CRT) Cathode rays tubes produce streams of electrons (originally called “cathode rays”. A cathode ray tube is a vacuumed tube equipped with at least two electrodes, a cathode (negative electrode that produces electrons) and an anode (positive electrode). When Thomson shot electrons from the cathode to the anode through a charged region in the middle of the tube, the “cathode ray” bent slightly towards the positive region. He concluded that the “cathode ray” was made up of tiny negative particles (that we now call electrons)electronselectrodes cathodeanode Mass Spectroscopy Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of ions. It is most generally used to find the composition of a physical sample by generating a mass spectrum representing the masses of sample components. The mass spectrum is measured by a mass spectrometer. (SEE NEXT PAGE)mass-to-charge ratioionsmass spectrum

3. This technique is applicable in: identifying unknown compounds by the mass of the compound molecules or their fragmentscompounds determining the isotopic composition of elements in a compoundisotopic determining the structure of a compound by observing its fragmentationstructure Mass spectrometers consist of three basic parts: an ion source, a mass analyzer, and a detector system. The stages within the mass spectrometer are:spectrometersion source Production of ions from the sample Separation of ions with different masses Detection of the number of ions of each mass produced Collection of data to generate the mass spectrum Mass Spectrometer Example of Mass Spectrometer results

4. Earnest Rutherford, 1871-1937 (England) Known as the "father" of nuclear physics, Rutherford pioneered the orbital theory of the atom through his discovery of alpha particles scattering off the nuclei of gold atoms in his gold foil experiment. Rutherford is known to have discovered the solid center of the atom known as the nucleus. He “modified” Thomson’s Plum Pudding Model by adding a solid center to the atom. He was awarded the Nobel Prize in 1908nuclear physics orbital theoryatomalpha particles scatteringgold foil experiment Top: Expected results: alpha particles passing through the plum pudding model of the atom undisturbed. Bottom: Observed results: a small portion of the particles were deflected, indicating a small, concentrated positive charge. Rutherford’s model of the Atom It differs from Bohr’s Planetary Model of the atom in that Rutherford does not define Electrons as having any sort of orbitals Rutherford’s Gold Foil Experiment Gold foil atom’s up-close "For concreteness, consider the passage of a high speed a particle through an atom having a positive central charge, and surrounded by a compensating charge of electrons." Rutherford didn’t use the word “nucleus”. The following is his actual terminology

5. Niels Bohr, 1885-1962 (Denmark) Bohr’s Planetary Model of the Atom (The top model is closest to his model) Bohr asked the question, “Why don’t the negative electrons end up in the positive center of the atom in Rutherford’s model?” He concluded that the electrons must be moving around the positive nucleus in “orbits”, much like how the planets orbit around the sun. Their individual velocities keep them at certain distances from the positive nucleus. These distances are called energy levels. The closer an electron (negative) is to the nucleus (positive) the happier it is. (LOW POTENTIAL ENERGY electron) The farther away an electron (negative) is from the nucleus (positive) the sadder it is (HIGH POTENTIAL ENERGY electron)  *Electrons far from the nucleus want to be near the positive nucleus. They have great “potential energy” because they have such a large fall (release of kinetic energy) to undergo to get to the positive nucleus! When the fall they release ENERGY! This ENERGY sometimes can be seen by the naked eye as COLOR! When an element (such as Hydrogen) is excited with energy it glows. The light emitted is actually mixture of certain set of color wavelengths; a set that is unique to that particular element. The wavelengths released from the element are produced from the excited electrons (that were excited with energy into outer orbits) relaxing back to orbits closer to the nucleus. (ground state) They release energy in this process (light!) + Positive Nucleus is HERE Far away from Nucleus Each line is an orbit that electrons may be in

6. Louis de Broglie, 1892-1987 (France) Erwin Schrödinger, 1887-1961 (Austria-Hungary) Ernest Heisenberg, 1901-1976 (Germany) The New Age Model of the Atom Albert Einstein, 1879-1955 (Germany) Electron Cloud Model of the Atom (Present day model of the atom) De Broglie suggested that electrons could be considered waves since they seemed to behave as waves in some experiments. As waves, electrons could only have certain frequencies, corresponding with the energy levels in which they are found. Schrödinger was a close friend of Einstein. He claimed that waves have discrete energy packets (called quanta – meaning “quantity”) that behave in a manner similar to particles. This led to the branch of physics that deals with atomic and subatomic systems which we call quantum mechanics. Heisenberg is one of the founders of quantum mechanics (The idea that energy exists in certain quantities “quanta”) He is also known for the Heisenberg Uncertainty Principle, which states that locating a particle in a small region of space makes the momentum of the particle uncertain; and conversely, that measuring the momentum of a particle precisely makes the position uncertain. This is the area of probability or “probability density” of where an electron most likely will be found in only the 1s orbital In 1905, Einstein proposed that light behaves as both a wave and a particle. This is the dual theory of light.

7. Present Day Model of the Atom (orbital shapes) -consists of areas of probability (probability density) where electrons were calculated to most likely will be found at particular energy levels within an atom s orbital d orbital components p orbital components Complete p orbital