Atomic Theory Past and Present

Matter is made up of particles called “atomos” or atoms I. Early Atomic Theory A. Democritus (400 BCE) http://www.timelineindex.com/content/view/1228 Matter is made up of particles called “atomos” or atoms

B. John Dalton’s Atomic Model (1808) Oxygen Hydrogen •1) Atoms are hard spheres which cannot be further broken down. •2) Atoms of one element are exactly alike, but different from atoms of another element.

3)Law of Multiple Proportions H2O Water H2O2 Hydrogen Peroxide 2 grams H + 16 grams O 2 grams H + 32 grams O Compounds always break down into simple ratios of elements by mass.

C. Thomson’s Atomic Model “Plum Pudding” (1897) http://www.aip.org/history/electron/jjcrooke.htm JJ Thomson discovered all elements contain ELECTRONS by using the cathode ray tube; Electrons are small, negatively charged particles.

D. Eugene Goldstein (1900) Discovered the proton using the cathode ray tube. The “canal rays” traveled in the opposite direction of the electron beam. Protons are positively charged as compared to electrons but protons are much more massive.

E. Rutherford’s Atomic Model “Planetary” (1911) •Rutherford discovered that all atoms contain a dense, positively-charged nucleus. •Most of the atom is empty space.

Gold Foil Experiment http://www.regentsprep.org/Regents/physics/phys05/catomodel/ruther.htm

F. Bohr’s Atomic Model “Quantized Electrons” (1913) For now, the electron pattern is 2: 8: 8: 2 •Electrons are only allowed certain, specific energies. •Electrons can be on the rings but not in-between.

Emission and Absorption Spectra H Hg Ne http://www.cbu.edu/~jvarrian/252/emspex.jpg

G. Sir James Chadwick (1932) Discovered the neutron Chadwick found that the particles given off by polonium increased the mass of the atoms but not their charge—evidence of the neutron (same mass as a proton but neutral)

If this is your vision of an atom, you are back in the 1920’s!! IN SUMMARY Atoms contain a dense nucleus with protons and neutrons. Atoms are mostly empty space. The nucleus has electrons orbiting around the nucleus in discrete energy levels. Electron-small mass, negative Proton-much larger mass, positive Neutron-about same mass as proton, no charge If this is your vision of an atom, you are back in the 1920’s!!

II. Atomic Model moves from the planetary view into Quantum Theory (1920’s) Quantum Theory is based on waves (electrons behave like both particles and waves) and probabilities

Light as Waves http://ircamera.as.arizona.edu/NatSci102/lectures/light.htm

Several scientists have contributed to Quantum Theory

A. Max Planck (1900) Light had been thought of as WAVES. Higher energy means higher frequency. Light is composed of tiny packets (photons) of quantized energy. Light can also act like particles!

B. Louis de Broglie (1923) Quantum numbers could describe BOTH the orbits of electrons and the spectral lines produced. Particles can act like waves

C. Werner Heisenberg (1925) Discovered that we can’t know exactly both speed and position of electrons—the “Uncertainty Principle”—because our methods of measuring atoms disturbs them.

D. Erwin Schrödinger (1926) Did “The Math” and put it all together He treated electrons mathematically as waves We can’t know exactly where an electron is, but we can give a location of high probability

E. Max Born (1926) Electron Probability Born introduced the idea of probability in describing the atomic world—we don’t know exactly where the electron is but we have a pretty good idea where it probably is. http://www.regentsprep.org/Regents/physics/phys05/catomodel/ruther.htm

Modern Atomic Model Summary—“Quantum Mechanics” •Electrons are like waves. •We can’t know exactly where electrons are, only where they are most likely.

Orbital Quantum Number Gives the shape of the orbital cloud (within 90% probability) s orbital— spherical

Orbital Quantum Number p orbitals—”dumbbell” shaped or similar to a “figure 8”

Orbital Quantum Number d orbitals—shape has 4 “lobes”

Orbital Quantum Number f orbitals—some people describe a “daisy-like” http://www.d.umn.edu/~pkiprof/ChemWebV2/AOs/index.html www.chem.ous.ac.jp

III. What makes up protons and neutrons? Quarks! During the 1960’s various physicists discovered fundamental particles, known as quarks Protons and neutrons are made up of quarks Electrons are also fundamental particles

IV. What is the most recent theory IV. What is the most recent theory? String Theory (late 20th Century to present) All fundamental particles are made of vibrating strings/strands of energy http://www.columbia.edu/cu/record/23/18/14.html