Scientists & Their Contributions to the Atomic Model (rev. 10/11)

Democritus Greece Democritus was a Greek philosopher who “…was one of two founders of ancient atomist theory”. “The atomists held that there are smallest indivisible bodies from which everything else is composed, and that these move about in an infinite void space”. “The atomists held that there are two fundamentally different kinds of realities composing the natural world, atoms and void. Atoms, from the Greek adjective atomos or atomon, ‘indivisible,’ are infinite in number and various in size and shape, and perfectly solid, with no internal gaps” Text http://plato.stanford.edu/entries/democritus/ image http://reich-chemistry.wikispaces.com/file/view/demo._atom_model.gif/97847685/demo._atom_model.gif

John Dalton’s: Atomic Theory of Matter England; 19th Century 4 Postulates: elements are composed of atoms all atoms of an element are identical, but different from atoms in other elements atoms are neither created nor destroyed a given compound always has the same relative number and kind of atoms.

J.J. Thomson England; late 1890’s called the negative particles electrons determined the charge to mass ratio of an electron The Plum Pudding Model is Thomson’s name for his model of the atom

JJ Thomson’s Plum Pudding Model of the Atom www.tamucc.edu

Cathode Ray Tube (CRT) negative end is the cathode positive end is the anode A cathode ray is radiation streaming from a cathode to an anode in a CRT it is a stream of particles a magnet can deflect the ray cathode ray particles have a negative charge

Cathode Ray Tube and Magnet Demo http://www.chem.uiuc.edu/clcwebsite/cathode.html

Robert Millikan Early 1900’s; American measured the charge of an electron using the Oil Drop Experiment. His experiment measured the force on tiny charged droplets of oil suspended against gravity between two metal electrodes. 1.60x10 -19 coulomb is the charge of an electron using Thomson’s charge to mass ratio, he determined the mass of the electron is 9.11x10 -28 g

Robert Millikan’s Oil Drop Experiment received the Nobel Prize for his work www.68pair.com

Ernest Rutherford 1911-1920, New Zealand Gold Foil Experiment (alpha scattering) he determined that an atom’s positive charge, and most of its mass, was concentrated in the core (most of the atom is empty space) In 1920, Ernest Rutherford postulated that there were neutral, massive particles in the nucleus of atoms.  he named the core of the atom “the nucleus”

Rutherford’s Model of the Atom The atom contains a tiny dense center called the nucleus The nucleus is essentially the entire mass of the atom http://abyss.uoregon.edu/~js/ast123/lectures/lec04.html

Rutherford’s Gold Foil Experiment Alpha particles were directed at a sheet of gold foil. Over 98% of the particles went straight through Approx. 2% of the particles were deflected Approx. 0.01% of the particles bounced off the gold foil. www.sci.tamucc.edu

Rutherford’s Experiment www.sci.tamucc.edu

Neils Bohr 1913, Danish Bohr introduced the theory of electrons traveling in orbits around the atom's nucleus, the chemical properties of each element being largely determined by the number of electrons in the outer orbits of its atoms. Model is called the Planetary Model. Bohr also introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a photon (light quantum) of discrete energy. http://upload.wikimedia.org/wikipedia/commons/6/6d/Niels_Bohr.jpg

Bohr Planetary Model of the atom

Heisenberg & Schrödinger 1925, 1926 – Germany (Heisenberg) and Austria (Schrödinger) Schrödinger built on Heisenberg’s Uncertainty Principle to develop the Mechanical Model of the Atom Their ideas together gave us the ELECTRON CLOUD model. This is our current atomic model and is known as the Quantum Mechanical Model of the Atom

Quantum Mechanical Model of the Atom. This is the current model of the atom.

Quantum Mechanical Model This image is a representation of Schrodinger’s Quantum Mechanical Model of the atom. This model shows the nucleus in the center surrounded by electrons in different energy levels, but there is no distinct energy level shown as in the Bohr Model. http://www.physicalworld.org/restless_universe/figs/fig_1_30.gif

Modern Atomic Theory * Atoms are composed of protons, neutrons and electrons * Recently, scientists discovered these particles contain particles (i.e. quarks, gluons) * A nucleus is a positively charged central core containing protons and neutrons * An individual proton has a charge that is equal to the charge of an electron, but the charge is positive * In a neutral atom: the number of protons equals the number of electrons * A proton is much more massive than an electron