1. The Structure of the Atom Chemistry – Chapter 4

2. Early Theories of Matter Philosophers ◦ Democritus was first to propose Atomic Theory:  Matter composed of empty space through which atoms move  Atoms are indivisible ◦ Aristotle rejected Atomic Theory  Respected for ideas on nature, physics, astronomy, etc., so most ignored Democritus’ ideas

3. John Dalton ◦ All matter composed of atoms ◦ Atoms can not be divided ◦ Different atoms combine to form compounds ◦ Atoms are separated, combined, or rearranged in chemical reactions ◦ Conducted convincing experiments Atom – smallest particle of an element that still retains the properties of the element ◦ Can move individual atoms around to form shapes, patterns, and simple machines  Nanotechnology

4. Subatomic Particles and the Nuclear Atom Electron discovery ◦ Sir William Crooks discovered the cathode ray  Led to invention of TV  Cathode ray particles carry negative charge ◦ J.J. Thomson found that mass of charged particle was much less than that of a H atom  This meant Dalton was wrong and atoms are divisible  Identified the electron  Plum pudding model (p. 94 fig 4-9)

5. ◦ Robert Millikan determined charge of electron  Single electron carries charge of -1 Nuclear atom ◦ Ernest Rutherford concluded plum pudding model was incorrect  Calculated atom consists of mostly empty space through which electrons move  Concluded there is a small, dense region in the center that contains all positive charge and virtually all mass (nucleus)  Nuclear model (p. 95 fig 4-12)

6. Discovery of protons and neutrons ◦ Rutherford concluded the nucleus contains positively charged particles (protons)  Protons carry a charge of +1 ◦ James Chadwick showed nucleus also contains a neutral particle in the nucleus (neutron)  Mass nearly equal to proton  Neutral charge ◦ Electrons are held within atom by attraction to positively charged nucleus ◦ Number of protons equals number of electrons

7. How Atoms Differ Atomic number ◦ Defined as the number of protons in an atom ◦ Determines element’s position on periodic table ◦ Atomic number = proton # = electron # Isotopes - all atoms of an element have same number of protons and electrons, but number of neutrons differ (isotopes) Mass number – sum of proton # and neutron # ◦ Number of neutrons = mass # - atomic #

8. Mass of individual atoms ◦ Protons and neutrons have approx. same mass ◦ Electrons are MUCH smaller ◦ B/c the masses are so small (must use scientific notation, which is cumbersome), chemists developed a standard for measurement  Carbon-12 atom  Exactly 12 atomic mass units (amu)  1 amu is 1/12 the mass of carbon-12 atom ◦ Atomic mass of an element is weighted average mass of the isotopes of that element

9. Unstable Nuclei and Radioactive Decay Radioactivity ◦ Chemical reactions involve only an atom’s electrons  Nucleus remains unchanged  Atom identity does not change ◦ Nuclear reactions involve change in atom’s nucleus  Atom of one element changes into atom of another element ◦ Radioactivity – some substances spontaneously emit radiation ◦ Radiation – rays and particles emitted by the radioactive material

10. ◦ Radioactive atoms emit radiation b/c their nuclei are unstable ◦ Radioactive decay - unstable nuclei lose energy by emitting radiation spontaneously Types of radiation ◦ Experiment conducted by scientists in late 1800s determined some radiation was deflected toward positively charged plate, some toward negatively charged plate, some not at all ◦ Alpha radiation – radiation deflected toward negatively charged plate  Alpha particles  2 protons, 2 neutrons  +2 charge  Equivalent to He-4 nucleus  Represented by α

11. ◦ Beta radiation – radiation deflected toward positively charged plate  Fast-moving electrons called beta particles  Beta particle is 1 electron with -1 charge  Represented by β ◦ Gamma rays – high-energy radiation that possess no mass and no charge  Represented by γ  B/c massless, emission of gamma rays can not result in formation of new atom ◦ Nuclear stability  Primary factor is ratio of neutrons to protons  Atoms w/ too few/many neutrons are unstable  Few radioactive atoms in nature ◦ Nuclear equation – shows atomic #, mass #, and particles involved  Both mass # and atomic # are conserved