Lewis Diagrams

• Lewis Diagrams are a simplified model of atoms and ions that show only valence elections
• shorthand for chemists to help visualize and predict how atoms will behave in chemical reactions
• the number of dots in a Lewis Diagram is equal to the number of valence electrons and the group number for main group elements
• some rules for drawing a Lewis diagram:
  • the first 2 electrons in a shell pair up
  • if there are exactly 4 electrons, draw 1 dot on each side
• for ions, the number of dots drawn is relative to the neutral atom’s number of valence electrons
  • anions are drawn with extra dots because they gain electrons
  • cations are drawn with fewer or no dots because they lose valence electrons

The octet rule

• atoms tend to gain, lose, or share electrons until their outer most shell has 8 electrons
• helium $(\mathrm{He})$ is an exception as its outermost shell is full/stable with 2 electrons

Periodic table patterns

• for the main element groups, you can quickly determine the number of valence electrons based on their group numbers
  • group 1 = 1 valence electron
  • group 2 = 2
  • group 3-8 = atomic number - 10