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2/5/2014 Early Atomic Theory A Historical Perspective What do Dinosaurs and Atoms have in Common? No one has seen an atom or a dinosaur directly. We know of their existence only by indirect evidence. Our theories of both dinosaurs and atoms has changed over time based on this indirect evidence 1 2/5/2014 Democritus, Greek philosopher, 460 to ca. 370 BC held that everything is composed of "atoms", which are invisible, indestructible fundamental units of matter • that between atoms 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. • Matter is composed of tiny particles called atoms Titus Lucretius Carus (died c. 50 BC) was an Epicurean poet writing in the middle years of the first century BC. He wrote a six-book Latin hexameter poem De rerum natura translated On the nature of things The universe is infinite, Lucretius argues, consisting of infinitely extended space and an infinite number of atoms. 2 2/5/2014 What is an ‘Atom’ • The smallest particle in an element that retains its identity in a chemical reaction. • From the Greek adjective atomos or atomon, ‘indivisible’ PIECES OF CARBON CARBON ATOM Antoine Lavoisier (France 1782) • Law of Conservation of Mass • In a chemical reaction mass is conserved. 3 2/5/2014 His experiments… When mercury is heated in oxygen at a moderate temperature, a red substance, calx of mercury, (mercuric oxide) is obtained. (A calx is the ash left when a substance burns in air.) At a higher temperature this calx decomposes into mercury and oxygen. 2 Hg(l) + O2(g) → 2 HgO (s) His experiments also revealed that the combined masses of mercury (200.59) and oxygen (16.00) were exactly equal to the mass of calx of mercury (216.59). That is, there was no change in mass upon formation or decomposition of the calx. Lavoisier hypothesized that this should be true of all chemical changes, and further experiments showed that he was right. This principle is now called the law of conservation of mass 4 2/5/2014 Joseph Proust (France 1754-1826) Based on his studies on copper carbonate reactions, Proust discovered that each pure compound has its own characteristic elemental composition Law of Definite Proportions: The elements that comprise a compound are always in a fixed proportion by mass. H2O 2 g/mol : 16 g/mol John Dalton (England 1766-1844) • School teacher • Studied the ratios in which elements combine in chemical reactions • Formulated the first modern Atomic Theory 5 2/5/2014 Dalton’s Atomic Theory 1. All matter is made of atoms. 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 3. Atoms of different elements can chemically combine in simple whole number ratios to form compounds. Example: CO2 Dalton’s Atomic Theory 4. Chemical reactions occur when atoms are rearranged. Chemical reactions do not change atoms of one element to another. Dalton’s Atomic model 6 2/5/2014 Atomic Theory • Much of Dalton’s theory still holds today • However, we now know that atoms are not indivisible Fermi Lab http://www.youtube.com/v/IrrxNxDKtd8 Particle collider Result of particle collisions 7 2/5/2014 Scanning Tunneling Microscope (STM) http://www.youtube.com/v/BUKTIwL1Uy8 Scanning Tunneling Microscope (STM) The scanning tunneling microscope (STM) is an instrument with which one can image surfaces so finely that individual atoms are resolved. The STM can even move atoms about. The images were obtained by scanning the surface with the same instrument. 8 2/5/2014 Iron Atom Arrangement - STM Atomic Force Microscope (AFM) is one type of scanning probe microscopes, which is used to image surface structures (on a nm or even sub-nm scale scale) and to measure surface forces http://www.youtube.com/v/MZb8C0f7Kdg http://www.youtube.com/v/Ha53tFTsmW8 9 2/5/2014 What is the size of an atom? • If you placed 100,000,000 Cu atoms side by side they would form a line only 1 cm long • Radius of most atoms is about 5x10-11 to 2x10-10m STM Image The Discovery of the Electron and the Proton 10 2/5/2014 History • Electron means “amber” in Greek • Properties discovered by the Greek Thales of Miletos 600 BC. Rubbed the mineral amber with cat fur and attracted feathers. Benjamin Franklin (America 1740’s) • Law of conservation of charge. • Viewed electricity as a flowing fluid and called the flow direction positive. 11 2/5/2014 Law of Charges • Like charges repel • Opposite charges attract The Electron 12 2/5/2014 Sir William Crookes (1855) invented the Crookes tube or cathode ray tube (CRT) Crookes could actually see some sort of particle beam going from the negative cathode to the positive anode. It wasn’t until 1897 that this “tube” was used to change Dalton’s Billiard Ball Model. 