XII.  Atoms

1. element – material mad of a single type of atom
2. basic components
3. structure
   1. nucleus
   2. electron cloud
4. Atomic number (# of protons)
5. Variations
   1. normal
   2. ions (anions, cations)
   3. isotopes
6. Mass of an atom
   1. mass number mass # - atomic # = # neutrons
   2. atomic mass (average of all isotopes)

1. Bohr atom (planetary model)- focuses on energy levels in electron cloud
   1. series of discrete energy levels (steps) where an electron can be located
   2. low potential energy near the nucleus (more stable), high potential energy far from nucleus
   3. requires energy input to shift electron to higher level (away from nucleus), but will spontaneously drop to any lower level and release energy
   4. in this model, energy is gained and lost as photons of light
   5. each wavelength has different energy per photon (UV is high, red is lower energy)
   6. each energy level step is given its own principal quantum number
2. shell model – focuses on position of electrons in electron cloud
   1. act like a series of concentric shells (really think they have funny shapes)
   2. inner ones filled first, then successively outwards
   3. outer shell may not be completely filled – valence electrons
   4. 2-8-8-18-18-32-32

1. rows- periods - number of shells
2. columns – groups - number of valence electrons
   1. left side –
      • only a few valence electrons
      • wants to lose them for “completeness”
      • form cations
      • metals
      • good conductors
      • size decreases as more electrons and protons are added (stronger electrical attractions)
   2. right side –
      • nonmetals
      • rightmost column is completely full and stable – inert noble gases
      • other right columns are nearly full
      • want to add electrons – form anions
      • poor conductors