The Missing Manual of Historical Figures Atomic History The Nuts and Bolts of Chemistry – The Missing Manual of Historical Figures

OVERVIEW Early Greeks - Democritus v Aristotle Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom ket09232004

OVERVIEW Early Greeks - Democritus v Aristotle Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom ket09232004

Early Greeks What is the nature of matter? -Infinitely divisible pieces of “stuff” -Earth, Air, Fire ,and Water No experiments ket09232004

Early Greeks Democritus Aristotle vs ket09232004

Aristotle vs. Democritus New idea “atomos” - indivisible Eventually, can’t divide matter any more First “atomic theory” Aristotle Didn’t buy it… All things are infinitely divisible Guess who won? ket09232004

For 2000 years, scientists thought all matter was infinitely divisible CHAMPION (Aristotle) For 2000 years, scientists thought all matter was infinitely divisible ket09232004

OVERVIEW Early Greeks - Democritus v Aristotle Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom ket09232004

Dalton’s Atomic Theory John Dalton 1807 Idea of “atom” Solid spheres-indestructible Unique to each element Combine evenly Reactions are rearrangements ket09232004

Dalton’s Atomic Theory Atoms combine evenly in compounds Small, whole number proportions (e.g.) water - H20 H O H O ket09232004

OVERVIEW Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom ket09232004

Thomson’s Discovery John Dalton Cathode Ray Tube Sends a “ray” of particles Used magnet to deflect beam first So, beam was made of particles Thomson then passed ray through +/- charged plates J.J. Thomson 1897 ket09232004

Thomson’s Discovery The beam was attracted to the positive plate. So, cathode rays are negative He called them ELECTRONS ket09232004

Thomson’s Discovery Most books give Thomson credit for discovering proton He and Millikan found the mass of an electron to be much smaller than an atom So, electrons are VERY VERY small Protons must be large in comparison plum pudding model ket09232004

Plum Pudding model Preface: “Plum Pudding” atomic model atoms are solid made of positively-charged material with negative “bits” scattered throughout (like raisins in plum pudding) (or raisin bread) ket09232004

OVERVIEW Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom ket09232004

Rutherford’s Discovery Ernest Rutherford Image courtesty of http://www.nobel.se/chemistry/laureates/1908/rutherford-bio.html ket09232004

Gold Foil Experiment To test the plum pudding model (1907): Shot alpha rays at thin gold foil (about 2000 atoms thick) EXPECTED to see the ray scatter as it hit all the solid atoms Like spray from a nozzle ket09232004

Gold Foil Experiment Here is what they saw: ket09232004

Click here to see an animation. Gold Foil Experiment In Detail: Click here to see an animation. ket09232004

Gold Foil Experiment RESULTS Most of the particles were not deflected Some were minimally deflected VERY few (1 in 20,000) bounced back “as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” - Rutherford ket09232004

Gold Foil Experiment CONCLUSIONS Plum pudding model wrong A “nucleus” exists It is tiny It is densely-packed and positively-charged Empty spaces exist in atoms LOTS of it!!!! ket09232004

Gold Foil Experiment How much empty space? Use a billiard ball to represent a nucleus The electrons occupy a volume one kilometer in ALL DIRECTIONS Most of that space is EMPTY. ket09232004

OVERVIEW Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom ket09232004

Chadwick’s Discovery (1932) PROBLEM There was more mass in nucleus than explained by protons alone Where did it come from? NEUTRONS Chadwick ket09232004

OVERVIEW Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom ket09232004

Bohr’s Atomic Model Nucleus has + charge Electrons have - charge Why don’t electrons simply “fall into” the nucleus? Bohr ket09232004

Bohr’s Atomic Model Bohr pictured atoms as little solar systems Nucleus in center Electrons “orbiting” in circles We now know this isn’t correct But it helps to explain many things about atoms ket09232004

HOMEWORK Read: Section 3.2 Do: p. 89; Q’s 1-3 p. 107; Q’s 17, 18 ket09232004

Protons, Neutrons, and Electrons Three Main Subatomic Particles Protons Located in nucleus Positively charged (+1) Mass = “1” mass unit (or 1 gram/mole) Number of protons = atomic number (Z) ket09232004

Protons, Neutrons, and Electrons Three Main Subatomic Particles Neutrons Located in nucleus no charge (0) Mass = “1” mass unit (or 1 gram/mole) ket09232004

Protons, Neutrons, and Electrons Three Main Subatomic Particles Electrons Located outside nucleus negative charge (-1) So small we assume mass = 0 Actual mass of 0.000 5 g/mol ket09232004

Protons, Neutrons, and Electrons Atomic number (Z) shown in lower left ALWAYS equals the number of protons Equals the number of electrons in a neutral atom Isotope Notation C 6 ket09232004

Protons, Neutrons, and Electrons Mass Number (A) shown in upper left Is the sum: neutrons + protons In this example, carbon has 6 protons and 7 neutrons Isotope Notation C 13 6 ket09232004

Protons, Neutrons, and Electrons We call this atom “carbon-13” Isotope Notation C 13 6 ket09232004

Protons, Neutrons, and Electrons Atoms with the same number of protons can have different numbers of neutrons. We call such atoms isotopes ket09232004

Protons, Neutrons, and Electrons Atoms with the same number of protons can have different numbers of neutrons. We call such atoms isotopes C C C 12 13 14 6 6 6 Carbon -12 Carbon -13 Carbon -14 ket09232004

Protons, Neutrons, and Electrons 12 13 14 6 6 6 6 protons 6 electrons 6 neutrons 6 protons 6 electrons 7 neutrons 6 protons 6 electrons 8 neutrons All three kinds of carbon atoms have the same chemistry! ket09232004

Protons, Neutrons, and Electrons H Pb F 1 208 +2 19 1 82 9 1 proton 1 electron 0 neutrons Neutrons = A - Z 1 - 1 = 0 82 protons 80 electrons 126 neutrons Neutrons = A - Z 208 - 82 = 126 9 protons 9 electrons 10 neutrons Neutrons = A - Z 19 - 9 = 10 ket09232004

Protons, Neutrons, and Electrons ? 7 3 What element is this? How do you know? ket09232004

Protons, Neutrons, and Electrons Li 7 3 What element is this? How do you know? Lithium the atomic number is 3 look on the periodic table ket09232004

Protons, Neutrons, and Electrons Pb ? K 207 235 ? ? 92 19 # of protons (Z) # of neutrons # of electrons mass number (A) name 82 125 207 lead-207 92 143 235 uranium-235 19 21 40 potassium-40 ket09232004