13 2/5/2014 Joseph John Thomson (England 1897) • J.J. Thomson took the Crookes tube and experimented with it. He held a magnet to the side of the tube and tried to direct the beam of light. • He discovered the electron while experimenting with cathode rays. Thomson took Crookes’ tube and used a magnet to bend the ray. The ray bent towards the positive side of the magnet. He concluded that the ray must have had a negative charge if it was attracted to the positive side. These negative charges became known as electrons 14 2/5/2014 Thomson’s Cathode Ray Tube http://www.youtube.com/v/GzMh4q-2HjM Thomson Experiment https://sites.google.com/site/physicsflash/home/thomson 15 2/5/2014 Thomson’s Discovery He determined that the cathode ray was made of negatively charged particles – electrons Thompson showed that the production of the cathode ray was not dependent on the type of gas in the tube, or the type of metal used for the electrodes. His discovery of the electron won the Nobel Prize in 1906 The Plum Pudding Model Thomson did not know how the electrons in an atom were arranged. He believed they were mixed throughout an atom. He proposed that the atom was a sphere of positively charged material. Spread throughout the atom were the negatively charged electrons similar to plums in a pudding or chocolate chips in ice cream. 16 2/5/2014 Robert Millikan - 1909 determined the size of the charge on an electron. He also determined that there was a smallest 'unit' charge, or that charge is 'quantized'. He received the Nobel Prize for his work. http://www.youtube.com/v/XMfYHag7Liw Electrical Charge • Charge of Electron 1.6 x 10-19 C (coulombs) • Rest Mass of Electron 9.11 x 10-28 g • Electric charges always exist in whole number multiples of a single basic unit, the electron 17 2/5/2014 The Proton and Atomic Nucleus Eugen Goldstein (German, 1886) Observed that cathode-ray tubes with a perforated cathode emit a glow from the end of the tube near the cathode. Concluded that there is another ray (positive particle) that travels in the opposite direction (opposite the cathode ray), from the anode toward the cathode. Because these rays pass through the holes, or channels, in the cathode, he called them canal rays. 18 2/5/2014 Canal Rays passed through holes, or channels, in the reverse direction as the cathode ray https://www.youtube.com/v/3WIjCtZLMDg Canal Rays 19 2/5/2014 Ernest Rutherford (England, 1911) • Tested Thomson’s theory of atomic structure with the “gold foil” experiment in 1910. • Proposed the name proton for the positively charged particles in the nucleus of an atom Gold Foil Experiment • Bombarded thin gold foil with a beam of ‘alpha’ particles (Helium nucleus). • If the positive charge was evenly spread out, the beam should have easily passed through. 20 2/5/2014 Rutherford's Gold Foil Experiment http://www.youtube.com/v/5pZj0u_XMbc Targets in the Clouds... Give it a try -Find target hidden by a black cloud -Shoot beams into the cloud -Record where the beams come out 21 2/5/2014 Targets in the Clouds Revealed the first target was: a wedge the second target was: a circle Expected Found 22 2/5/2014 Rutherford's Experiment Most particles passed through with no deflection, while some were highly deflected Rutherford concluded that most particles passed through because the atom is mostly empty space. Rutherford’s Conclusions • All of the positive charge, and most of the mass of an atom are concentrated in a small, dense core called the nucleus. • Atoms are mostly empty space. 23 2/5/2014 Rutherford’s Nuclear Model The Neutron 24 2/5/2014 • 1932, James Chadwick proved the existence of the neutron • Chadwick bombarded beryllium with alpha particles. • The beryllium emitted a neutral particle which in turn was used to bombarded the hydrogen atoms in paraffin (and other elements as targets). Chadwick’s Apparatus 25 2/5/2014 • Determine that the neutron did exist and that its mass was about 0.1 percent more than the proton's. • No electrical charge. • His work lead the way to neutron bombardment of the uranium atom, splitting its nucleus and releasing the huge amounts of energy. Subatomic Particles – Fundamental Constants Particle Symbol Relative Charge Mass Relative to the Proton Rest Mass (g) Electron e- 1- 1/1840 9.11 x 10-28 g Proton p+ 1+ 1 1.67 x 10-24g Neutron n0 0 1 1.67 x 10-24g 26 2/5/2014 http://www.pbs.org/wgbh/aso/databank/entries/dp32ne.html http://www.vzhang.com/vzfiles/joseph_proust.htm http://plato.stanford.edu/entries/democritus/ http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/TheAtomic-Theory-687.html 27