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Atomic Structure Dalton’s Atomic Theory John Dalton performed experiments to study the ratio in which elements combine in chemical reactions He then formulated a hypotheses and theories to explain his observations Daltons Atomic Theory All elements are composed of indivisible particles called ATOMS Atoms of the same ELEMENT are identical ATOMS of any one ELEMENT are different from those of any other element Atoms of different elements can physically mix together or chemically combine to form compounds Chemical reactions occur when atoms are joined, separated or rearranged Atoms of one element are never changed into atoms of another by a chemical reaction What did Dalton base his theory on? Law of Conservation of Mass (Antoine Lavosier, 1789) Law of Definite Proportion, (Proust's Law) Atoms are neither created or destroyed (under normal chemical reactions) Atom ratio is fixed, so mass must be constant. Law of Multiple Proportions (Dalton) Applies where two elements, A and B, form more than one compound. How Big is an Atom Imagine a penny, made up of copper (Cu) If we ground up the penny into dust, the dust would still have the same properties as copper If you placed atoms of copper in a row, 100 000 000 atoms would make a line 1 centimeter long 1 2 3 4 5 How Big is an Atom? A penny contains around 2.4 x 1022 atoms of copper Can we see atoms? To see atoms we need a scanning tunneling electron microscope or an ion beam microscope A micrograph of a NickelMolybdenum (Ni4Mo) bimetallic compound, each dot is a single atom The specimen is in the form of a needle that is over 1000 times sharper than an ordinary household sewing needle. In fact, the end of the needle is so sharp that it cannot be seen by the naked eye or even a standard optical microscope http://www.ornl.gov Atomic Diameter Units The typical atom has a diameter on the order of 1 - 3 x 10-10 m Angstrom (Å) (non-standard) 10-10 m Nanometers (nm) 10-9 m Picometers (pm) 10-12 m Typical atom diameter = 1 - 3 Å, 0.1 – 0.3 nm or 100 – 300 pm What’s Wrong with DAT? Most of Dalton’s theory is accepted today However, We now know that atoms ARE divisible Atoms contain sub-atomic particles ELECTRONS PROTONS Trinity NEUTRONS Electrons Discovered in 1887 by J. J. Thomson Thompson initially called electrons CATHODE RAYS Negative Positive Vacuum High Voltage Thomson then added electrical plates above and below the tube He found that positively charged plates ATTRACTED the ‘ray’, negatively charged plates repel Cathode ray made up of tiny negatively charged particles 1900 – Electron defined as a small charged particle approximately 1/2000 the mass of a Hydrogen atom 1916 Robert A. Millikan determined the charge carried by an electron and the charge to mass ratio Look up Millikan’s Oil Drop Experiment The Electron Today Symbol Relative mass (compared to a proton) Actual mass Relative electrical charge Actual electrical charge e1/1840 9.11 x 10-28g 1-1.602 x 10-19 C Protons 1866 E. Goldstein found evidence of positively charged particles traveling in the opposite direction to electrons in a cathode ray tube He named them CANAL RAYS He deduced that canal rays were positively charged particles we now know as PROTONS Proton Summary Symbol Relative mass Actual mass p+ 1 1.67 x 10-24g (1840 x me) Relative electrical charge 1+ Actual electrical charge +1.602 x 10-19 C Neutron Sir James Chadwick (1891 – 1974) Discovered the neutron in 1932 Has no charge but a mass close to that of the proton electron neutron proton Neutrons Symbol Relative mass Actual mass Relative electrical charge Actual electrical charge n0 1 1.67 x 10-24g (1840 x me) 0 0C Rutherfords Experiment 1911, University of Manchester, England A beam of a-particles (a Helium nucleus – He – 2e-) aimed at a gold foil Foil surrounded by fluorescent screen which flashed when hit by an a-particle Experiment done in a vacuum Vacuum Flourescent Screen Au Foil a-source c. Rutherford Appleton Laboratories Rutherford’s Results Conclusions Rutherford concluded that the atom is mostly empty space However, the atom contains a nucleus where the mass and positive charge of the atom are concentrated We now know that the nucleus contains PROTONS and NEUTRONS Electrons ORBIT the nucleus but are not part of it Review What do you think of these statements? All atoms are identical Chemical Reactions occur when atoms of one element change into atoms of another An atom has protons, neutrons and electrons in the nucleus