Particles, Nuclei, and Isotopes The Nuts and Bolts of Chemistry

Atomic History: An Overview Early Greeks - Democritus vs Aristotle Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom

Atomic History: An Overview Early Greeks - Democritus vs. Aristotle Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom

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

Early Greeks Aristotle Democritus vs

Democritus vs. Aristotle 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?

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

Atomic History: An Overview Early Greeks - Democritus vs Aristotle Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom

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

Dalton’s Atomic Theory Atoms combine evenly in compounds Small, whole number proportions (e.g.) water - H20 (not H1O1/2 ) H O H O

Atomic History: An Overview Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom

Thomson’s Discovery Studied electricity not atomic structure His 1st observation was that a magnet could deflect the current and realized that it was made of particles So to study current he pumped all the air out of a glass tube and applied voltage to two metal plates + (anode)/- (cathode) J.J. Thomson 1897

Thomson’s Discovery (cont…) He then sent a ray of particles through the tube and saw that they came out of the cathode side and were attracted to the anode side and the tube glowed This told him that the particles were negative… he called his discovery a Cathode Ray Tube Sends a “ray” of - particles Used in TVs and Computers J.J. Thomson 1897

Thomson’s Discovery The beam was attracted to the positive plate. So, cathode rays are negative He realized that these negative particle must be ELECTRONS

Thomson’s Discovery Most books give Thomson credit for discovering the 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

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)

Atomic History: An Overview Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom

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

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 positive atoms Like spray from a nozzle

Gold Foil Experiment Here is what they saw:

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

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

Gold Foil Experiment CONCLUSIONS Plum pudding model wrong A “nucleus” exists It is tiny It is densely-packed and positively-charged (this is the only way a strong positive particle could be deflected) Empty spaces exist in atoms LOTS of it!!!!

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.

Atomic History: An Overview Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom

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

Atomic History: An Overview Dalton’s Atomic Theory Thomson - discovery of electrons Rutherford - discovery of nucleus Chadwick - discovery of neutron Bohr - planetary model of atom

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

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

What Theory Do We Currently Support? Electron Cloud Model Nucleus is composed of Protons and Neutrons Electrons spin very quickly around the nucleus forming a “giant cloud” of negative energy These electrons can move further away or closer to the nucleus at anytime Their distance and space is infinite!

HOMEWORK Notes 4.2

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)

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

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

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

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

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

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

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

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!

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

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

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

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

Homework… Complete the Atomic Structure Worksheets ((((( Back AND Front )))))